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TRANSMISSION OF HllGH POWER 9 MHZ SIGNALS NEAR IONOSPHERIC CUT-OFF FROM 'SURA' TOWARDS THE MOON AND RECEPTION OF THE ECHO WITH THE RADIO TELESCOPE 'UTRZ'.

Y

C.G.M. van 't Klooster, Yu.Tokarev, Yu.1. Belov, ** A.A. Konovalenko, P. Rodriguez

*

*** ****

* ESA-Estec, The Netherlands ** Radio-Physics Institute, NIRFI, Russia. *** Institute of Radio Astronomy, RAI, Ukraine **** Naval Research Laboratory, USA

Abstract Two unique broad-band decametric arrays SURA and UTR2 are described in outline format. They are used for radio astronomy aind radiophysical research (solar system investigations, ionospheric research, near-Earth plasma, etc). Both facilities were used last summer in 'bistatic' configuration, transmitting from SURA and receiving with UTR2. Moon and Sun were used as targets, INASA!; WIND spacecraft was detecting with the RAD2 receiver [5]. Preliminary results are shown.

Introduction Radiophysics Research Institute (RRI or NIRFI) carries out research related to ionosphere and near- Earth plasma. It realised large facilities in support, amongst which the transmitlreceive station SURA near Varsel'sursk (56'7' N, 46" E:) in the early 80's. The 'SURA' facility (named after a nearby river) can have Aeff>50.000 m2 for an antenna beam near zenith. It is also used as radio-telescope [ l ] . The UTRS, (= Ukrainian T-shaped Radio telescope) is the world's largest radio telescope at decametric wavelength. It is located near Kharkov (Ukraine) and has an effective receiving area Aeff>l00.000 m2. It provides sensitive and accurate receive capability in the band of 10 to 25 MHz with 5 simultaneous beams in phased array configuration. It is used for radio astronomy, for investigations of SUN and solar system,etc.[2,4]. It is operated by the Radio Astron- omy Institute in Kharkov. Results were obtained with SURA transmitting to WIND [3] and bistatic experiments were carried out in the past, a specific experience has been built up at both institutes. Suggestions were made by NRL to use 'SURA' and 'UTR2' in a bistatic radar mode for further investigations (related to solar system and radio communications). ,A special target scenario was indicated by NRL la!jt year: Moon, Sun and WIND were within a few degrees angular separation (1 5/7/1996), thus providing a situation with different targets within the transmit beam of SURA.

Echo signals from the Moon were easily detected by both UTR2 and SURA. Detection of an echo from the Sun is not confirmed now, further study of results continues. During the consecutive days of transmission with SURA, the signals were detected by WIND.

The 'SURA' facility The facility was realised early 80's to explore effects of irradiating the ionosphere with high power signals. Ideas were suggested for application of the induced inhomogenities in the ionosphere, like collimation (artificial lens) or the creation of bounded regions, perhaps as communication channels ('ionospheric transmission-lines' for 'discrete' communication). The frequencies were selected near critical ionospheric frequencies. An antenna was realised

The antenna consists of a 12*12 element array. It is subdivided into 3 sub-sections, each of which is connected to one of the 3 transmitters (250 kW SW transmitters for 4 to 30 MHz). Synchronisation and modulation is possible. Coherent combination of the power in space gives, with 3 transmitters synchron- ised and connected to the 3 parallel subsections, about 300MWatt effectively radiated near zenith. Subdivision into sections gives a flexibility to adapt to different scenarios (Tx/Rx, RX or Tx at 3 closely spaced frequencies, Tx and Rx, etc.). Fig.1 shows a scheme of the configuration. Switching of polarisation (RH-LH) occurs by switches at sub-section level (manual). Airlines connect 3 Tx outputs to respective subsections, within which the 4 radiating columns are connected to a binary feed-network. Time control and time delay elements gives frequency independent scan capability. The scanning is in principle over f40" from zenith. At higher frequencies grating lobes appear for larger 'off-zenith' angles. A reduced gain results as has been observed [l]. But observations at low elevation have also been done, exploiting such grating lobe.

(4.6-9.3 MHz).

10th International Conference on Antennas and Propagation, 14-17 April 1997, Conference Publication No. 436 0 IEE 1997

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7 x x x x x x x x x x x x Iil;l,..,;;;;;i .. x x x x x x x x x x x x

c

rhnaa

-

1 7 3 1607 8 3 i O i i i P

x x x x x x x x x x x x 7 I l l - ~ x x x x x x x x x x x x

~~dum., , - I I ~ x x x x x x x x x x x x N X X X X X X X X X X X X

Frequency 4.6 - 9.3 MHZ

I Polarisation I LH-RH manually switchable I

Array size

Subsection

I I I I

12*12 elements,300*300 m2

3 (4 * 12 elements)

I Scan volume I 2 40 ', from zenith I Array orientation:

Scanning EW

t I I I

along magnetic meridian (at height of F-layer locally)

Manually installed

Element

VSWR

Broad-band turnstile-dipole,

<2 (500Q twinline) I 1

I I Pol isolation

Step phasers

Beamwidth

better than 25 dB

4", time delay phasing N-S

I O " * I O " (4.785 MHZ) 5" * 5" (9.310 MHZ)

Gain (calc) Gain-(meas)

201(4.785MHz), 61 I(9.31MHz) 200(4.785MHz), 380(9.31 MHz)

Table 1 Parameters for SURA facility

Directivity

A recording of a received Moon echo is shown as example in fig.3 (1 987), without calibrated vertical scale (pulse width was 2.2 sec, the period 5 sec). Indicative relative pattern information for the SURA antenna was obtained from an earlier transmission to WIND (a sinc-like pattern shape was observed in WIND data [3]). SURA is in the list of important R+D facilities of Min. of Education in Russia.

305(4.785MHz) 1072(9.31 MHz

Fig.2 Moon echo observed with SURA facility. Time axis horizontal, sampled response, Relative signal recording, no calibration vertical.

The UTR2 Radio telescope. UTR2 permits a detection of fluxes down to 10 Jansky (1 Jansky = 10-26WHz-'m-2) with angular resolution of 25'. It operates usually at frequencies in the 10 - 25 MHz band, it can receive the SURA signals near 9 MHz. Obviously effects due to ionosphere and man-made interferences complicate observation, clearly a more quiet environment is needed (the backside of the Moon has been suggested as a good location for disturbance-free radio astronomy). A T-shaped aperture is used with 2 long sections of 900 m in the long dimension in the North-South (NS) direction, with a 50 m width, so containing 240 by 6 elements separated by 9 m in EW and 7.5 m in NS. The EW arm is 900m with 600 elements spaced 9m in EW direction and 7.5m in NS direction. Robust, zinc-coated shunt-type dipoles are used as elements, linearly polarised (in EW direction), with a length of 8 m and 1.8 m in diameter, located 3.5 m above the ground. The geometrical area filled is ca 1800*50+900*50, thus leading to a large receiving area Aeff at frequencies considered. The telescope uses a phasing system based on time delay principles. The six dipoles along the narrow dimension in each of the arms are phased with phase shifters having four positions (16 beam directions). Phasing along the NS direction is performed in 256 steps using three stages. Along the NS direction a subdivision is done into 8 groups of 180 elements each, with the three stage phasing capability resulting into 256 beam positions. Along the EW direction 4 groups are found of 150 elements, within a group a two stage phasing is used, providing 128 beam positions. A further phasing of the sections is done to combine and to interrelate NS and EW branches, as needed also for differences in phase centre location. Amplification of signals is at section level. Details are found in [4]. The building with control equipment is located in the centre at the intersection of NS and EW branches. Summation of the signals provides 5 simultaneous beams. UTR2 provides with this multiple beam

10th International Conference on Antennas and Propagation, 14-17 April 1997, Conference Publication No. 436 0 IEE 1997

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capability (5 beams) a unique tool for instance to monitor the Sun as a heliograph, where the five beams shifting over the Sun provide a localisation posibility of solar phenomena. Solar observations were also carried out during the bistatic experiment, to monitor the solar activity during the reception period. Fig 3 below shows one hallf branch of the NS arrav of UTR2.

Fig

WlND Spacecraft. NASA‘s spacecraft WIND carries instruments providing in situ data about the E,arth’s environment and its responses to solar wind (For details see for instance ‘www.sel.noaa.gsv/wintl/windlinks. html’). A sampling receiver RAD-2 operates in program- mable mode between 1 arid 12 h4Hz [5]. WIND is in an interesting orbit, it was near a Lagrange point. It follows far-out orbits. WIND WEIS about 1.500.000 km away from the Earth during the experiment last year, in an angular direction near to that of the Sun. RAD2 detected the pulsed signal transmissions (CW was used earlier [3]). As an experiment also a trans-mission at lower frequency (5.8 MHz) was detected. Daily plots ( W W ) show transmissions. Some examples will be shown in the presentation.

Preliminary Results One subsection of SURA was used for the trans- mission to the Moon, with an effectively radiated power PG=20*1 O6 Watt. The full array was used to transmit to the Sun. For the M o m a pulse duration of 1.2 sec was taken, at 5 sec interval. UTR2 received a direct signal (groundwave) and the Moon echo was observed at variabe levels to about 30 dB above the background level. ( Galactic Background has a brightness temperature Tb=4*105K at this frequency, much higher than the 1OOOK noise temperature of the receiver). Fig.4 shows a fragment of the signal as received by UTR2. The direct signal is seen as well as the Moon echo. The solar activity was monitored during the reception and indeed some solar burst:; were observed. lonograms were taken at SURA to monitor the plasma-frequency. At UTR.2, 3 oLit of the five beams were pointed towards the Moon (NS array). The strongest signal was received in a beam deviating about 1” from the expected direction, so indicating

ionospheric refraction. Spectral analysis of the echo shows 0.3 Hz Doppler-shift, compatible indicatively with a movement due to Earth rotation.

Fig5 shows a sample of a recording taken during the transmission towards the Sun, when the Moon was also within the SURA beam (15/7/1996). The high level of the direct signal is observed, related to the 10 sec pulse (20 sec period) to the Sun, followed by a lower level, which is the Moon echo. Again the flexibility of UTR2 was exploited, with one arm (NS) tracking the Moon and the other arm (EW) tracking the SUN. There is no confirmation now of a detection of an echo signal from the SUN.

A preliminary estimation for the power budget is of interest. With Tb=4*105 K, 30dB higher received signal at UTR2 corresponds to 2*10-’3Watt, as follows from Prec=kTb*Af*x, with k=l.38*1 0-23, Tb=105 K and Af=35Hz, where 30 dB above Tb is indicated with ‘x’. The power radiated from the Moon ignoring for a moment the medium (ionosphere) follows as:

4nR ‘ p ~

’moon-utr2 ret A

eff-utr2 fl

or about 4 Watt, with R=4*108m (distance to Moon), Aeff =0.96* I O 6 m2. SURA did radiate effectively 20 MWatt, it gives about 100 Watt as power reradiated by the Moon assuming a= 1013 m2 (perfect disk):

0 =PG- 4nR moon-sura P

An approximation of the radar cross-section of the Moon as if it were a disk (a= I O l 3 m2 ) is not good. A 0.46 times smaller value was determined near 9 MHz at Varsel’sursk. The ionosphere causes about q,,=0.4 (estimate for two-way absorption), but the ionosphere can be different above UTR2 and SURA. A polarisation mismatch is present (Cp on Tx, linear on receive) of U,, = 0.5 (near statistical, as medium, antennas and imperfect reflecting Moon contribute to depolarisation). It results: q,ot=qion*~po,*qmoon= 0.032 in comparing:

‘moon -utr2 =0.8 P

Some contributors have a large relative error. The experimentally determined value qmoon=O. 16 (NIRFI) is of interest. Above mentioned quotient should be ‘l’, but relative errors need better assessment.More

10th International Conference on Antennas and Propagation, 14-17 April 1997, Conference Publication No. 436 0 IEE 1997

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work is needed to explore this further as it impacts on the estimation

SUMMARY Two large, unique phased arrays UTR2 and SURA, operating at decametric wave!ength, were described and some results were shown of a bistatic radar experiment with Moon, Swi and WIND involved as targets. The Moon echo was observed, but a reception of the Sun echo was not confirmed. WIND detected all transmissions. The multibeam capability of UTR2 was exploited during the observations, it also gave the possibility to monitor the solar activity during the experiment. Results provide interesting information, a repetition of such experiment is anticipated, possibly exploiting other techniques like coding to explore improvements in dynamic range,

References: [ I ] Yu.P. Ilyasov, Yu.1. Belov, 'Large Phased

Arrays as Radio Telescopes in Russia', Estec Workshop Proceedings WPP-110, Noordwijk, Feb 1996.

Antennas and their Use in interferometer Configuration', EstecWorkshop Proceedings WPP-110, Noordwijk, Feb 1996.

'Experimental 9 MHZ Transmission from Varsel'sursk, Russia, to NASA WIND S/C' Preparing for the Future, ESA Vol 5 Nr 4.

Sharykin, I.N.Zhouck, 'Decametric Survey of Discrete Sources in the Northern Sky', Astrophysics and Space Science 54(1978)p3-36, D.Reidel Pub. Holland.

Investigation of the WIND Spacecraft', Space Science Reviews, 71, pp 231-263, Kluwer 1995.

[2] A.A. Konovalenko 'Large Broad-Band Array

[3] C. van 't Klooster, Yu. Belov, Yu Tokarev

[4] S.Ya Braude, A.V.Megn, B.P.Ryabov, N.K.

[5] J.Bougeret,e.a. 'Waves: The Radio and Plasma

Fia.4 - Left - Sample of signal transmitted to the Moon, the direct signal (high pulse level) and Moon echo are seen. Max echo signal in this plot ca 26 dB above Tb, with a bandwidth of 25 Hz for UTR2 receiver.

Fia.5 Sample of signal transmitted to the Sun with the direct signal and the Moon echo indicated.

10th International Conference on Antennas and Propagation, 14-17 April 1997, Conference hblication No. 436 0 IEE 1997