Current UK Research into HFSystems,

HF Propagation, and the Ionosphere

Technical Working Party on HF Propagation and Ionospheric Effects

National Radio Propagation Committee

Radiocommunications Agency

United Kingdom

1998

Introduction

This report gives a brief account of most of the research into high frequency (HF)systems and the ionosphere currently being undertaken in the UK, together withdescriptions of national research facilities.

It has been prepared by the Technical Working Party on HF Propagation andIonospheric Effects, which is a working party of the Radiocommunications AgencyNational Radio Propagation Committee. For information, the terms of reference ofthe National Radio Propagation Committee are given on the next page.

The report is the compilation of accounts written by the researchers on their ownwork. It is hoped that the report will help the Working Party and the Committee,and other interested parties, in their aim of maintaining an active and relevantresearch base in radio propagation, and that it may be updated occasionally.

The compilers thank the research community for their contributions, P. R. Green ofUMIST for the typographic compilation, and the Radiocommunications Agency forundertaking publication.

G. F. Gott (Chairman)L. W. BarclayP. S. Cannon

Technical Working Party on HF and Ionospheric Effects

June 1998

Terms of Reference of the National Radio Propagation Committee

1 The National Radio Propagation Committee provides a National forum forthe co-ordination of needs and study programmes in the field of pre-competitive radio propagation research and closely related matters.

2 The Committee advises the Radiocommunications Agency on the scopeand content of the National Radio Propagation Programme, including itsmajor element - the core programme of studies at the Rutherford AppletonLaboratory.

3 The National Radio Propagation Programme is intended to providepropagation information of value for spectrum utilisation and planning andfor the design and operation of radio systems and services.

4 The Committee may establish Technical Working Parties on defined topicswhere there is a number of related studies being undertaken within the UKorganisations.

Technical Working parties bring together a number of groups whichare undertaking relevant studies in order to co-ordinate studies, toexchange results and to identify topics where new studies areneeded.

5 The Committee shall receive reports annually of:

The core programme at the Rutherford Appleton Laboratory;Each of the Technical Working parties;The relevant interests of the members.

6 The Committee shall be chaired and administrated by theRadiocommunications Agency.

7 Membership of the Committee shall be by invitation from theRadiocommunications Agency, with the intention of includingrepresentatives from all major groups with significant interests in the studyand application of radio propagation.

20˚0˚GEOGRAPHIC

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CUTLASS field of view

CUTLASS radar, Hankasalmi, Finland (section 2.4)

Contents

page

1 Communications 1

2 Radar and direction finding 12

3 Channel measurements, modelling and simulation 18

4 Ionospheric science 33

5 National facilities 43

6 References 48

7 Contact personnel 54

FEI letter 57

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1 Communications

1.1 HF channel evaluation, selection and management (DERA)

Considerable background research done by the Defence Evaluation and ResearchAgency has resulted in the formulation of a concept for an automatic channelevaluation and selection process. When this concept was put forward in the relevantNATO HF forum (the HF Communications Systems Group - HFCSG), it evolved into onerealisation of an Automatic Radio Control System (ARCS).

The ARCS system is described in outline in Arthur and Maundrell (1.1.1), and comprisesthree main functions: Automatic Channel Selection, Automatic Link Establishment andAutomatic Link Maintenance. These three processes are interactive.

It is proposed that ARCS uses a probe signal on assigned frequencies to characterisethe propagation path in terms of Doppler shifts and spreads, time dispersion and signal-to-noise ratio at the receiver. Additionally, estimates of interference are made. Apreviously measured data library is held available for use in the ARCS terminal,tabulating the performance of the modulation waveforms. This enables a match to bemade and channels selected for the desired user communication requirements.

The ARCS system will then establish the link, using a robust protocol in order to ensurethat ARCS will work under severe path conditions (e.g. at high latitudes) and withdifficult mobile platforms. Monitoring processes will be incorporated to ensure thatwaveform or channel changing is accomplished in order to maintain the link.

Although a complete technical standard has not yet been agreed within NATO, thenecessary research work is well advanced, and is led by DERA.

1.2 Frequency management tools (GEC-Marconi Research Centre)

Software tools have been produced to generate frequency assignments for a varietyof applications in the HF and VHF bands. One particular example is the HF frequencyassignment algorithm (FAA).

The FAA is a sophisticated tool which allocates frequencies to many networks (>1500)distributed over large geographic areas. The FAA has been developed primarily for anHF application although it could be adapted for other frequency bands. The mainobjectives are:

· To provide each network with a set of usable frequencies (allocation set) forcommunications within the network for all times of the day; the day is dividedinto several (unevenly spaced, if required) time blocks, typically 12.

· To ensure that each allocation set comprises valid frequencies (i.e. frequenciesare within the specified user groups and not in adjacent (or forbidden) bands).

· To re-use frequencies in space and time.

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· To minimize interference between networks in both the signal bandwidth andadjacent channels.

· To eliminate intermodulation products and harmonics for co-sited networks. · To provide each net with a single primary allocation set and, if required, several

reserve allocation sets. · To provide allocation sets for simplex, multiple frequency (e.g. ALE) and

frequency hopping (e.g. secure) networks. · To enable the user to provide preferred allocation sets. · To enable allocation sets to be carried forward from one time block to the next

if propagation conditions permit and the frequencies remain valid. · To take into account the HF propagation effects pertinent to each network.

1.3 Intelligent channel selection (Racal Radio)

Racal Communications Systems Ltd., in association with Racal Research Ltd., havedesigned and produced a range of tactical HF radios which employ automaticchannel selection for customers in many countries.

The basic mode of Adaptive Channel Selection (which is also known as Free ChannelSearch) involves each end of a link passively monitoring a set of channels anddetermining the quietest one. Communication then takes place on this channel.

A Frequency Hopping mode, which gives improved Electronic Protective Measures(EPM), is also available. This operates in a similar manner to the basic mode exceptthat the quietest set of channels is determined, and these are then used as thechannels on which the radios hop.

1.4 Link establishment protocol (Roke Manor Research Ltd)

Emerging requirements for high capacity HF systems, integrated with fixedtelecommunications networks, have highlighted the inadequacy of existing HF linkestablishment protocols.

Roke Manor Research have designed a new protocol with the following key features:

· Compatibility with OSI Model. · Based on widely-used X.25 telecommunication standard. · Overcomes known deficiencies of other protocols. · Independent of modem waveform. · Suitable for synchronous and asynchronous operation. · Suitable for simplex, duplex and broadcast channels. · Suitable for common and separate Calling and Traffic Channel regimes. · Variable length and optional fields for maximum efficiency.

The proposed new protocol is well suited for future HF infrastructure projects, bothbecause it satisfies all the main requirements and because it has been designed withseveral features and expansion capabilities which will allow it to be readily adaptedto new traffic types, new HF waveforms and other future system requirements.

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1.5 Waveform design (DERA)

DERA has conducted and sponsored research into waveforms, and particularly intorobust waveforms designed for use with difficult mobile platforms under extremepropagation conditions (e.g. at high latitudes or on trans-equatorial paths). ‘Difficult’mobiles in this context means those mobiles which do not have large transmitterpowers and have inefficient antennas, e.g. small aircraft, where the electrical size ofthe platform is often a small part of a wavelength at HF. Additionally, such mobilesoften suffer with severe EMC problems.

A prime example of this work is the use of Multi-Frequency Shift Keying with appropriatecoding protection, and an initial prototype is outlined with its performance byMaundrell (1.5.1). This work has now resulted in further DSP-based implementations,permitting great flexibility of tone numbers, length and spacing, with various degreesof interleaving and coding (Clark et al. (1.5.2)).

1.6 High speed HF modem studies (GEC-Marconi Research Centre)

A number of studies into algorithms suitable for high HF modems have been carried out.The major emphasis has been, for serial modems, on the equaliser algorithm to be usedas equalisation is likely to be the most computationally intensive process within themodem. Studies have concentrated on algorithms for the numerous internationalstandards such as STANAG 4285, MIL-STD 188 110A etc. Some of these studies haveincluded simulation activities to investigate the performance of various algorithmsunder typical and extreme HF propagation conditions. Among the algorithms studiedwere: decision feedback equalisers; near-maximum likelihood sequence estimators anddata directed equalisers. Studies have been performed on advanced parallel tonemodems that use trellis coded modulation. These are reported to offer much improvedperformance over other parallel modems and even over serial modems under someconditions.

1.7 Neural network equalisation (GEC-Marconi Research Centre)

Much work has been carried out into the use of neural network equalisers. Severalneural network structures have been identified as candidates to perform the functionof an adaptive equaliser to counteract the effect of multipath distortion. The mostappropriate of these, the radial basis function (RBF), was chosen for a detailedsimulation study. Results indicate that under appropriate conditions, i.e. relatively lowmultipath and fast fading, the RBF can outperform the Viterbi algorithm and, by aconsiderable margin, especially at high signal-to-noise ratios. Thus it appears that gainsfrom the RBF are occurring in conditions which are unlikely to pertain for the case of highspeed HF modems.

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1.8 High speed HF modem studies (Racal Radio)

Racal Communications Systems Ltd., in conjunction with Racal Communications Inc.and Communications Research Centre Ottawa, has been involved in a number ofstudies concentrating on improving the performance of HF high speed serial modems.

Studies have included improvements into increased Doppler spread capability,algorithms using block decision equalisers, and time varying channel interpolation.

1.9 Very high data rate modem (DERA)

An application for high rate HF data transmission has arisen for medium ranges of a fewhundred kilometres. Error tolerant data transmission is required at rates up to 16 kbit/s.DERA and UMIST are investigating the characterisation of such channels usingDAMSON, and other techniques, with the intention of designing a modem specificallyfor this application.

1.10 Robust slow rate data transmission (UMIST)

The Department of Electrical Engineering and Electronics at UMIST, supported by DERA,has made theoretical studies and experimental link trials of medium and slow raterobust data transmission on HF links (Gott et al. (1.10.1)). Current work in this areainvolves an attempt to design, construct and experimentally evaluate theperformance of a slow rate modem which can operate wholly within the spectrum ofa significantly stronger interfering signal whose spectrum is varying rapidly with time,such as a voice signal. The transmitter uses a wideband signal (e.g. 2.7 kHz bandwidthfor a data rate of 50 bits/s or less), inserting a time gap between adjacent pulses. Thereceiver synchronises to the time gaps, and, in a time interval free of the desired signals,makes a spectral analysis of the inband interference. A high resolution rejection filter isthen synthesised to reject the interfering spectral components. The spectral analysisand rejection filter synthesis operations occur immediately before the reception of everydata bit. The data signal also has components rejected, but the signal-to-interferenceratio may be greatly improved.

1.11 Adaptive error control coding for HF communications(DERA, HW Communications, Lancaster University)

The highly variable nature of the HF radio channel makes the selection of a single errorcontrol code very difficult. During periods of poor propagation, a powerful code isrequired to ensure reliable communication. During periods of good propagation, theadditional redundancy of the more powerful code reduces the data capacity of thechannel. It can be very difficult to find an acceptable compromise between thesetwo situations and a system that can change code rate in response to changingchannel conditions is desirable. Flexible codecs have been designed which canoperate at a number of code rates with minimum software/hardware modification.Practical implementations have been developed using Reed-Solomon andReed-Muller codes. A second important component of an adaptive system is Real Time

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Channel Evaluation which allows the correct code rate to be selected beforeundetected errors occur in the data. Code-Combining is a second technique whichprovides a method of matching the code rate to the channel quality. This is a form ofadaptivity where additional redundancy is transmitted only when requested, toenable correction of a badly corrupted block. Work has been done using Reed-Mullercodes and an adaptive trellis decoder to implement a soft decision code-combiningsystem.

1.12 Novel synchronisation and channel estimation using auxiliary decodinginformation (HW Communications Ltd, Lancaster University)

The majority of the classical methods for synchronisation for digital communicationsystems require a hardware overhead, and additional data added to the informationstream, to provide effective synchronisation. Channel estimation techniques, ifprovided, also generally require some system overhead.

This project develops new digital techniques which are able to provide both theseoperations without any system overhead. They utilise auxiliary information obtainedfrom the actual decoding procedure to provide both synchronisation and channelinformation even at low signal-to-noise ratios.

The novel algorithms are examples of what may be described as multifunctionalcoding, as they combine the functions of demodulation, error control, synchronisationand channel estimation into a single functional unit. This highlights a major advantageof using DSP techniques in that the functionality of all these systems is software definedand hence offers adaptable systems which may be re-configured easily.

1.13 Robust compression techniques for transmission of images on HF radio channels(DERA, Lancaster University)

This project is an investigation into the transmission of binary and multi-bit plane imagesover HF, leading to an integrated transmitter and receiver system for mobilecommunications. The system implements joint source-channel coding, designed toaddress the characteristics and problems of the channel.

Using HF radio channels leads to various problems such as low data rates and high biterror rates. Errors due to signal fading, multi-path, and noise, can cause extensive visualdamage to the received image data. Therefore appropriate coding schemes havebeen developed.

Specialised compression algorithms, which provide a robust image coding stage,decrease the size of the data stream. Whilst achieving this, it is necessary to ensure thatthe compressed/coded information remains as sufficient and intact as possible whenburst errors occur.

To facilitate the transmission of multi-bit plane images, various bit plane codingschemes are investigated to not only separate the planes, so that the binarycompression stages can work more efficiently, but also to interleave colour informationacross the planes. Further stages have been implemented to improve the

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compressibility of the image and investigate the removal of non-critical pictureelements, thus speeding up transmission time.

Concatenated onto the compression process is additional channel coding in the formof a RM (16:11:4) block code. This adds to the robustness of the transmitted data. Bitinsertion or deletion during transmission affects data codeword synchronisation. It istherefore essential to re-achieve synchronisation as quickly as possible in order to limiterror propagation. To overcome this problem, a ‘smart decoder’ has beenimplemented to regain codeword alignment.

1.14 A testbed for automated HF communications: development and deployment(DERA, Lancaster University, Leeds University)

This project is concerned with the development of a radio system architecture suitablefor the testing and implementation of an Automatic Radio Control System (ARCS). Itmeets the requirements of such a system in terms of the basic processes of AutomaticChannel Selection (ACS), Automatic Link Establishment (ALE) and Automatic LinkMaintenance (ALM), as well as the need for a variety of actual transmission schemes.

The chosen design philosophy is that of a flexible, modular architecture, which allowsit to meet all of the above requirements, and also allows a degree of future systemevolution and enhancement. Within the context of new automated, adaptable HFradio systems, this architecture allows the development of isolated system elements(modem, codec etc.), which may be easily integrated to form a complete radiosystem. This approach allows different modules to be tested within an otherwise fixedsystem to judge comparative performances. It will also allow an intelligent system toreconfigure its mode of operation by switching individual elements, to provide a set-upoptimised for prevailing channel conditions.

Both theoretical and practical aspects of designing this system architecture are beingconsidered.

1.15 HF communication system performance at high latitudes (University of Leicester)

Several experimental HF links have been deployed over a number of high and veryhigh latitude paths and an evaluation of data signalling errors undertaken (FSKmodulation). Particular attention has been given to the relationship between Dopplerspread, path geometry in relation to the auroral oval and the time distribution of biterrors (Dhanda et al (1.15.1)).

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1.16 Meteor scatter research (DERA)

DERA has had a continuing interest in meteor scatter (MS) based beyond line-of-sight(BLOS) communication systems for about a decade. The interest has stemmed primarilyfrom the secure and robust nature of MS systems, qualities particularly attractive froma military point of view. Previous MS research activity at DERA has culminated in thedevelopment of the BLOSSOM simplex system (Cannon and Dickson (1.16.1)) andcharacterisation of the effects of spatial diversity on the performance of MS systems(Shukla et al. (1.16.2)).

Current DERA MS research focuses on techniques to improve the data throughput.Typical throughput for conventional simplex systems is around few tens of bits/s owingto the high losses associated with the signal scattering process. The philosophy ofilluminating with increased antenna gain just those regions of the sky which aregeometrically favoured for meteor signal return, the so called hotspots, has beenadopted as a means of increasing the throughput. Increased antenna gain isachieved through beam synthesis, either passively or adaptively combining the signalsfrom a uniform linear array of antennas. A detailed MS computer prediction model(Akram and Cannon (1.16.3)) has been developed to aid the process of efficientlydirecting the extra derived antenna gain.

Relative to a single Yagi reference system, the method of passive beam formation usinga 4-element Butler matrix was found to increase the signal availability by a factorranging between 1.6-1.8 over the signal-to-noise ratio threshold range 20-30 dB in a 300Hz bandwidth (Akram and Cannon (1.16.4)). DERA is in the process of investigating theadvantages of adaptive beam formation techniques whereby the direction of asingle high gain beam is updated depending on the arrival distribution of incomingmeteor trails.

The major implementation overhead of adaptive schemes is the component ofaccurate direction finding of meteor trails. This overhead can be removed if priorknowledge about the arrival distribution of incoming trails is available. Results from thetestbed suggest that the MS computer prediction model is capable of accuratelypredicting the diurnal arrival distribution of the sporadic meteors (Akram and Cannon(1.16.5)). This opens the route for a strategy whereby a high gain adaptive beam isguided by the MS computer model. Work is currently proceeding to realise such asystem.

1.17 A study of HF NVIS antennas on vehicles (University of Liverpool)

The Department of Electrical Engineering and Electronics at the University of Liverpool,supported by DERA Malvern, has made an in-depth study of the problem of producingthe high-angle radiation necessary for near vertical incidence sky wave (NVIS)propagation from vehicle-mounted antennas. The technique used was based on themethod of Characteristic Modes with an appropriate wire-grid model of a typicalvehicle such as a Land Rover and the Numerical Electromagnetics Code (NEC2) foranalysis (Murray and Austin (1.17.1)). It was found that the conducting body of thevehicle, plus its antenna, support a number of modes of current. These are a functionof the excitation frequency and the dimensions of the vehicle/antenna combinationbut are independent of the method of excitation. Their so-called modal significancevaries with frequency and indicates the ease with which each may be excited. The

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vehicle with antenna, and the four most significant modes are shown in figure 1.17.1.It is evident that mode 1 is entirely unsuitable for NVIS applications because of the nulltoward the zenith, while modes 2 and 3 are ideal. Mode 4 also has appropriatecharacteristics but is the least likely to be excited. This study confirmed the inherentunsuitability of the conventional vertical whip antenna in the NVIS role while indicatingthat a loop in the vertical plane radiated predominantly toward the zenith.

Of particular importance is the finding that a dual feed system, where the loopantenna is fed by in-phase signals at both its front and rear bases produced theoptimum NVIS radiation pattern. A series of pattern measurements performed at DERAPershore using a balloon-mounted receiver confirmed the accuracy of the computerpredictions.

1.18 Adaptive antennas for HF reception (University of Leicester)

One of the major limitations in the performance of HF long range radio communicationsis the presence at the receiver of additional signals on the same frequency as thewanted signal. Such co-channel signals may severely degrade the data throughputcapabilities of communication systems. The work on adaptive antenna techniques forDF purposes is being extended to include techniques whereby the signals from severalreceivers connected to a spaced element antenna array are combined in such away as to form a beam in the direction of the wanted signal and nulls in the directionsof the unwanted signals (spatial filtering).

1.19 Broadband HF Surveillance Receiver (Roke Manor Research Ltd)

An HF surveillance receiver has recently been developed at Roke Manor. This provides‘staring’ reception of a full 3.3 MHz sub-band of the HF spectrum. This bandwidth isdigitised at 10 MS/s with 16 bit resolution, after which analysis functions, such as realtime FFT, equalisation and demodulation, are performed.

Several sub-band receivers can be configured in parallel to provide correspondinglywider bandwidth analysis if required.

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Figure 1.17.1 Wire-grid model of vehicle with loop antenna, and polar diagramsof the four most significant modes at 7 MHz.

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1.20 Digital HF receivers and exciters (GEC-Marconi Research Centre)

GEC-Marconi Communications Ltd. have designed and produced a range of veryhigh performance digitally implemented HF communication receivers and transmitterdrives (exciters), the H2550/1550 series. The key A-D/D-A conversion and digital signalprocessing elements were developed at MRC.

The H2550 receiver uses a very linear bandpass sigma-delta A-D converter at thesecond IF, followed by a digital mix to baseband, and subsequent signal processingto allow a wide range of digital channel filters and demodulation functions to beimplemented. It also features digitally implemented feedforward AGC for improvedtransient response and levelling. The high linearity A-D conversion scheme employedin the receiver provides improved in-band dynamic range compared withconventional analogue receiver designs; also the digitally implemented FIR channelfilters provide far superior channel response, both in-band and out-of-band. There are256 programmable channels available. All the software is held in flash memory andfacilities are provided to allow all the receiver functions, including DSP functions, to beupdated by remote control via a serial link. The in-band third order intercept point istypically +11 dBm. With the preselector module fitted the out-of-band third orderintercept point is typically +55 dBm. Image rejection is better than 100 dB.

A complementary digitally implemented high performance HF exciter has beendesigned and produced to meet a range of communication requirements. It convertsthe input signals to digital signals and uses a digital-to-analogue converter at itssecond IF stage. Digitally modulated ISB, FSK, FEK and CW are available together witha wide range of digitally implemented sideband filters.

1.21 Digital HF receivers (Racal Radio)

Racal Radio Ltd. have designed and produced a range of very high performance HFsurveillance and communications receivers which combine Racal’s expertise in highperformance RF design with a powerful array of digital signal processors (DSP). TheRA3790 series has units with full ‘operator’ front panels (RA3791 and RA3792) as well asunits that are intended for remote control use only (RA3793, RA3794 and RA3796).

The RA3790 series of digital HF receivers uses a very linear sigma-delta analogue todigital converter (ADC) at a second IF of 32 kHz to digitize the analogue signals. This isfollowed by a digital mix to baseband and decimation within the DSP array. The highlylinear sigma-delta ADC gives the receiver an improved in-band dynamic range whencompared with a conventional analogue receiver. The IF filtering, AGC anddemodulation functions are all implemented digitally. A very wide range of userprogrammable IF filter bandwidths is available as well as an adjustable IF notch filter.All of the IF filters have a 100 dB stopband, and have linear phase, zero differentialgroup delay characteristics. The AGC is implemented within the DSP array using a feedforward technique. The gain of the analogue circuits before the ADC are also underthe control of the DSP array. This results in a gain control system with excellent transientresponse, and effectively no overshoot, which automatically adjusts the relative gainsof the digital and analogue sections of the receiver to maximize the in-band dynamicrange of the receiver. The AGC hang and decay times may also be programmed bythe user. A dual RS-423A (ASCII) remote control interface is also included within thebasic receiver to allow the user to control the receiver from a computer or from any

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RA3790 series receiver that has a front panel.

This combination of high performance analogue circuits coupled with a powerful DSParray has also been used in Racal’s latest generation of VHF/UHF receivers, the RA3720series.

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2 Radar and direction finding

2.1 Jindalee operational radar network - JORN (GEC-Marconi Research Centre)

The JORN is an over-the-horizon HF sky wave radar system currently being developedfor the Commonwealth of Australia. The prime contractor is Telecom Australia and GEC-Marconi have been subcontracted to design the radar. The GEC-Marconi ResearchCentre has developed some of the critical components of the final radar system. Oneof the most critical components in any HF sky wave radar system is a model of theionosphere. This model is necessary so that geographic target positions can be inferredfrom measured radar returns using raytracing techniques. This process is known ascoordinate registration (CR).

The real time Ionospheric Model (RTIM) predicts the values of ionospheric parametersat any location using as its input real time sounder data and ionospheric medianmodels. The model has two aspects: the fitting of the ionospheric median models to thesounder data and a statistical correction applied in the locality of each sounding.

Raytracing techniques enable a mapping to be established between measured radarcoordinates and operationally useful coordinates. Three-dimensional raytracing iscapable of modelling the passage of radio waves. A simpler raytracing method hasbeen evaluated using analytical techniques.

The preceding paragraphs have been extracted from Wheadon et al. (2.1.1).

2.2 HF sky wave radar modelling (DERA)

DERA Malvern has developed a comprehensive computer model for the performanceand availability of HF sky wave radars (Howland and Walbridge (2.2.1)). Performancemeasures include signal-to-noise ratio, signal-to-clutter ratio, sub-clutter visibility, signallevel, noise level, etc. The model is able to predict the median value of thesemeasures, or the probability of the radar exceeding a certain value of these measures.The model can model a monostatic or bistatic HF radar, with a definable systemconfiguration, for radar and target regions located anywhere in the world. The modelis currently being verified and validated against real data.

2.3 Relocatable antennas for HF sky wave radars (DERA)

A major disadvantage with existing HF sky wave radar systems is the cost andengineering difficulties associated with the preparation of a suitable site for the radarantenna (which may typically require a very carefully flattened stretch of land 2 kmin length). DERA is currently investigating techniques that will allow an HF radarantenna to be rapidly deployed on unprepared ground, with advanced signalprocessing techniques being used to compensate for the irregularities of the ground.

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2.4 HF coherent backscatter radars (University of Leicester)

The group has been involved in the construction of CUTLASS (Collaborative UK TwinLocated Auroral Sounding System), (Jones and Thomas (2.4.1). These radars arelocated in Finland and Iceland. Each radar has a 16 beam phased array(frontispiece), with interferometer capabilities for determining elevation angles ofarrival. The viewing areas of the radars covers 3,000,000 km2 (roughly the area ofWestern Europe) in which a resolution cell of 45 x 45 km can be achieved. These radarsform part of the SuperDARN chain, and at the moment are used exclusively forgeophysics research into the auroral ionosphere and magnetosphere. The field viewof CUTLASS is shown in figure 2.4.1.

A substantial part of the group’s technical effort is devoted to development of newinstrumentation. This includes HF receivers, transmitters and phase changes for beamsteering applications, sounding and error rate systems, DF systems and variousapplications of HF Doppler techniques.

2.5 Modelling OTH radar systems (University of Leicester)

The group is involved in modelling the performance of OTH systems similar to theCUTLASS radar. Ray tracing is undertaken through various ionospheric models todetermine ground illumination etc.

2.6 HF surface wave radars (GEC-Marconi Research Centre)

A demonstration surface wave radar system has recently been deployed to track shipsalong the coast of Essex and the Thames estuary. A sea spectrum inversion algorithmhas also been implemented so that wind and sea-state parameters can be extractedfrom the sea Doppler spectrum and displayed to the user in real time. In particular,progress has recently been made on a method of resolving the wind ambiguity whichexists when independent data are not available.

2.7 Frequency management for HF surface wave radars (DERA)

The selection of a suitable frequency is crucial for the successful operation of HF surfacewave radars. Although the radar is intended for surface wave operation, it may stillsuffer from ionospheric clutter and other effects associated with operating a sky waveradar. This work is addressing the techniques required to optimise the frequency atwhich the radar operates.

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20˚0˚GEOGRAPHIC

60˚

80˚

ΓΓΓΓΓΓ

Svalbard

Tromsø

HankasalmiPykkvibær

CUTLASS field of view

Figure 2.4.1 Field view of CUTLASS, with the locations of the EISCAT and ESRradars.

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2.8 Target height estimation in HF surface wave radars (DERA)

Existing HF surface wave radars are unable to provide information on the height ofdetected targets due to the poor vertical pattern of the radar’s antennas. This workis considering the use of non-linear estimation techniques to determine the target’saltitude from the target parameters that can be measured by a surface wave radar.

2.9 Superresolution HF direction finding (Roke Manor Research Ltd)

In practical direction finding scenarios, there are frequently multiple ‘rays’ incident onthe DF antenna. Some are from the target transmitter, some from co-channelinterference and some from multipath reflections.

Superresolution DF and related maximum likelihood methods provide a multiple signalDF capability and can thus offer considerably superior DF performance in the signalenvironment of the real world.

The techniques employed at Roke Manor Research Ltd. employ modern multichanneldigital receivers and associated fast digital signal processing.

The array description (or array manifold) is a table of antenna element gain and phasepatterns. SR-DF does not rely on any particular antenna geometry; it is only necessaryto know the relative positions of the elements and their patterns. It has demonstratedsuperior accuracy in operational environments, even with small apertures and at lowsignal-to-noise ratios.

The HF band is particularly suited to SR-DF techniques, as multiple signals on a channeland multimode propagation are commonplace. To this end, field trials havedemonstrated the ability to provide superresolution DF mode separation. This results inmuch more accurate azimuth and elevation bearings and thus much enhancedsingle site location operation.

Field evaluations of Superresolution HF DF have demonstrated excellent results. Theexample plot of results shown in figure 2.9.1 (for a 1235 km path) clearly shows the 1Fand 2F reflections with only a 2.5º spread in azimuth (Tarran (2.9.1)).

A number of operational SR-DF systems have now been commissioned by Roke Manorin conjunction with Watkins Johnson Ltd. For example, an HF system has been installedat the Radiocommunications Agency’s Baldock Radio site (section 5.1). This equipmentcomprises a VXI multichannel receiver and real time DSP to provide over 30 MUSIC DFsper second in both azimuth and elevation.

2.10 The accuracy and performance of HF/DF systems (University of Leicester)

This project has been in progress for a number of years and was originally concernedexclusively with wide aperture goniometric systems. The instrumental and propagationerrors associated with these systems have been investigated at length and work in thisarea still continues. This presently involves the evaluation of propagation effects on theperformance of goniometric DF systems at very high latitudes. In recent years moreattention has been devoted to adaptive DF systems using MUSIC and DOSE/IMP

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methods of determining the angular directions of arrival. Such systems have beendeployed at mid and high latitudes and extensive data sets collected to evaluatesystem performance.

A special study has been made of the problems of HF direction finding in the auroraland polar cap zones. In these regions very large bearing errors of up to ±100º arefrequently observed. For polar path caps, it has been possible to relate the incidenceof such errors to the convection of ionospheric ‘plasma blobs’ which break away fromthe dayside (sun lit) auroral oval and convect across the polar cap to the night side(Warrington (2.10.1)). Similarly, large errors are also observed on sub-auoral oval pathswhen propagation is along the ionospheric trough (Rogers et al (2.10.2).

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Signal Details

Spanish Transmitter location = 40.07N, 3.28W

Distance from Tx to Rx = 1235 km

Transmitter Frequency = 12.035 MHz

Receiver Bandwidth = 1 kHz

Number of samples per DF = 64

Number of Rx channels used = 8

Time = 1200 hours

Date = 25 January 1995

Figure 2.9.1 Results of superresolution DF for a 1235 km path. The example showsthe 1F and 2F reflections with only a 2.5° spread in azimuth.

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3 Channel measurements, modelling and simulation

3.1 DAMSON (DERA)

DERA, in collaboration with the Communications Research Centre of Canada, hasdeveloped a channel sounder known as DAMSON (Doppler And Multipath SOundingNetwork) to characterise narrow band channels (3 kHz) by measuring their scatteringfunctions. The real time nature of the DAMSON processing makes it unique in thefrequency range but, in its approach to measuring the channel its operation is similarto that of Wagner et al. (3.1.1). The DAMSON system is described in more detail in Davies and Cannon (3.1.2) and initial results are described in Cannon et al (3.1.3).

DAMSON operates from remote sites, on pre-selected frequencies. The system is basedon commercially available equipment (such as HF communication receivers andtransmitters, computers etc.) and makes extensive use of digital signal processing (DSP)techniques. High accuracy system timing is derived using the Global PositioningSatellite (GPS) system. The latter ensures that transmit and receive stations areaccurately synchronised and also allows absolute time-of-flight measurements to bemade.

DAMSON characterises the propagation path using a number of sounding waveformswhich can be flexibly scheduled. Of these, the delay-Doppler (DD) waveform hasbeen specifically designed to measure accurately the channel scattering function asdescribed above. Typically, a Barker-13 code is used, modulated at 2400 baud ontoa bi-phase PSK carrier. Each code lasts 5 ms with a pulse repetition interval of 15 ms.The codes are sent 128 times giving an integration time of 1.92 s and a frequencyresolution of 0.52 Hz. The Doppler frequency range for the measurements is typically ±33 Hz of the central frequency. The processing gain for such DD measurements is < 32dB.

The other operating modes include a noise measurement period lasting 2.8 s and 4 s ofCW transmissions. The CW measurement mode is a very sensitive detector of signalsdue to the long integration time and low receive bandwidth, but is unable todistinguish between different propagation modes in time. A time of flight (TOF) modealso exists which is similar to the DD mode but which is able to unambiguously measurethe signal TOF and large multipath delays due to its low pulse repetition frequency(PRF). Like the DD mode, the TOF mode also uses pulse compression waveforms butwith a shorter integration time. Due to its low PRF the TOF mode is unable to provide anaccurate measure of the Doppler shift and spread.

The DAMSON equipment has been deployed within Norway with transmitters at IsfjordRadio on Svalbard (78.06ºN, 13.63ºE) and at Harstad (68.8ºN, 16.5ºE) and a receiverat Tuentangen, near Oslo (59.94ºN, 11.09ºE). The path from Svalbard to Tuentangenlies with its mid-point in the auroral zone or polar cap region and the path from Harstadto Tuentangen lies with its mid-point in the auroral zone. The path lies close to thegeophysical observatory at Tromsø, Norway. A further receiver is located in Sweden ateither Kiruna (67.87ºN, 20.25ºE) or at Lycksele (64.63ºN, 18.67ºE).

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Figure 3.1.1 illustrates a tri-modal scattering function recorded on the Harstad - Kirunalink, where each mode exhibits different Doppler spreads and shifts. Conditions such asthese can pose serious problems for HF modems.

3.2 ROSE and IRIS ionosondes (DERA)

Throughout the world extensive use is made of HF radio sounding systems to remotelyprobe the ionosphere and acquire information to assist frequency management of HFcommunications and radar assets. One category of remote sensing systems is knownas the ionosonde, which is essentially a radar which emits pulses, chirps or otherwaveforms, to measure the group delay of the return signal reflected from theionosphere.

The BR Corporation of California, USA, manufacture a chirp sounder receiver (RCS-5)which is able to receive transmissions from a number of compatible transmitters (RCS-4)distributed throughout the world. As a result of their success they have become verypopular with UK and other NATO forces. The output of the RCS-5 receiver is a simplemonochrome ionogram, displaying relative time delay against transmission frequency.

DERA has developed a system known as ROSE (Radio Oblique Sounding Equipment)which provides an enhanced performance over the RCS-5 (Arthur and Cannon(3.2.1)). ROSE-200 consists of an RCS-5 receiver and a PC with integral DSP. This systemprovides better resolution, signal amplitude colour coded display for all the modes andan archive facility.

A problem with ROSE is the continuing need for the RCS-5 receiver which is bothexpensive and bulky. To meet the need for a smaller and cheaper system DERA havenow developed IRIS (Improved Radio Ionospheric Sounder). IRIS provides the samefunctionality as ROSE-200 but uses only a PC (with two integral DSP cards) and astandard HF communications receiver; a Racal RA3711.

Both ROSE and IRIS produce similar quality ionograms. Figure 3.2.1 shows an exampleof an IRIS oblique ionogram for the path Norway to the UK.

3.3 Oblique ionospheric sounding (RAL, Radio Communications Research Unit)

Many radiowave applications require real time determination of the 2-dimensionalstructure of the ionospheric electron density along the path, or at least of the midpathelectron density profile. Since it is often impractical to deploy a vertical ionosonde atthe point of interest, the idea of reconstructing an average midpath profile from a 2-dimensional oblique sounding is very appealing. Oblique inversion algorithms are ofinterest for applications such as over-the-horizon radar, surface-wave radar or HFcommunications. Total electron content from oblique sounding might also be used toimprove the accuracy of single frequency GPS navigational information.

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Figure 3.1.1 Damson display, illustrating a tri-modal scattering function recordedon the Harstad – Kiruna link, where each mode exhibits differentDoppler spreads and shifts.

21

Figure 3.2.1 ROSE/IRIS oblique ionogram for a Norway/UK path.

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It is planned to use a recently developed oblique sounding system to assist in thedevelopment of suitable inversion algorithms, and to test them by using a midpathvertical sounding as control data. The system will also be used to test short-termionospheric forecasting models. The work involves collaboration with the Radio Scienceand Propagation Group at DERA Malvern, which has been instrumental in thedevelopment of the IRIS oblique sounder, and with the University of Leicester who areproviding the transmitting equipment. The transmitter and receiver are now equippedwith GPS timing, which allows absolute delays to be measured. In the first instance thesystem will be tested at mid-latitudes. It is hoped that the work can be extended tohigh latitudes, as oblique sounding could provide useful real time input data for high-latitude ionospheric modelling.

3.4 Channel evaluation and oblique sounding (University of Leicester)

The complexity of the HF propagation environment can seriously affect theperformance of long range radio communication circuits. Furthermore, the increasedsophistication of the modulation schemes employed by modern data modemsnecessitates a better understanding of the propagation environment in order toestablish reliable links. A project is being undertaken in which techniques to evaluatethe ionospheric channel in real time, with particular reference to the requirements ofmodern modulation schemes, are being investigated.

New experiments are being undertaken combining channel sounding and directionfinding techniques. In these experiments, measurements of the directionalcharacteristics of the Doppler and time delay components of signals received from theDAMSON transmissions are made. Variations in bearing with Doppler frequency arefrequently evident in these data, an effect attributed to Doppler shifts imposed on thesignal when scattered from ionospheric irregularities drifting across the reflection points.For a trans-auroral path from Isfjord to Cricklade, UK, for which the ionospheric reflectionpoints were sub-auroral, standard deviations of up to around 2.5º were observed inthe azimuthal power distribution of the received energy. Much greater disturbanceswere recorded on the signals received over a polar cap path from Isfjord to Alert forwhich very large azimuthal standard deviations, of up to around 35º, were measured(Warrington (3.4.1)).

Oblique sounding is often undertaken with FMCW (chirp) sounders for the productionof ionograms in support of these and our other studies.

3.5 Neural network analysis of ionograms (GEC-Marconi Research Centre)

A preliminary investigation has been conducted into the inversion of backscatterionograms using neural network techniques. The extraction of ionospheric data fromwide-sweep sounder systems is potentially useful in both HF radar frequencymanagement systems and in wide-area HF broadcast and circuit applications. Neuralnetworks are, in general, particularly suitable for image recognition and analysis tasks,and so lend themselves to ionogram trace extraction and parameter determination.A suitable neural network was implemented and trained on a set of simulatedionograms. A second set of simulated and real ionograms was presented to the neuralnetwork in order to extract the layer parameters. The output parameters were then

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compared to the known layer parameters; the results indicate that the method hasmerit under certain conditions and works as well as conventional full backscatterinversion techniques.

3.6 HF communication system modelling (DERA)

The jamming, interception and direction finding (DF) capabilities of HF, Beyond Line OfSight (BLOS), and Electronic Warfare (EW) systems, are influenced by the ionosphericenvironment through which radio signals must propagate. Although the vulnerabilityof HF systems can be reduced by using sophisticated systems techniques (e.g. spreadspectrum, antenna nulling), electromagnetic energy may still propagate to the enemyand be exploited. There is, however, an alternative technique. The technique is basedon the tactical use of signal propagation (Goodman et al, (3.6.1), Shukla and Cannon(3.6.2)) which exploits knowledge of the ionosphere and ray-tracing techniques tominimise signal coverage and deny, or minimise, access of the enemy to the radiatedelectromagnetic energy. The improvements obtained may be achieved withoutresorting to sophisticated systems techniques.

DERA is investigating the tactical propagation technique via the development of aBLOS-HF Jamming and Interception Vulnerability Estimator (JIVE). This tactical HFdecision aid employs an ionospheric environmental model and a communicationsmodel to predict both the interception and jamming vulnerability of HFcommunications links. Based on these models JIVE recommends systems configurations(e.g. best frequency, ground station) which minimise communications vulnerability. Theoutput of JIVE can then be used by a HF operator, or by a communications systemautomatically to configure the least vulnerable communications link. The latterapproach is particularly powerful if used in conjunction with the next generation of HFcommunications systems (Arthur and Maundrell (1.1.1)) incorporating AutomatedRadio Control Systems (ARCS). JIVE systems may also be used off-line duringcommunications scenario modelling.

A test-bed HF-BLOS JIVE system which uses simple frequency management techniquesto minimise signal interception has been developed at DERA. At present the system usesa median based ionospheric model and a simple mirror reflection propagationtechnique. As more sophisticated models become available (e.g. analytic ray-tracing)these will be incorporated into the system to provide improved vulnerabilitypredictions. The JIVE system developed (JIVE-PC-1.0) is based on a PC and may beused by conventional HF operators prior to link establishment, or duringcommunications scenario modelling. Figure 3.6.1 is a typical signal coverage outputfrom JIVE. A 12 hour frequency management schedule can be used by an operator toconfigure the system for minimum signal interception.

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Figure 3.6.1 A typical signal coverage output from JIVE. The transmitter islocated at the centre with ground wave signal level contourssurrounding it. Also shown are the locations of desired receivers(black crosses) together with intercept sites (red boxes).

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3.7 HF propagation modelling (DERA)

The management of Beyond the Line Of Sight, (BLOS), HF communications dependscritically on realistic ionospheric modelling and accurate ray tracing through thesemodels.

Ray tracing is a powerful tool and is particularly useful when a detailed knowledge ofpropagation through the ionosphere is required, for instance when using Over-The-Horizon Radar systems, single station location systems and HF direction finding systems.

Numerical ray tracing provides the highest accuracy but it is limited in application bythe computational time required, especially when a large number of rays must betraced. Analytic ray tracing is much faster, however, until now it has been limited tosimple and unrealistic ionospheric models.

DERA is developing a versatile package (Platt and Cannon (3.7.1)) in which real datais combined with synoptic models to produce an accurate grid-point model. At eachgrid point location a quasi-parabolic segment model (QPS), is constructed. The beautyof the QPS model is that there are explicit equations for the ray parameters. This meansthat we can now analytically ray trace through very complicated and, just asimportant, real time ionospheric profiles. The DERA ray tracing technique uses novelways to incorporate the effects of ionospheric tilts.

DERA is pursuing a number of activities to develop or enhance ionospheric models witha view to increasing their accuracy. One activity is the use of real time obliqueionosonde data to update automatically statistical median ionospheric models bygenerating pseudo-parameters. This technique has shown some promise in the mid-latitude environment during geomagnetic quiet periods (Shukla and Cannon (3.7.2)).Another activity is the incorporation of real time ionospheric data (e.g. from verticalincidence ionosondes) into parameterised models (e.g. PIM) to produce real timeionospheric specifications.

3.8 HF simulator using DSP (GEC-Marconi Research Centre)

Work has commenced on a GEC-Marconi funded project to develop a flexible HFchannel simulator using a 300 MFLOPS floating point processor under PC control. Thesimulator will model the multipath and frequency dispersion channel characteristicsbased on the ITU-R recommendations, i.e. using the Watterson method. The keyfeatures will include a user friendly interface to enter the model data; select apropagation model from which to derive path characteristics; select multiple pathcomponents (within a single channel); perform multiple channel simulations (includingfrequency hopping and ALE systems) and display channel impulse functions. Inaddition to the user selected scenarios, the standard ITU-R and proposed NATOchannel scenarios will be available.

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3.9 HF simulators (DERA)

It has become apparent over the years, that the performance of HF path simulatorimplementations can vary. This can be due to the degree of accuracy in theimplementation, but can also be affected by the dynamic range of the modem undertest, unless great care is taken in the simulator system. Additionally, there has been nointernational standard for simulator testing.

DERA has now built DSP-based versions of propagation path simulators, and one ofthese has recently been validated to the satisfaction of the NATO HFCSG for use inevaluating waveforms. One use is in the establishment of the ARCS waveformperformance database referred to in section 1.1, and measurements are progressingusing an agreed set of simulator parameters. These will have standard interference testsadded in the near future. Figure 3.9.1 shows an example of a 3D modemcharacterisation plot obtained using the simulator.

3.10 ITU models (RAL, Radio Communications Research Unit)

RCRU develops and maintains ionospheric prediction tools for the InternationalTelecommunication Union. Up-to-date versions of MUFFY (MUF predictions), REC 533(field-strength forecasting), WOMAP and HRMNTH are available.

3.11 HF broadcast program (GEC-Marconi Research Centre)

An HF sky wave broadcast propagation prediction program has been developed tofacilitate the design and development of such systems. ITU-R methods for calculatingpropagation paths are employed and the program is capable of providing predictionsfor single or multiple sites worldwide. Incorporated into the program are the ITU-Rantenna patterns which allow the user to configure a suitable antenna. Colourcontour plots of signal strength are produced overlaid on geographic coastline mapswith several view projections available, e.g. azimuthal equidistant, Mercator, etc. Anexample is given in figure 3.11.1.

3.12 MF coverage program (GEC-Marconi Research Centre)

An MF ground wave broadcast program has recently been developed to calculatesignal strength contours from single and multiple sites and the mutual interferencebetween broadcast transmitters. The method uses an MF ground wave path-losspropagation algorithm which when combined with a map of the ground conductivityis used to determine the mixed path signal strength and plots contours of signal strengthoverlaid on a geographic map. Interference from one or more additional broadcastsites can be determined and regions of mutual interference may also be superimposedon the maps. Combined signal strength contours from several transmitters may also bedisplayed. An example is given in figure 3.12.1

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Figure 3.9.1 An example of a 3D modem characterisation plot obtained usingthe simulator. Surface of BER = 10-3 at 1200 bps for implementationof a serial tone Mil-Std-188-110A with long interleaver setting.

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Figure 3.11.1 Contour plot of signal strength overlaid on coastline map.

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Figure 3.12.1 An example of combined signal strength contours, and region ofmutual interference.

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3.13 Short-term ionospheric forecasting service - STIFS(GEC-Marconi Research Centre)

Communicators often experience adverse propagation conditions during periods ofenhanced solar and magnetic activity. The STIFS provides early warning of short-termionospheric disturbances and is a valuable aid to maximizing circuit availability.Forecasts are valid for the European environment and they are issued via fax orelectronic mail and are available from a Bulletin Board facility. The daily message iscompiled from European ionosonde data, global geomagnetic data, propagationreports and locally recorded magnetic, VLF and signal strength measurements. TheSTIFS message is issued on a daily basis, 365 days a year, using a fully automated systemfor data collection, message compilation and distribution.

3.14 Modelling of HF spectral occupancy over northern Europe (UMIST)

UMIST has undertaken systematic measurements of occupancy across the entire HFspectrum since January 1982, with the support of DERA and the EPSRC. This experimenthas developed from a single measurement site at DERA Pershore in central England, tothe present international system, which includes continuously operating automaticmeasurement sites in England (RA Baldock Radio Station described in section 5.1),Sweden (Defence Research Establishment, Linköping) and Germany (Rohde andSchwarz, Munich). These three sites are approximately at the corners of an equilateraltriangle of side length 1000 km, and provide effective measurement of HF spectraloccupancy over northern Europe. The systems are linked to UMIST for system controland data transfer.

The main measurement antenna is a Rohde and Schwarz HE010 active monopole,whose polar diagram is appropriate for the reception of signals arriving at low tomedium angles of elevation. This is mounted to give an antenna factor of 10 dB, whichis constant across the HF spectrum. The measurement receiver is the Rohde andSchwarz ESH3, which is incremented in frequency through each ITU allocation of theHF spectrum, and the RMS value of the IF output measured. The fraction ofmeasurements which exceeds a given threshold defines the occupancy for thatallocation (Gott et al. (3.14.1)).

Occupancy is measured across each ITU defined frequency allocation, andmathematical models have been derived which give very good fits to theexperimental data, and which enable the accurate prediction of future occupancy(Pantjiaros et al. (3.14.2)).

Rohde and Schwarz HE004 active turnstile antennas are also installed at the threemeasurement sites, and are appropriate for the reception of signals arriving at mediumto high angles of elevation. Spectral occupancy measurements made using theturnstile antennas show important differences when compared to measurementsmade using the monopole antennas (Pantjiaros et al. (3.14.3)).

The present effort is concerned with the derivation of occupancy models for northernEurope, incorporating interpolation and extrapolation of measured occupancy values.An example of modelled occupancy contours over northern Europe, for a monopoleantenna, is shown in figure 3.14.1 (Gott et al. (3.14.4)). The models are fitted using NAGGENSTAT 5 statistical software, and a Cray CS64000 supercomputer. Typical modelling

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requires numerous regression operations on perhaps 400,000 experimental values ofoccupancy.

In 1997, a further HF occupancy measurement system was installed at Kiruna innorthern Sweden (inside the Arctic circle) by the National Defence ResearchEstablishment, Linköping, and the occupancy modelling is now being extended tohigher latitudes.

3.15 Variation of HF spectral occupancy with azimuth and elevation (UMIST)

The DERA Cobbett Hill Radio Station in Surrey, described briefly in section 5.2, isdedicated to HF radio research. One facility is an array of 20 terminated sloping-Veeantennas, of leg length 99 m, arranged in a circular configuration with a commoncentre and commutated feeder. The antennas are thus positioned every 18º inazimuth.

Under an agreement with DERA, UMIST has installed a data gathering system atCobbett Hill Radio Station for the automatic measurement of HF occupancy withazimuth, using the Vee array. A Rohde and Schwarz HE010 active monopole has alsobeen installed for calibration purposes, and the system is linked to UMIST for systemcontrol and data transfer. This system has been operational since July 1995. Results arepresently being obtained, and mathematical models are being developed(Pantjiaros et al. (3.15.1)).

It is intended to measure and model the variation of HF spectral occupancy, using thesloping Vee antennas in the UK, and the active monopole and active turnstileantennas over northern Europe, until at least the next sunspot maximum.

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Figure 3.14.1 Modelled HF occupancy contours over northern Europe.

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4 Ionospheric science

4.1 Ionospheric science (DERA)

DERA owns and operates a Digital Portable Sounder (DPS) (developed by theUniversity of Massachusetts) at Tromsø in Northern Norway. The instrument is used bothto continue a statistical data base of measurements in that area going back overmany decades, and also to provide data for real time updates of ionospheric maps.By this approach, area coverage for HF systems can be better estimated andfrequency management generally improved. An example of a vertical ionogram fromTromsø together with its true height electron density profile (in white) is shown in figure4.1.1. The Digisonde can also be used to measure movements in the ionosphere.

4.2 Amateur radio research (RSGB)

The ‘Six and Ten Reporting Club’ is a number of interested amateurs which maintainslistening watches and reports to a central body. The data is analysed and monthlyreports are issued. The types of topic covered include: monthly 28 MHz beaconanalysis reports, analysis of 28 MHz reception in South Africa, solar indices data for thecurrent month, (including S.S., Geo-physical, and K-indices, with a breakdown ofdisturbed days), 50 MHz monthly propagation data from 14 European countries notingAuroral and Sporadic-E type events, details of tropospheric propagation tests carriedout by UK amateurs, and daily comparisons of 28 MHz and 50 MHz. There are alsoactivity reports from various parties and topical news from overseas amateurs.

The ITU Beacon Project reports on the reception of the only operating station presentlyworking on this project, LN2A. Received data is compiled, analysed and distributed tothe Radiocommunications Agency.

The IARU Region Beacon Co-ordinator is currently a UK amateur, who maintains a listof all operating beacons. There are special projects in operation at the moment in the14 MHz and 21 MHz bands, and there is a move to install beacons in areas so faromitted, such as India. When these new beacons are working they will assist inextending propagation knowledge in those areas.

The RSGB has begun transmissions of geophysical data on 3.518 MHz (CW at 12 wpm).The service is currently operated on Sundays at 0900, 1200, and 1800 local time. It isexpected that the service will be expanded to cover other days in the near future.

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Figure 4.1.1 An example of a Digisonde vertical ionogram from Tromsø, with thetrue height electron density profile shown in white. The ordinaryand extraordinary polarisations are shown in red and greenrespectively.

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4.3 Short-term forecasting of the ionosphere(RAL, Radio Communications Research Unit)

Broadcasters and official users require up-to-date information on the state of theionosphere. Several approaches are possible, ranging from physically-based globalnumerical modelling to operationally oriented methods like real time channel sounding.The short-term forecasting research project uses an intermediate approach based onempirical interpolation between a network of vertical ionosondes. The goal is toprovide a practical tool for hourly ionospheric predictions over Europe.

As well as offering a useful on-line resource for National RadiocommunicationsProgramme Committee members, this work fits in well with the objectives of Europeanproject COST 251, which is concerned with ‘improved quality of service in ionospherictelecommunication systems planning and operation’. The short-term forecasting toolwill also supply cross-validation data for global numerical modelling of theionosphere/thermosphere system.

4.4 Propagation of very high power radio waves (University of Leicester)

Ionospheric modification by means of high power radio waves (heating) is animportant technique for investigating the ionosphere and has led to the discovery ofnumerous effects. These include phenomena which, in addition to their pure scientificsignificance, may have practical importance in the field of radio communications. Thecreation of Artificial Ionospheric Mirrors (AIMs) can open up possibilities for novelcommunications channels. The generation of heater induced radar scatteringirregularities is illustrated in figure 4.4.1.

The most advanced high power system currently available is located at Tromsø innorthern Norway (now operated by the EISCAT (European Incoherent SCATter)Scientific Association). Extensive studies into the propagation of very high power radiowaves (ERP ~1 GW) have been undertaken by Leicester scientists using the ionosphericmodification facilities at Tromsø and also with facilities at Arecibo, Puerto Rico (see, forexample, Robinson (4.4.1) and Stubbe et al (4.4.2)). Diagnostics for the heatingexperiments include a wide range of HF systems.

4.5 HF diagnostics for geophysics (University of Leicester)

Extensive use has been made of low power HF diagnostic signals for measuringionospheric disturbances such as those produced naturally by internal acoustic gravitywaves and artificially by ionospheric modification. The techniques involved includeDoppler, phase and anomalous absorption measurements as well as direct radarbackscatter.

As an example of these types of measurement, an HF Doppler sounder has beenrunning almost continuously in northern Norway since the middle of 1995. Thisexperiment (the Leicester Doppler Pulsation Experiment (DOPE)) consists of a transmitterat Seljelvnes and a receiver at Ramfjordmoen.

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Figure 4.4.1 The results of a recent Leicester University heating experimentwhich, in addition to the EISCAT heater, utilises the CUTLASSradars. The beams from the Finland CUTLASS radar are depictedin the left panel of the figure fanning out over the Norwegiancoast. The upper right panel shows an artificial disturbance,created by the heater, in the ionosphere above Tromsø whichwas detected by CUTLASS. The lower right panel clearly shows thedisappearance of the man-made patch when the heater isturned off. This patch in the ionosphere acts as a target fromwhich radar signals can be reflected.

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Ionospheric motions at the reflection points associated with acoustic gravity wavestravelling through the ionosphere (TIDs) impose Doppler shifts onto the received signal.With this equipment, high spatial and temporal resolution observations of theionospheric signatures of ULF waves at high latitudes have been undertaken (Wrightet al (4.5.1)).

4.6 Ionospheric tomography (The University of Wales Aberystwyth)

Ionospheric tomography has developed during recent years to become a powerfulmeans of imaging space plasma, complementary to other ionospheric soundingtechniques. The method uses radio transmissions from satellites which are received ata limited number of ground stations to monitor the electron content along intersectingray paths. The resultant data set is inverted in a reconstruction algorithm to provide atwo-dimensional image of electron density. The method is particularly adept atimaging horizontal structures in the plasma on scales of tens to hundreds of kilometres.The use of satellite transmissions and a few receiving stations makes the technique well-suited for observations in remote parts of the globe. It shows potential for monitoringfeatures, such as the mid-latitude trough and the patches and holes in the polar cap,that give rise to problems to the practical use of HF radio communications andtransionospheric propagation applications like navigation.

The group at The University of Wales, Aberystwyth has played a leading role in thedevelopment of ionospheric tomography. Observations have been made at sub-auroral, auroral and polar latitudes (Kersley et al. (4.6.1)). The sub-auroral observationsallow the equatorward progression of the mid-latitude trough during the course of thenight and its subsequent retraction to higher latitudes to be monitored (Pryse et al.(4.6.2)). Statistical studies enable the dependency on geomagnetic conditions of thetrough characteristics, such as the position of the minimum density and gradient of thewalls, to be determined (Kersley et al. (4.6.1)). Images from a region co-located withthe European Incoherent Scatter Radar (EISCAT) facility show that the technique isable to image latitudinally narrow troughs and boundary blobs (Walker et al. (4.6.3),Mitchell et al. (4.6.4)). Verification of the tomographic images has been obtained bycomparison with density measurements by the radar, an example of which is shownin Figure 4.6.1. Observations in the polar cap show field-aligned density structures in thecusp region, patches in the region of antisunward flow, and the severely depletedionosphere of the polar hole (Pryse et al. (4.6.5). Measurements by an extended chainof six receivers covering latitudes from Svalbard to southern Scandinavia yield atomographic image covering some 30 degrees of latitude that extends from polar tosub-auroral regions (Kersley et al. (4.6.1)).

Four receivers have been deployed at Ny-Ålesund, Longyearbyen, Bjornoya, andTromsø for routine observations of the polar ionosphere, with a chain of five stationsnow operational in the UK. Measurements over an extended period of time providemaps of the high-latitude electron density for different geophysical conditions. Work isin progress on the use of ionospheric tomography to provide a spatial complement tocurrent methods of ionospheric monitoring, for application to the operation of HFsystems and ionospheric modelling and mapping at mid to high latitudes over Europe.

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Figure 4.6.1 An example of the comparison of tomographic images with densitymeasurements obtained by radar.

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4.7 Propagation effects on GPS signals (The University of Wales Aberystwyth)

Signals from GPS navigational satellites are subject to propagation effects thatdegrade the performance of the system. The long-established experience oftransionospheric propagation at the University of Wales Aberystwyth is now beingapplied to studies of GPS. Measurements are being made of ionospheric electroncontent using both group delay and carrier phase techniques, while attention is alsobeing devoted to the determination of the largely unknown contribution arising fromplasma in the high protonospheric section of the path. Both topics will take onincreasing importance over the next few years with the rise in the solar cycle, when thegreatly enhanced electron contents may cause limitations to the performance of someGPS applications. Experimental studies are also being made of scintillation effects onGPS reception caused by small-scale irregularities in the ionised propagation medium.

4.8 Real time modelling of the effects of high latitude processes (RAL)

Recent statistical work has clearly demonstrated how the ionospheric densities at mid-latitudes are influenced by southward turnings of the interplanetary magnetic field(Davis et al. (4.8.1)). Theoretically, such links have been predicted because the powerextracted from the solar wind is greater under such circumstances and this power isthought to generate disturbed winds in the neutral gas of the thermosphere. Theseblow to lower latitudes, where they move the plasma up or down field lines dependingon their direction. As the rate of loss of the plasma depends on altitude, this modulatesionospheric densities. However, a longer-lasting and more significant effect is that thecomposition of the thermospheric gas is altered at a given pressure level because ofthe upwelling effect of the heating at high latitudes. These composition changes arebrought to mid latitudes by the enhanced winds and have a major effect on theionisation loss rates and densities. Recent work has shown that this behaviour is wellreproduced by global models of the coupled ionosphere-thermosphere system (Fuller-Rowell et al. (4.8.2)). Initially these models were used to predict features of theionosphere only at high-latitudes (e.g. Quegan et al. (4.8.3)) but now they havedeveloped to the stage where they have real operational implications for real timemapping of the global ionosphere (Millward et al. (4.8.4)). The major driver of the windsand of the thermospheric heating is the convection of the high-latitude ionosphericplasma caused by the solar wind flow. In recent years our understanding of thetemporal variations of these ionospheric flows, and their response to changes in theinterplanetary medium, has grown (Lockwood (4.8.5), Cowley and Lockwood (4.8.6)).The energy deposition is higher when the convection pattern changes rapidly than forsteady conditions (Idenden et al. (4.8.7)) and the evolution of high latitude plasmadensities is altered in subtle ways (Lockwood (4.8.8)) such that features like polar cappatches are seen (Lockwood and Carlson (4.8.9). These effects are caused by bodilymotion of the ionospheric plasma into and out of regions of high production and loss- they are more complex and structured than the changes at mid-latitudes. The latterare caused by the associated thermospheric disturbances and tend to be anintegrated response of the power input at high latitudes (Davis et al. (4.8.1)).

To try to exploit these major steps in our understanding of ionospheric behaviour, aproject at RAL is using real time, automatically-scaled ionospheric data, along with acoupled model of the global ionosphere-thermosphere system, to generate real timemaps of the ionosphere which could be used in the operation of HF systems. The modelused is that developed at Sheffield University and University College London in the UK

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and the Solar Environment Laboratory in Boulder, USA (Millward et al. (4.8.4)). This hasbeen installed in a dedicated DEC Alfa-Server 2100 at RAL which allows it to run eighttimes faster than real time. The model is global but, in order to minimise run times, theplasmasphere and equatorial ionosphere part is not in use in the first instance. Themodel is controlled by the date and time, the current sunspot number and an indexof the high-latitude power input called the ‘TIROS’ activity level. The aim is to use thereal time data from a number of ionosondes (including those run by RAL at Chilton andLerwick) to control the model iteratively. The TIROS level will be adjusted so as to bestmatch the most recent soundings and the global maps produced will be continuouslymade available on the world-wide web. In effect, the physical model will be used toextrapolate the ionospheric densities between the observation points. The TIROS levelsused to fit the data will be published as an index of ionospheric disturbance.

4.9 Statistical studies of ionospheric behaviour and morphology (RAL)

Statistical research into ionospheric behaviour, made possible by the long sequencesof data stored in WDC-C1, is carried out as part of RAL’s in-house research programme,funded by a PPARC rolling grant award. A survey of 24 years of simultaneous data onthe interplanetary medium and the ionosphere (Davis et al. (4.8.1)) has demonstratedthe effects following swing of the interplanetary magnetic field (IMF) from northward tosouthward. Such changes in the IMF’s orientation are important because the energyextracted from the solar wind is higher when it has a large southward component. Thestudies also demonstrate the role of the solar wind energy density, which is itselfcorrelated with the component of the IMF normal to the ecliptic (northward orsouthward) (Bowe et al. (4.9.1), Hapgood et al. (4.9.2)). In the hour following asouthward turning of the IMF, a rise in the auroral electrojet index AE is seen, indicativeof a pulse of energy deposition at high latitudes called a substorm. This is thought tocause global changes in the winds and composition of the neutral thermospherewhich lead to increased rate of loss of the ionospheric plasma. For about a dayfollowing each of these southward turnings, a significant depletion of the ionospherewas indeed seen. The survey has been extended to cover other European stationsand a coherent picture is now emerging of the response of the ionosphere, as afunction of latitude and the local time at the storm onset.

4.10 Physics of ionospheric storms (University of Southampton)

The research (Field and Rishbeth (4.10.1), Field et al. (4.10.2)) has two parts:

(1) Analysis of archived ionospheric data.(2) Computational modelling using the UCL/Sheffield/NOAA ‘Coupled

Thermosphere-Ionosphere-Plasmasphere Model’.

In part (1), the F2-layer response to geomagnetic activity has been studied, using datafor 53 stations spanning 33 years to derive ‘daily mean’ and ‘local-time’ variations of(disturbed/quiet) ratio of the critical frequency foF2. The variation of the ‘daily mean’level agrees well with the latitude variation of the (atomic oxygen/molecular nitrogen)concentration ratio in the neutral air, as derived both from the published globalempirical ‘MSIS’ model (Mass Spectrometer/Incoherent Scatter), and from thetheoretical modelling of part (2).

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In part (2) of the work, the three-dimensional, time-dependent ‘CTIPM’ (which solvesthe basic conservation equations for mass, momentum and energy to compute theglobal distributions of neutral and charged species at heights of 85-500 km) is used tosimulate the changes in the mid-latitude F2-layer that follow a high-latitudegeomagnetic storm.

The results of (1) and (2) broadly confirm the ‘composition change’ theory of the F2-layer storms i.e. that F2 layer storm effects at mid-latitudes are mainly caused bycomposition changes in the ambient neutral air. These changes are produced by aglobal ‘storm circulation’ in the thermosphere, driven by particle precipitation andelectric fields at high latitudes.

The data analysis (1) also reveals systematic differences in the F2-layer response tomagnetic disturbance during three solar cycles, which may indicate long-termchanges in chemical composition, perhaps due to varying solar activity. This workrelates to studies of long-term ‘global change’ in the upper atmosphere (Rishbeth(4.10.3)), and is also providing a new physical description of the annual andsemiannual variations of the normal F2 layer.

An example of results is shown in figure 4.10.1.

4.11 Auroral Research (University of Southampton)

Observations of the aurora with high spatial resolution (typically 100 m with opticalimagers, 1 km with the EISCAT radar), and with time resolution of 0.2 s, are combinedwith computational modelling of the particle acceleration and excitation processesin highly structured auroral arcs. The results give new insight into the physicalmechanisms of the aurora. The Southampton work is part of an internationalcollaboration involving Finland, Germany and the U.S.A. (Lanchester et al, (4.11.1)).

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Figure 4.10.1 Percentage changes in NmF2 (full curves) on [O/N2] ratio (dashedcurves) versus ‘storm time’ ST [time elapsed from the start of thesimulated geomagnetic storm], for nine zones of magnetic latitudeas shown.

(a) June solstice, geographic longitude 234 (American sector).(b) December solstice, geographic longitude 0 (European/African sector).

These results simulate, in both hemispheres, the well-known ‘magnetic storm effect’(decrease of foF2) in summer mid-latitudes, and the smaller ‘positive storm effect’(increase of foF2) in winter mid-latitudes. Some of the features are local-time effects.

Computed from the Coupled-Thermosphere-Ionosphere-Plasmasphere model(University College London, University of Sheffield, N.O.A.A., Boulder Colorado).

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5 National facilities

5.1 Radiocommunications Agency Baldock Radio Station

Baldock radio station is located on an electrically quiet site some 60 km north of thecentre of London. It is operational 24 hours a day, primarily for the alleviation ofinterference problems, but also offering it’s facilities on a commercial basis.

The monitoring and direction finding facilities are concentrated in the frequency range9 kHz to 30 MHz with restricted monitoring facilities up to 1 GHz.

There is the capability of taking field strength measurements, using calibrated invertedcone antennas, on any type of signal over the range 9 kHz to 30 MHz with anaccuracy of ±1 dB in the VLF and LF bands, and ±2 dB for higher frequencies up to 30MHz.

Frequency measurements in the same bands can be made to an accuracy of ±1 Hzon amplitude modulated or plain carrier signals. For frequency modulated signals,results are given to the nearest 10 Hz. Frequency measurements between 30 MHz and1 GHz can also be made with an accuracy dependent on the modulation format.

Direction finding facilities are available over two ranges, 100 kHz to 2 MHz and 1.5 MHzto 30 MHz. The lower range uses a microprocessor controlled Adcock array and the highrange comprises of a superresolution direction finding system using 8 antennas from thePlessey ‘Pusher’ array.

Plots of spectrum occupancy are available in a variety of formats using threedimensional ‘waterfall’ plots. These can be produced in either ‘max hold’ or ‘videoaverage’ form, and can cover a maximum duration of up to 7 days. The spectrumanalysers producing these plots are computer controlled and programmes can betailored to individual requirements.

A wide range of demodulation facilities are available for RTT circuits, together withstandard AM, FM, CW, and SSB.

5.2 DERA Cobbett Hill Radio Station

DERA Cobbett Hill is an HF radio station dedicated to research activities. The radiostation operates from two sites; a low noise receive and laboratory site at Cobbett Hill,near Pirbright in Surrey, and a remote transmitter station located on the main DERA siteat Farnborough.

Cobbett Hill is electrically very quiet, being located in an environmentally protectedarea, well away from industrial and residential noise sources. The measured site noiselevel is comparable with the atmospheric noise figures given in CCIR Report 322,indicating virtually zero contribution from man made noise. A variety of full sized HFantennas are available on the 90 hectare site. The laboratories are equipped withmodern communications and measurement receivers.

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The associated transmitter station at Farnborough is furnished with 4 x 1 kW and 2 x 10kW transmitters together with a selection of HF antennas. This site is unmanned andremotely controlled from Cobbett Hill over a land line data link. The two siteconfiguration permits several experiments/trials to be conducted concurrently withoutthe risk of local transmitter interference influencing results.

Cobbett Hill Radio Station supports a number of propagation, occupancy and radiosystems research programmes. A calibrated range facility is used to plot polar diagramsand measure the efficiency of aircraft HF antennas. The station has several frequenciespermanently allocated across the HF band. A unique combination of good facilities,stable site noise level, easy access to frequencies and an experienced staff, make thisradio station a valuable asset for the HF research community.

Many years ago DERA designed and built an array of sloping-Vee antennas radiatingoutward from a central vertical supporting structure. This was initially a transmittingand receiving antenna, but was rebuilt some ten years ago as a receive only antennaand a computer control and switching system incorporated in order to provide afacility to measure azimuthal occupancy. This was phase one in an intended twophase programme, with the second phase providing measurements of occupancyvertical arrival angles, using a separate array.

Phase one has recently been updated and UMIST have been commissioned toconduct occupancy measurements and rationalise them with previousomnidirectional occupancy work, as outlined in section 3.15.

5.3 World Data Centre C1 (Rutherford Appleton Laboratory)

The World Data Centre C1 for Solar-Terrestrial Physics maintains and updates anarchive of ionospheric soundings from around the globe. For some of these stations thedata sequence is very long; the longest being for Slough which extends back to 1931.In addition, the WDC collects, stores and distributes supporting data on geomagneticand solar activity and on the interplanetary medium. Data are exchanged under rulesset by ICSU (International Council of Scientific Unions).

The WDC also provides forecasts and warnings of solar, geomagnetic and ionosphericactivity on a range of time scales. Recently, neural network procedures have beendeveloped which allow improved prediction of the solar-cycle variation in the globalionospheric indices IF2 and IG (Stamper (5.3.1)). Figure 5.3.1 contrasts the results of theold and the new methods. The old method had two problems: firstly the predictionsbecome markedly less accurate the further into the future the predictions areprojected, for example the predicted size and timing of the cycle peak is increasinglyin error. Secondly, the method is not stable and can cause wildly fluctuating andinaccurate values, particularly in the descending phase of the solar cycle. Both theseproblems are avoided by the new algorithm which has now been implemented.

The WDC is partly funded by the Particle Physics and Astronomy Research Council(PPARC) and partly by the Radiocommunications Agency. Some work has beensponsored by other agencies, for example the European Space Agency, ESA. The WDCtakes in and distributes data in many forms, ranging from paper records to file transferby the internet. Table 5.3.1 gives a breakdown of the present usage of the WDC withinthe UK. The total number of data accesses made by organisations and research

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groups are given for the 6-month period April-September 1996. Note that there is awide variation in the volume of data in each access. The numbers are also splitaccording to the method used to access the data. At present, users can initiate access to the digital database either by a menu system or by the world-wide web(WWW): in the next year the menu system will be phased out.

Table 5.3.1 Numbers of WDC data accesses in the period April-September 1996

e-mail paper(mail/fax)

Menu WWW Total

Academic 2196 257 226 334 3013 (43%)Government 0 265 0 10 275 (4%)

Industry 555 20 8 19 602 (9%)System - - 104 887 991 (15%)Others 1104 236 131 571 2042 (29%)

Total 3855 778 469 1821 6923

5.4 Ionosonde service (Rutherford Appleton Laboratory)

RAL operates three digital ionosondes, at Chilton, Lerwick and Port Stanley. At all threesites, a Lowell DPS-1 Digisonde takes soundings every 15 minutes and the data areautomatically scaled by the ARTIST software. Ionograms from Lerwick and Chilton forthe past week, as well as the automatically-scaled data, are now available in realtime via the internet. All data from all three sites is checked and corrected using theADEP editor before being stored in a database at the WDC C1 at RAL. Data aresupplied to a number of users (see WDC usage statistics), both in real time and in finalform. The real time data are used in operations control and frequency managementfor communication, broadcast, radar and navigation systems. The fully-analysed andcorrected data are used for ionospheric research and mapping, retrospective analysisof system performance and to generate statistics for HF system design.

The Chilton ionosonde replaces the one at Slough which had operated continuouslysince 1931 but ceased taking measurements in December 1995. The distance fromSlough to Chilton is small compared to the characteristic correlation lengths of the mid-latitude ionosphere. Thus the data from Chilton can be used to extend the uniquesequence of data from Slough. Both sounders were operated simultaneously fromDecember 1993 to December 1995 to allow cross-calibration of the data. Thesecomparisons reveal the differences to arise predominantly from the new sounderhardware and new scaling methods in use at Chilton. The regressions and correlations,and their seasonal variations, will soon be published on the WDC world-wide webpages (http:// wdcc1.bnsc.rl.ac.uk/).

The new sounders and scaling procedures offer exciting new possibilities for mappingand understanding the ionosphere. In addition to the ADEP facility for scaling andanalysing data from the Digisondes, software has been written to process data fromthe dynasonde sounders at Tromsø in northern Scandinavia and Halley Bay inAntarctica (operated by DERA (Malvern), and British Antarctic Survey, respectively).

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83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 0

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Figure 5.3.1 Predictions of the solar-cycle variations of the global ionosphericIF2 index by the old (top) and new (bottom) methods in use bythe WDC. Each plot shows the predicted value for years 1982-1997 for periods 1-20 months ahead. The curve for zero monthsahead gives the values actually observed and is highlighted withred circles.

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6 References

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1.5.1Maundrell M.J., ‘An implementation of multi-frequency shift keying (MFSK)providing a robust modulation system for air/ground communications’, 5th IEEInternational Conference on ‘HF Radio Systems and Techniques’, Conf. Pub.339, Edinburgh, July 1991, pp 199-205.

1.5.2Clark P.D.J., Horley N., Darnell M., Honary B. and Maundrell M., ‘Design andperformance of an adaptive MFSK HF terminal’, 6th IEE InternationalConference on ‘HF Radio Systems and Techniques’, Conf. Pub. 392, York, July1994, pp 330-332.

1.10.1Gott G.F., Darbyshire E.P., Brydon A.N., Oodit B.P. and Rubenstein R.H., ‘Robustslow and medium rate data transmission’, 5th IEE International Conference on‘HF Radio Systems and Techniques’, Conf. Pub. 339, Edinburgh, July 1991, pp212-216.

1.15.1Dhanda B.S., Warrington E.M. and Jones T.B., ‘Observations of Dopplerspreading and FSK signalling errors on HF signals propagating over high latitudepaths’, Proceedings of AGARD Specialists Meeting on Digital CommunicationSystems: Propagation Effects, Technical Solutions, Systems Design, Athens,September 1995, pp 18/1-18/12.

1.16.1Cannon P.S. and Dickson A.H., ‘BLOSSOM - an air-to-ground meteor burstsystem’, Proc. IEEE Milcom’86 Conf., Monterey, California, USA, 1986.

1.16.2Shukla A. K., Cannon P.S. and Lester M., ‘Spaced antenna diversity intemperate latitude meteor burst systems operating near 40 MHz: variation ofsignal cross-correlation coefficient with time’, Radio Sci., 27(6), 1992, pp 841-849.

1.16.3Akram A. and Cannon P.S., ‘A meteor scatter communications system datathroughput model’, 6th IEE International Conference on ‘HF Radio Systems andTechniques’, Conf. Pub. 392, York, 1994, pp 343-347.

1.16.4Akram A. and Cannon P.S., ‘Passive beam forming and spatial diversity inmeteor scatter communication systems’, Radio Science, Vol.31, No.2, 1996, pp367-380.

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1.16.5Akram A. and Cannon P.S., ‘A new meteor scatter prediction model and itsapplication to adaptive beam steering’, accepted for publication in RadioScience, 1997.

1.17.1Murray K.P. and Austin B.A., ‘Synthesis of vehicular radiation patterns using themethod of Characteristic Modes’, Proc. IEE - Microwaves, Ant. and Prop., 141,(3), 1994, pp 151-154.

2.1.1Wheadon N.S., Whitehouse J.C., Milsom J.D. and Herring R.N., ‘Ionosphericmodelling and target coordinate registration for HF sky wave radars’, 6th IEEInternational Conference on ‘HF Radio Systems and Techniques’, Conf. Pub.392, York, 1994, pp 258-266.

2.2.1Howland P.E. and Walbridge M.R., ‘An HF sky wave radar availability model’,6th IEE International Conference on ‘HF Radio Systems and Techniques’, Conf.Pub. 392, York, 1994, pp 273-277.

2.4.1Jones T.B. and Thomas E.C., ‘CUTLASS: A new HF radar for auroral research’,Proceedings of the IEE Seventh International Conference on HF Systems andTechniques, July 1997, pp 325-329.

2.9.1Tarran C.J., ‘Operational HF DF systems employing real time superresolutionprocessing’, 7th IEE International Conference on ‘HF Radio Systems andTechniques’, Conf. Pub. 441, Nottingham, 1997, pp 311-319.

2.10.1Warrington E.M., Rogers N.C. and Jones T.B., ‘Large HF bearing errors forpropagation paths contained within the polar cap’, Proc. IEE - Microwaves,Ant. and Prop., 144, (4), 1997, pp 241-249.

2.10.2Rogers N.C., Warrington E.M. and Jones T.B., ‘Large HF bearing errors forpropagation paths tangential to the auroral oval’, Proc. IEE - Microwaves, Ant.and Prop., 144, (2), 1997, pp 91-96.

3.1.1Wagner L. S., Goldstein J.A., Rupar M.A. and Kennedy E.J., ‘Delay, Doppler andamplitude characteristics of HF signals received over a 1300 km transauroral skywave channel’, submitted to Radio Science.

3.1.2Davies, N. C. and Cannon P.S., ‘DAMSON - A system to measure multipathdispersion, Doppler spread and Doppler shift on multi-mechanismcommunications channels’, AGARD Conf. on ‘Multiple Mechanism PropagationPaths’, Rotterdam, Netherlands, Sept 1993.

3.1.3Cannon P. S., Davies N.C., Angling M.J., Jodalen V., Moreland K.W. and

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Lundborg B, ‘Initial results from DAMSON - a system to measure multipath,Doppler spread and Doppler shift on disturbed HF channels’, 9th IEEInternational Conference on Antennas and Propagation, Eindhoven,Netherlands, April 1995.

3.2.1Arthur P. C. and Cannon P.S., ‘ROSE, A high performance oblique ionosondeproviding new opportunities for ionospheric research’, Annali di Geofisica, VolXXXVII, No 2, 1994, pp 135-144.

3.4.1Warrington E.M., ‘Observations of the directional characteristics ofionospherically propagated HF radio channel sounding signals over two highlatitude paths’, Proceedings of the Second Symposium on Radiolocation andDirection Finding. Southwest Research Institute, San Antonio, Texas, 4-6November 1997.

3.6.1Goodman J.M., Uffleman D.R. and Fahlsing F., ‘The role of the propagationenvironment in HF electronic warfare’, AGARD Conference ‘Propagation effectson ECM-resistant systems in communications and navigation’, CP331, London,1982.

3.6.2Shukla A. K. Cannon P. S. ‘A Jamming and interception vulnerability estimatorfor HF communications systems’, AGARD Symposium ‘Environmental factors inelectronic warfare related to aerospace systems’, Rome, May 1995.

3.7.1Platt I. G. and Cannon P.S., ‘A propagation model for the high and mid-latitudeover Europe’, 6th IEE International Conference on ‘HF Radio Systems andTechniques’, Conf. Pub. 392, York, 1994, pp 86-90.

3.7.2Shukla A. K. and Cannon P. S., ‘Prediction model updating using the ROSE-200oblique sounder at mid and higher latitudes’, 6th IEE International Conference on ‘HFRadio Systems and Techniques’, Conf. Pub. 392, York, 1994, pp 99-103.

3.14.1Gott G.F., Chan S.K., Pantjiaros C.A. and Laycock P.J., ‘High frequency spectraloccupancy at the solstices’, Proc. IEE-Communications, 144 (1), February 1997, pp 24-32.

3.14.2Pantjiaros C.A. and Laycock P.J., Gott G.F. and Chan S.K., ‘Development of theLaycock-Gott occupancy model’, Proc. IEE-Communications, 144 (1), February 1997,pp 33-39.

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3.14.3Pantjiaros C.A., Wylie J.A., Gott G.F. and Laycock P.J.’Spectral occupancy onHF NVIS links in central Europe’, 7th IEE International Conference on ‘HF RadioSystems and Techniques’, Conf. Pub. 441, Nottingham, 1997, pp 403-407.

3.14.4Gott G.F., Pantjiaros C.A., Wylie J.A. and Laycock P.J., ‘HF spectral occupancymeasurements in Europe’, IEE colloquium on ‘Frequency selection andmanagement techniques for HF communications’, (digest 1996/024), 1996.

3.15.1Pantjiaros C.A., Economou L.V., Gott G.F. and Laycock P.J.’Variation of HFspectral occupancy with azimuth in the UK’, 7th IEE International Conferenceon ‘HF Radio Systems and Techniques’, Conf. Pub. 441, Nottingham, 1997, pp19-24.

4.4.1Robinson T.R., ‘The heating of the high latitude ionosphere by high power radiowaves’, Physics Reports, 179, 1989, pp 79-209.

4.4.2Stubbe P., Stocker A.J., Honary F., Robinson T.R. and Jones T.B., ‘Stimulatedemissions (SEE) and anomalous HF wave absorption near electrongyroharmonics’, Journal of Geophysical Research, 99, 1994, pp 6233-6246.

4.5.1Wright D.M., Yeoman T.K. and Chapman P.J., ‘High-latitude HF Dopplerobservations of ULF waves: 1. Waves with large spatial scale sizes’, in press,Annales Geophysicae, 1997.

4.6.1Kersley L., Pryse S.E., Walker I.K., Heaton J.A.T., Mitchell C.N., Williams M.J. andWillson C.A., ‘Imaging of electron density troughs by tomographic techniques’,Radio Sci., 32, 1997, pp 1607-1621.

4.6.2Pryse S.E., Kersley L., Rice D.L., Russell C.D. and Walker I.K., ‘Tomographicimaging of the ionospheric mid-latitude trough’, Anns. Geophysicae, 11, 1993,pp 144-149.

4.6.3Walker I.K., Heaton J.A.T., Kersley L., Mitchell C.N., Pryse S.E. and Williams M.J.,‘EISCAT verification in the development of ionospheric tomography’, Anns.Geophysicae, 14, 1996, pp 1413-1421.

4.6.4Mitchell C.N., Jones D.G., Kersley L., Pryse S.E. and Walker I.K.,’Imaging of field-aligned structures in the auroral ionosphere’, Anns. Geophysicae, 13, 1995, pp1311-1319.

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4.6.5Pryse S.E., Kersley L., Williams M.J., Walker I.K. and Willson C.A.,’Tomographicimaging of the polar-cap ionosphere over Svalbard’, J. Atmos. Terr. Phys., 59,1997, pp 1953-1959.

4.8.1Davis, C.J., Wild M.N., Lockwood M. and Tulunay Y.K.,’Ionospheric andgeomagnetic response to changes in IMF Bz: a super-posed epoch study’,Annales Geophys., in press, 1996.

4.8.2Fuller-Rowell, T.J., Codrescu M.V., Rishbeth H., Moffet R.J. and Quegan S., ‘Onthe seasonal response of the thermosphere and ionosphere to geomagneticstorms’, J. Geophys. Res., 101, 1996, pp 2343-2353.

4.8.3Quegan, S., Gill R.S., and Lockwood M., ‘Comparisons between EISCATobservations and model calculations of the high latitude ionosphere’, J. Atmos.Terr. Phys., 50,1988, pp 1057-1076.

4.8.4Millward, G.H., Rishbeth H., Fuller-Rowell T.J., Aylward A.D., Quegan S. andMoffett R.J., ‘Ionospheric F2 layer seasonal and semi-annual variations’, J.Geophys. Res., 101, 1996, pp 5149-5156.

4.8.5Lockwood, M., ‘The excitation of ionospheric convection’, J. Atmos. Terr.Phys., 53, 1991, pp 177-199.

4.8.6Cowley, S.W.H. and Lockwood M., ‘Excitation and decay of solar-wind drivenflows in the magnetosphere-ionosphere system’, Annales Geophys., 10, 1992, pp103-115.

4.8.7Idenden, D.W., Moffett R.J., Quegan S. and Fuller-Rowell T.J., ‘Time-dependentconvection at high-latitudes’, Annales Geophys., 14, 1996, pp 1159-1169.

4.8.8Lockwood, M., ‘Modelling the high-latitude ionosphere for time-varying plasmaconvection’, Proc. IEE - Microwaves, Ant. and Prop., 140, 1993, pp 91-100.

4.8.9Lockwood, M. and Carlson H.C., Jr.,’Production of polar cap electron densitypatches by transient magnetopause reconnection’, Geophys. Res. Lett., 19,1992, pp 1731-1734.

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4.9.1Bowe, G.A., Hapgood M.A., Lockwood M., and Willis D.M., ‘Variability of solarwind number density, speed and dynamic pressure as a function of theinterplanetary magnetic field components: a survey over two solar cycles’,Geophys. Res. Lett., 17, 1990, pp 1825-1828.

4.9.2Hapgood, M.A., Bowe G., Lockwood M., Willis D.M. and Tulunay Y., ‘Variability of the interplanetary magnetic field at 1 A.U. over 24 years: 1963-1986’, Planet.Space Sci., 39, 1991, pp 411-423.

4.10.1Field P.R. and Rishbeth H. (1997), ‘The response of the ionospheric F2-layer togeomagnetic activity: An analysis of worldwide data’, J. Atmos. Solar-Terrest.Phys., 59, pp 163-180.

4.10.2Field P.R., Rishbeth H., Moffett R.J., Idenden D.W., Fuller-Rowell T.J., MillwardG.H. and Aylward A.D. (1998), ‘Modelling composition changes in F-layer storms’,J. Atmos. Solar-Terrest. Phys., 60, in press.

4.10.3Rishbeth H., ‘Long term changes in the ionosphere’, Adv. Space Res., inpress.

4.11.1Lanchester B.S., Rees M.H., Lummerzheim D., Otto A., Frey H.U. and Kaila K.(1997), ‘Evidence for large fluxes of auroral electrons in narrow 100 m filamentscaused by transient parallel electric fields’, J. Geophys. Res., 102, pp 9741-9748.

5.3.1Stamper R., ‘Improved prediction of IF2 and IG indices using neural networks’,Proc. IEE - Microwaves, Ant. and Prop., 143, 1996, pp 341-346.

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7 Contact personnel

Dr C.P. Ash (chris.ash@rrl.co.uk)Racal Research Ltd.Worton Grange Industrial EstateReading, Berks RG2 0SB

Dr B.A. Austin (ee104@liv.ac.uk)Department of Electrical Engineering and ElectronicsUniversity of LiverpoolBrownlow HillLiverpool L69 3GJ

Mr R.C. Bagwell (bbagwell@dra.hmg.gb)Defence Evaluation and Research Agency (DERA)Cobbett Hill Radio StationNormandy, Surrey GU3 2AA

Professor L.W.Barclay OBE (les@barclay.demon.co.uk)Barclay Associates Ltd.12 St Stephen’s RoadCold NortonChelmsford, Essex CM3 6JE

Dr P.S. Cannon (pcannon@dra.hmg.gb)Dr S.G. FarquharMr P.E. Howland (pehowland@dra.hmg.gb)Mr M.J. Maundrell (mjmaundrell@dra.hmg.gb)Mrs A.P. NobleMr M.R. WalbridgeDefence Evaluation and Research Agency (DERA)St Andrews RoadMalvern, Worcs WR14 3PS

Professor M. Darnell (miked@elect-eng.leeds.ac.uk)Department of Electronic and Electrical EngineeringUniversity of LeedsLeeds LS2 9JT

Mr R.H. Edmundson (redmndsn@fei.org.uk)The Federation of The Electronics IndustryRussell Square House10-12 Russell SquareLondon WC1B 5EE

Mr C.J. Tarran (chris.tarran@roke.co.uk)Mr. R.J. GoodwinRoke Manor Research Ltd.Roke ManorRomsey, Hants SO51 0ZN

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Professor G.F. Gott (gott@umist.ac.uk)Department of Electrical Engineering and ElectronicsUMISTPO Box 88Manchester M60 1QD

Professor B. Honary (b.honary@lancaster.ac.uk)Communications Research CentreLancaster UniversityLancaster LA1 4YR

Professor T.B. Jones (tbj@ion.le.ac.uk)Department of Physics and AstronomyUniversity of LeicesterUniversity RoadLeicester LE1 7RH

Professor L. Kersley (lek@aber.ac.uk)Dr S.E. PryseDepartment of PhysicsThe University of Wales AberystwythAberystwythCeredigion SY23 3BZ

Dr M.F. Levy (m.levy@rl.ac.uk)Professor M. Lockwood (m.lockwood@rl.ac.uk)Rutherford Appleton LaboratoryChiltonDidcot, Oxfordshire OX11 0QX

Mr J.D. MilsomMr N.S. Wheadon (nigel.wheadon@gmrc.gecm.com)GEC-Marconi Research CentreGreat BaddowChemlsford, Essex CM2 8HN

Professor H. Rishbeth (hr@phys.soton.ac.uk)Professor M.H. Rees (M.H.Rees@soton.ac.uk)Dr B.S. LanchesterDepartment of PhysicsThe UniversitySouthampton SO17 1BJ

Mr G. Williams (G4FKH) (gwyn@g4fkh.demon.co.uk)Propagation Studies CommitteeRSGB, Lambda HousePotters Bar, Herts EN6 3JE

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Dr E.M. Warrington (emw@leicester.ac.uk)Department of EngineeringUniversity of LeicesterUniversity RoadLeicester LE1 7RH

Mr R. Wiltshire (rms#u#rw@ra.gtnet.gov.uk)Baldock Radio StationRoyston RoadBaldock, Herts SG7 6SH

56

THE FEDERATIONOF THEELECTRONICS INDUSTRY

RUSSELL SQUARE HOUSE10-12 RUSSELL SQUARELONDON WC1B 5EETELEPHONE: (0171) 331 2000FAX: (0171) 331 2040

R H Edmondson, Assistant Director ICT Tel: (0171) 331 2028 (Direct Line)email: redmndsn@fei.org.uk

FEI Input for DTI NRPC TWP Report on Current Research into HF Systems, HFPropagation, and the Ionosphere

The Federation of the Electronics Industry (FEI) is the DTI recognised ‘lead’ TradeAssociation for the information technology, electronics, and communications industries.The industry represented is now the fastest growing sector in the world, set to representsome 10% of global GDP by the turn of the century. The activities of the Federation aremanaged under five business areas each having it’s own Director, Executives,Secretarial support, and committee structure.

In a marketplace as varied and dynamic as that represented by FEI, members’interests continually change with the economic and political environment. TheFederation’s activities are constantly adapted to meet these changing demands;ensuring that its some 250 member companies, with the assistance of 34 permanentstaff members, flourish at a time of unprecedented change.

Within the Federation’s Information and Communication Technologies (ICT) BusinessArea there is a Radio Communications Co-ordination Group (RCCG) which has sevencommittees reporting to it, one of which is the High Frequency CommunicationsCommittee (HFCC).

For the past few years the FEI HFCC has provided the UK coordination forum for thedevelopment of frequency agile, adaptive systems for both fixed and mobile services.Broad band antennas and amplifiers, used in conjunction with frequency synthesisersand DSP based control systems, have enabled a range of communication systems tobe developed which utilise the spectrum efficiently and which provide improved linkavailability and performance. However, although the use of these systems is within theRadio Regulations, this is not expressed clearly and appropriate procedures withminimum administrative requirements are not included. This has been identified as anagenda item for the ITU WRC-97. In recent months the FEI HFCC has been supportingand contributing to the studies on this matter. Further work will be undertaken for ITU-RWorking Parties 9C and 1A and in briefing for the WRC.

It is expected that the widespread introduction of frequency adaptive systems would

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result in a very substantial reduction in spectrum occupancy at HF and would providehigh quality communications through this difficult propagation environment.