SATELLITE

Satellite

SATELLITE

OVERVIEW OF SATELLITE COMMUNICATION

COMMUNICATION THROUGH SATELLITE

Introduction

Long distance communication using conventional techniques like coaxial cable or microwave radio relay links involves a large number of repeaters. For radio relay links of repeater spacing is limited by line of sight and is of the order of tens of kms. As the number of repeaters increase system performance and reliability are degraded. Tropo scatter propoagation can cover several hundred kms. but the channel capacity is limited and costs are high due to necessity of large antennas and high transmit power. HF communication is subject to fading due to ionospheric disturbances and channel capacity is severely restricted due to limited bandwidth available. Large areas could be covered if the height of microwave repeater could be increased by putting it on board an artificial earth satellite (Fig.1). Science Fiction writer Arthur C. Clarke in an article in Wireless World in 1945 proposed that worldwide coverage could be obtained by using three microwave repeaters placed in a geostationary orbit at the height of about 36000 kms. with a period of 24 hours (Fig.2).

Fig. 1

Modes of Communication

Fig. 2

Global Coverage with Geostationary Satellite

Satellite communication provide a practical and economical means of long haul communication traffic in a country with a large geographical area.

It also enables communication service to those areas which are virtually INACCESSIBLE by other conventional forms of communication system due to natural physical barriers.

Principles and Features of Satellite Communications

Principles

Figure 2 shows the principles of satellite communications. Here, a geostationary satellite with microwave radio repeater equipment receives and amplifies radio waves sent from earth stations and returns them to the earth.

A geostationary satellite is launched above the equator 36,000 km high above the earth. Its period round the earth coincides with that of the earth rotation. Therefore, the satellite looks as if it is stationary from the earth. If three (3) communication satellites are launched equidistantly above the equator (See Fig.2), it can serve almost all communication network round the world. Therefore, to facilitate public international telecommunications, INTELSATS IV and V have been launched above the Atlantic, Pacific, and Indian Oceans. These networks cover almost all countries around the world.

Features

For international communication, a submarine cable along the Atlantic Ocean was installed in 1857. Also, shortwave radio communication (invented by Marconi in 1886) has been in use. However, short wave radio communication has disadvantages of :

(1) Small transmission capacity; only small telephone channels can be used to transmit.

(2) Fading in wave propagation; interferes with stability of transmission. Although overthehorizon propagation is used for short distance international communications, it is impossible to apply it to transoceanic long distance communications.

Unlike other system, geostationary satellite communication systems summarize as follows :

(1) Stable and large capacity communication.

(2) Costs of establishment and maintenance do not depend on communication distance. The costs of submarine and overthehorizon systems are proportional to the length, but those of the satellite system do not affect the communication distance. Therefore, the satellite system is ideal for long distance communications.

(3) Multiple access is possible. Signals sent from an earth station can be received at several earth stations simultaneously. Therefore, it can transmit signals to many stations simultaneously, such as TV. Actually, increasing of submarine cable's capacity and distance between repeaters, can make submarine cables competitive to satellite communication specially when very large capacity is required but for small traffic size countries, satellite communication is unavailable for the independent communication services.

Advantages of Satellite Communications

(i) Large coverage : Almost onethird of the earth with exception of polar regions is visible from geostationary orbit. It is, thus, possible to cover about 10,000 kms. distance irrespective of intervening terrain with a single satellite.

(ii) High quality : Satellite links can be designed for high quality performance. The link performance is highly stable since it is free from ionospheric disturbances, multipath effects or fading.

(iii) High reliability : Reliability is high since there is only one repeater in the link.

(iv) High capacity : With microwave frequencies, wide bandwidths are available and large communication capacity can be obtained.

(v) Flexibility : In a terrestrial system, communication is tied down to the links installed. On the other hand, satellite communication is well suited for changing traffic requirements, locations and channel capacities.

(vi) Speed of installation : Installation of earth terminals can be achieved in a short time as compared to laying of cables or radio relay links.

(vii) Mobile, shortterm or emergency communications : With ariliftable or road transportable terminals, shortterm or emergency communications can be quickly provided. Reliable long distance land mobile, maritime mobile and aeronautical mobile services are feasible only by means of satellite.

(viii) Satellite communication is ideally suited for point to multipoint transmission on broadcasting over large areas. Application of satellites for TV broadcasting, audio and video distribution and teleconferencing, facsimile, data and news dissemination is, therefore, increasing rapidly.

(ix) All types of common services are possible.

Satellite Communication Network

Satellite Communication Network could be defined as an ensemble of earth stations of predetermined size spread over a predefined coverage area, interconnected through a suitably designed satellite, placed at a predetermined location in properly chosen orbit around the earth. Thus, two important elements of a satellite communication network are :

(i) Space Segment

(ii) Ground Segment

Uplinks and Down Link

Uplink is the radio path from Ground segment, i.e. earth station to the Space segment, i.e. satellite, whereas Downlink is the radio path from space segment, i.e. satellite to the ground segment, i.e. earth station.

Frequency Bands

Choice of Frequency band for space communication depends upon

Bandwidth required.

Noise consideration

Propagation factors

Technological developments with regard to component and device.

As the signal levels from the satellite are expected to be very low, any natural phenomenon to aid the reception of the incoming signals must be exploited. Note in Figure 3 that between the frequencies of 2 GHz to 10 GHz, the level of the skynoise reduces and this band of frequencies is known as the 'microwave window'.

The most of the communication satellites as on today are using a frequency of 6 GHz for "Up link" and 4 GHz for "Down link" transmission.

These frequencies are preferred because of

Less atmospheric absorption than higher frequency.

Less noise both galactic and manmade.

Less space loss compared to higher frequency.

A well developed technology available at these frequencies.

6 GHz/4 GHz bands are shared with terrestrial services, creating interference problem.

As equatorial orbit is filling with geostationary satellites, RF interference is increasing from one satellite system to another is increasing.

14/11 and 30/20 GHz systems for telecommunication and broadcasting satellite services are slowly coming being.

Frequency bands in use for satellite communication are :

"L" BAND18302700 MHz

"S" BAND25002700 MHzINSAT IS USING

"C" BAND59256425 MHz UP

37004200 MHz DOWNINSAT IS USING

"X" BAND79008400 UP

72507750 DOWN

"KU" BAND14.00014.500 Hz. UP

1095011200 GHz/DN.

1145011700 GHz/DN.

"K" BAND27.530 GHz UP

17.721.2 GHz DOWN

EXTENDED C BAND67257025 UP

45004800 DOWNINSAT IS USING

V BAND4051 GHz UP

4041 GHz DOWN

V Band Intersatellite5964 GHz

5458 GHz

Time Delay

The total earthsatelliteearth path length may be as much as 74,000 km thus giving a oneway propagation delay of 250 ms. The effect of this delay on telephone conversations, where a 500 ms gap can arise between one person asking a question and hearing the other person reply, has been widely investigated, and was found to be less of a problem than had been anticipated. With geostationary satellites, twohop operation sometimes unavoidable and gives rise to a delay of over one second.

Fig. 3

Geographical Advantage

A station which is located closer to the subsatellite point, as demonstrated in Fig.4 will have an advantage in received signal level with respect to one at the edge of the service area of the satellite. For a global coverage satellite, this can be as much as 4.3 dB.

Fig. 4

Example of Geographical Advantage

Communication Systems

Satellite communication systems classify that :

(a) Communication system

(1)Multiplexed telephone channels with one

(i) carrier frequency system, and

(iii) Single channel per carrier system.

(b) Modulation system

(1) Analog modulation (Frequency Modulation system), and

(2) Digital modulation system

(c) System configuration

(1) Preassignment system

(2) Demand assignment system, and

(3) Various other systems combined with those above.

Kinds and Systems of Communication Satellite

(1) Kinds of Communication Satellites depends on type of orbit and freq. band used.

During the early experimental stage of communication satellites, a passive satellite was used without any amplifiers and it only reflected radio waves sent from the earth station. But, later on active satellite with amplifiers was developed and put into practical use. Communication Satellite can be classified by the orbit used and also by frequency band used. Before discussing satellite orbits in a more generalized manner, however, it is necessary to be aware of the natural laws that control the movement of satellites. These are based on Kepler's laws and basically stated are :

(i) The orbit plane of any earth satellite must bisect the Earth centrally.

(ii) The Earth must be at the centre of any orbit.

The choice of orbit is restricted to three basic types, namely : polar, equatorial and inclined as illustrated in Fig.5. The actual shape of the orbit is limited to circular and elliptical. Any combination of type and shape is possible but observations are made only of the circular polar, elliptically inclined and the circular equatorial.

Fig. 5

Three Basic Orbits

Circular polar orbit

This is the only orbit that can provide full global coverage by one satellite, but requires a number of orbits to do so. In a communications sense where instantaneous transfer of information is required, full global coverage could be achieved with a series of satellites, where each satellite is separated in time and angle of its orbit. However, this produces economic, technical and operational disadvantages and is thus not used for telecommunications though it is favoured for some navigation, meteorological and land resource satellite system.

Elliptically inclined orbit

An orbit of this type has unique properties that have been successfully used for some communications satellite system, notably the Russian domestic system. For this system, the elliptical orbit has an angle of inclination of 63 degrees and a 12hour orbit period. By design, the satellite is made to be visible for eight of its 12hour orbit period to minimize the handover problem while providing substantial coverage of the temperate and polar regions. By using three satellites, suitably phased, continuous coverage of particular temperate region can be provided that would not be covered by other orbits.

The elliptically inclined orbit is used exclusively by the Russians for their Orbital and Molniya systems, but since coverage is limited to particular areas (higher latitutdes), it is, therefore, not suitable for a global network.

Circular Equatorial Orbit

Circular orbits in the equatorial plane permit fewer satellites and ground stations to be used, and satellites with long orbital periods (at high altitudes) have greater mutual visibility. A satellite in a circular orbit at 35,800 km has a period of 24 hours and consequently appears stationary over a fixed point on the earth's surface. The satellite is visible from one third of the earth's surface, up to the Arctic circle, and this orbit is almost universally preferred for satellite communications system.

Stabilization of the satellite is necessary since the earth is not truly spherical, and the moon, sun and the earth's tidal motion have gravitational effects on the satellite, tending to make it drift from its correct position. Inclination to the equatorial plane produces a sinusoidal variation in longitude, seen from earth as motion around an ellipse once every 24 hours, with peak deviation equal to the inclination angle. Incorrect velocity results incorrect altitude, and a drift to the east or to the west. When a non reusable launcher is utilized, injection of the satellite into geostationary orbit requires two rocket burns : the first to get the vehicle into a parking orbit, and the second via an elliptical transfer orbit to geostationary altitude. The spacecraft's own apogee motor then increases its velocity to about 10,000 fps to maintain the geostationary orbit. When launched from the Space Transportation System (Shuttle), a booster rocket is attached to the satellite to boost it to the geostationary orbit.

The satellite must then be correctly positioned, and held in position for its required lifetime (typically 7 to 10 years). This is done by using hydrazine (liquid nitrogen plus ammonia) and cold gas jets. About 40 lbs. of hydrazine are required for corrections to maintain geostationary position within q 0.1x for five years, but since hydrazine is also used for initial positioning, the quantity available depends on the accuracy of the launch. To extend the life of the satellites, less frequent corrections may be made allowing the satellite to drift.

F = Noise Figure of Receiver. The antenna noise is expressed in degrees kelvin and is called noise temperature of antenna. It can be converted to familiar units of power, watts by multiplying it with Boltzmann's constant K = 1.38 x 1029 joule/kelvin and the bandwidth. Noise temperature of an antenna is of the order of 2050oK.

Geostationary Satellite

This satellite revolves above the equator round the earth at a height of 35,790 km. Its period of revolving round the earth is same as that of the earth rotation on its own axis. Therefore, it looks as if it is stationary. This system was contributed to the "WIRELESS WORLD" by Mr. A.C.Clark, Dr. Rosen (an American) and others. It launched a Syncom communication satellite in 1963. Syncom No. 1 failed to launch in February, 1963. But, Syncon No. 2 finally succeeded in July 1963. This satellite centered the equator and moved like a figure eight (8). This was not a complete geostationary satellite, but it came into practical use (24 hours) as synchronous satellite. This satellite is advantageous because :

(1) Its large antenna at an earth station is easy to track.

(2) Twentyfour (24) hours communication can be made with even only one satellite.

(3) The satellite looks at the earth as if it were stationary, and it radiates highly effective wave power.

(4) Visibility from one (1) satellite is very wide, and global communication can be made using only three (3) satellites.

Its drawback, however, is its delay caused in long distance transmission. But, the system is economical and accordingly, it is widely used for both international and regional domestic communications.

Figure of Merit (G/T)

The earth station are classified on the basis of figure of merit of earth station which is defined by the parameter 'G' by 'T' (G/T). G is the receive gain of the earth station antenna and T is the equivalent noise at LNA input. This noise includes :

(i) Antenna noise

(ii) Equivalent noise of receive chain (LNA, down converter) referred to LNA input.

The total noise is expressed in terms of noise temperature (Kelvin). Thus, G/T of an earth station in dB/K is given by.

G/T = GR 10 log T in dB

where G is the receive gain of earth station antenna and T is noise temperature of the receive chain. A high G/T implies that an earth station can receive very weak signal because antenna gain is high and noise is low. Note that an LNA is specified by its noise temperature, i.e. by noise its generates.

Noise Temperature

The amount of receiver noise present is defined as receiver noise temperature To eq. The parameter To eq is an effective equivalent temperature that an external noise source would have to produce the same amount of receiver noise. The equivalent temperature is written as

To eq = Tb0 + (F1) 290owhere, Tb = back ground noise temperature accounting for contribution collected by antenna.

MULTIPLE ACCESS TECHNIQUES

1.Introduction

Each transponder of a communication satellite with its bandwidth of 36 MHz can, in principle, be used as a repeater for an RF channel between two earth stations. But considering that

the number of earth stations to be interconnected is such larger than the number of transponders;

the capacity of a transponder is much more than what is required between a pair of earth stations;

the capacity of a transponder needs to be shared by several earth stations. It is a problem of multiplexing, to which conventional frequency division and time division methods or more specialised solutions can be applied. The techniques employed for sharing a common transponder by multiple stations are called multiple access techniques.

Multiple access can be feasible only if the input and output of the repeater are accessible to all the stations. A communication satellite meets this requirement because it operates in broadcast mode, i.e. output of a transponder is received by all the stations irrespective of their geographical locations (Fig.1). The conventional microwave system do not have this property.

Fig. 1

2. Types of Multiple Access

The transponder bandwidth may be shared by multiple earth stations using one of the following techniques :

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

In this handout, we shall describe FDMA and TDMA multiple access techniques and cover only the principle of CDMA.

3. Frequency Division Multiple Access (FDMA)

FDMA is the most frequently used technique because it is simplest to implement. It involves :

dividing the transponder bandwidth into smaller frequency bands and assigning them to different earth stations (Fig.2);

transmitting the carriers towards the satellite within the assigned frequency bands;

retransmitting the carriers from the satellite after frequency translation and amplification by a common transponder;

receiving in each earth station all the carriers which contain telephony channels meant for the earth station.

Fig. 2

3.1 FM/FDM/FDMA

Usually a carrier transmitted from an earth station is frequency modulated with a FDM band of telephony channels. The FDM band may contain channels for multiple destinations. At each destination earth station, the received carrier is demodulated and from the FDM band the channels meant for the station are stripped (Fig.3). This scheme is called FDM/FM/FDMA in short.

Fig. 3

Note that a station must receive the carriers of all the stations it has links with; and to receive the carriers it must have separate receive chain for each carrier.

3.2 Standard Carrier Sizes

The bandwidth and the intensity (transmitted power) of the carrier transmitted by an earth station depends on the size of modulating FDM band. Table 1 shows the standard RF carriers and corresponding number of accesses per transponder of 36 MHz bandwidth.

Table 1

RF Bandwidth

per Carrier (MHz)Voice CHLS.

per CarrierAccesses per

TransponderVoice CHLS.

per Transponder

2.52414336

5607420

101323.5*456

369721972

*3 192CHANNEL CARRIERS and

1 60CHANNEL CARRIER

In the above table, capacity of transponder has been divided for carriers of same size, but it is not essential. Usually there is a mix of carriers of various sizes in a transponder.

Capacity Reduction in FDMA

A very important point to be noted in Table 1 is that as the number of accesses to a transponder is increased, the total channel capacity actually realized from the transponder is reduced. This is primarily due to nonlinear characteristics of the transponder. When amplifying several carriers simultaneously, the transponder generates intermodulation noise. The noise can be reduced if the transponder is operated below its rated output. But each carrier is to be transmitted by the satellite at a minimum specified level so that it may be received properly by the earth station. Therefore, to reduce the net output of a transponder, it is necessary to reduce the number of carriers. Operating a transponder at output less than its rated power is called "back off". 3 dB back off means the transponder is used at half of its rated power capacity.

3.4Advantages of FDMA

It utilizes proven technology.

If the existing terrestrial trunks are analog and are already multiplexed on FDM basis, FDMA is the most suitable.

3.5 Disadvantages of FDMA

The full capacity of a transponder is not effectively utilised due to back off. Utilisation efficiency is reduced when number of the carriers is increased.

Each earth station needs to be equipped with as many receive chains as the number of carriers.

It is difficult to reconfigure the frequency and traffic plans. For example, even to change frequency of a carrier, the receive chains at all the earth stations need to be modified.

4. Single Channel Per Carrier (SCPC)

Remote earth stations require very few channels, even less than five. The traffic growth is also in increments of one channel. For such situations, Single Channel Per Carrier (SCPC) system is u sed. It is an extreme case of FDMA technique. Here, each carrier is modulated with a single channel. Thus, a station be required to transmit as many carriers as the number of channels it requires. The basic scheme is as under (Fig.4) :

Fig. 4

The carriers are spaced at 45 KHz (22.5 KHz spacing is also used).

In a 35 MHz transponder, there can be about 800 carriers. The number of carriers is limited by the transponder power rather than its bandwidth.

The carriers are frequency modulated by a single channel.

To make efficient use of transponder power, VOI mode of operation is used. In VOI mode, the carrier is transmitted only when speech is present. As we know, on a voice channel, speech is present on average for about 40% of the time. Therefore, each carrier is also switched on only for about 40% of the time.

Since the carriers are placed closely, it becomes necessary to introduce a pilot carrier for Automatic Frequency Control (AFC). The frequency drift in the local oscillator of the satellite transponder are taken care of by the AFC unit.

Fig. 5

Mixing of the carriers is avoided by time sharing the use of the transponder. Each station transmits a short "burst" of a modulated carrier (Fig.6)

Fig. 6

The digital signal carried by each burst has a preamble which identifies the origin of the burst.

The bursts are transmitted by the earth stations in a sequence. The bursts from different stations are so synchronized that they arrive at the satellite in separate nonoverlapping time slots (Fig.7).

Fig. 7

One of the earth stations called master station is assigned the job of maintaining the synchronization. It does so by transmitting reference bursts (Fig.8). Each earth station detects the reference burst, and adjust its clock so that it transmits the burst at the assigned instant of time.

Fig. 8

Guard time is provided between adjacent bursts to account for the timing errors.

The duration of the time slot assigned to a station determines its number of channels. If a station requires more channels, it is assigned a longer time slot.

Format of the digital signal which modulates the carrier to generate a burst is shown in Fig.9. The burst begins with unmodulated carrier so that the demodulator of the receive chain may lock its local oscillator frequency. The preamble starts with clock bits which enable synchronization of the local clock of the receive chain. Unique word identifies the beginning of the digital signal. The address of the burst originating station, order wire and digital signals for the various destination stations then follow.

Fig. 9

5.1 TDMA Configuration

Fig.10 shows the schematic of the transmit chain at an earth station. TDMA buffers are required to temporarily store the information bits to assemble the burst. From each of the input bit stream fixed number of bits are taken and time multiplexed. Then the possible containing the address of burst originating station is. added The digital burst so formed modulates a carrier to generate the carrier burst.

Fig. 10

Configuration at Earth Station B

The received signal from the satellite consists of series of bursts from all transmitting stations. All the bursts are sequentially demodulated as they arrive by a single demodulator. From each demodulated burst, the bits meant for the station (e.g. station B as shown in Fig.11) are separated out.

Fig. 11

Configuration at Earth Station B

5.2 Advantages of TDMA

Since there is only one carrier, the problem of intermodulation noise is avoided. The transponder can work at its full capacity without any backoff.

There are no frequency guard bands. So the bandwidth of the transponder is fully utilised.

There is a single receive chain with common demodulator for all the bursts. Reconfiguration of the network can be easily done just by changing the time allocations.

As the number of accesses is increased, there are more bursts but the transponder utilisation does not deteriorate as in FDMA.

It is equally suitable for digital voice (PCM) and data transmission.

5.3 Disadvantages of TDMA

The TDMA technology is costlier than FDMA.

If the existing trunks are multiplexed on FDM basis, additional equipment for analog to digital conversion is required.

6.0 Code Division Multiple Access (CDMA)

In CDMA, the transponder bandwidth is used by all the stations simultaneously and using the same carrier frequency. Before transmission, each carrier is so "coded" that it has no correlation with other carriers. If uncorrelated signals are mixed they can always be separated if their codes are known. A PRBS (Pseudo Random Binary Sequence) is used to "code" the carrier. A different PRBS is used for each carrier so that after coding the carriers are uncorrelated. When such carriers are transmitted by the earth stations to a common transponder, the signal received from the satellite by each earth station consists of a frequency band of mixed but uncorrelated carriers. Using the same PRBS, the carriers are decoded and separated.

SATELLITE LINK CALCULATIONS

1.0Introduction

1.1The objective of Satellite Link Design calculations is to maximise the channel capacity. Satellite communications is a very highly cost intensive proposition and so it is necessary to derive a maximum number of channels from the Satellite Link so as to minimise the cost per circuit. It is needless to emphasize that maximisation of channel capacity should be consistent with the performance quality of the circuit. In fact, the link should be designed with adequate margin such that the quality of the circuit does not degrade below a specified value even during anomalous propagation. It is also necessary to aim at utilizing the existing and proven technologies and should also aim at low cost implementation of the earth stations.

1.2The maximisation of the channel capacity requires the most efficient utilisation of the available satellite power and bandwidth. However, there are some practical constraints on the full utilisation of the available satellite power and bandwidth. Generally, the power and bandwidth available per satellite transponder will be much greater than that would be needed for meeting the traffic requirements of individual earth stations. Therefore, it would be necessary to permit more than one Earth station to access the same satellite transponder. If this access is by sharing the frequency of the transponder then this will lead to a cutback in usable bandwidth and power, thereby resulting in substantial reduction in the channel capacity.

The curve below shows a plot of the channel capacity of a satellite transponder as a function of the satellite EIRP + Earth station G/T for single access FDM/FM carrier. As the satellite EIRP + G/T is gradually increased, the number of channels increase linearly, indicating the availability of bandwidth. However, if the satellite EIRP + G/T is increased further a point is reached beyond which there is a departure from the linear relationship. At this point, the bandwidth limitation starts coming into play. If satellite EIRP + G/T is furthe rincreased, the curve goes through a peak and then flattens out, indicating that the bandwidth is fully utilised and that further increase in EIRP+G/T will not bring any further increase in channel capacity. A similar curve will be obtained if the channel capacity is plotted as a function of bandwidth.

Fig.

In this handout, general principles of link engineering are explained for single carrier, multicarrier and SCPC working.

2.0 Performance Standards

2.1Hypothetical Reference Circuit for FDM/FM Telephony CCIR Recommendation 3523

CCIR recommends that the hypothetical reference circuit for systems in Fixed Satellite Service using FDM/FM telephony should be as shown in figure below :

Fig.

(i) It should consist of one earthspaceearth link. The space portion may contain one or more satellite to satellite paths.

(ii) The circuit should include one pair of modulators and one pair of demodulators for transmision from top baseband to RF and from RF to baseband.

(iii) The multiplex telephony equipment and tail link between the earth station and the associated switching centre should not be included in the reference circuit.

2.2 Maximum Allowable Noise Power in the HRC for FDM/FM Telephony CCIR Rec. 3533.

CCIR recommends that the noise power at a point of zero relative level in any telephony channel in the HRC defined above should not exceed the provisional values given below :

(i) 10,000 pWOp one minute mean power for more than 20% of any month.

(ii) 50,000 pWOp one minute mean power for more than 0.3% of any month.

(iii) 1 in 10 pW unweighted (with an integrating time of 5 milliseconds) for more than 0.01% of any year.

The noise power of 10,000 pWOp includes the following :

(i) Up and Down Link Thermal Noises.

(ii) The intermodulation noise due to nonlinearity of the satellite transponder.

(iii) Earth station equipment noise including the intermodulation noise in the earth station HPA.

(iv) The interference noise from terrestrial microwave systems.

(v) Inter satellite system interference noise.

2.3 A merit of the satellite circuit is that fading due to multipath effect is extremely small. However, the noise will increase during rainfall which increases signal attenuation and the noise temperature in the earth station receiver. Moreover, when an Earth station antenna vibrates during severe storm the receive signal power drops momentarily which also increases the noise. Among the noise performances specified by the CCIR, 50,000 pWOp and 1 in 10 pW are safeguards in the circuit design for coping up with the degradation of circuit quality by meteorological conditions.

Three different criteria for three different percentages of time will have to be satisfied. The problem of designing the satellite link is how to choose the C/N properly during clear weather so as to have S/N above the three different limits for given percentages of time. At frequencies where fading is negligible, this is a straight forward process. In this case, the C/N remains constant and the deviation can be chosen so that the S/N is above the line corresponding to 10,000 pWOp.

2.4 Noise Budget

Maximum permissible noise power=10,000 pWOp

Inter satellite system interference noise power=2,000 pWOp

Interference noise power for terrestrial radio

relay systems=1,000 pWOp

The earth station equipment noise including

the multicarrier intermodulation noise in the

earth station high amplifier=1,500 pWOp

Therefore, the noise budget for contribution from

Up link and Down link thermal noise and the

intermodulation noise of the satellite transponder=10,000 4500

5,500 pWOp

2.5 Threshold Margin

FM systems are characterised by the threshold effect. The C/N at the input of the demodulator has a threshold level beyond which the wideband gain of the system is lost and the S/N deteriorates sharply. At the threshold which occurs at a C/N of about 10 dB or so the S/N suddenly jumps to very low value with even a small reduction in the C/N.

The satellite systems are very similar to the terrestrial radio relay systems in many respects but are quite different in respect of their margin above threshold. In terrestrial systems the RF C/N in the radio bandwidth is 30 to 60 dB above the threshold of the conventional demodulator. In terrestrial systems, this high threshold margin is necessary to ensure adequate system performance when the carrier fades are caused by atmospheric disturbances especially the multipath effects in a multihop cross country system. An important merit of the satellite system when compared to the terrestrial microwave systems, is that the fading due to multipath effect is negligible. The fading in satellite link is mainly due to rain attenuation and also due to momentary signal fades caused by the vibration of the earth station antenna due to severe windstorm conditions. It has been found that fading under the most adverse climatic conditions is only of the order of 4 to 6 dB. Therefore, the earlier satellite systems used an operating margin of 6 dB above threshold in the down link. However, one cannot make a general statement that 6 dB margin is appropriate and necessary for all earth stations. To meet the noise performance specified by the CCIR. In the case of some earth stations a much smaller margin would do. Therefore, the margin above threshold to be adopted for any particular case is very much the prerogative of the earth station system engineer. The margin chosen should be adequate so that the C/N would remain above threshold even under the most adverse weather conditions. For example, in INTELSAT IV Link Design, a minimum C/N of 12.7 dB has been used which gives margin of 2.7 dB above threshold.

2.6 Threshold Extension

The threshold of a conventional FM demodulator can be extended by a threshold extension demodulator so that the threshold occurs at about 4 to 5 dB lower than in a conventional demodulator. Thus, for chosen operating margin, it is possible to fix the operating C/N at a corresponding lower value. The use of threshold extension demodulator in FM system does not modify the basic relation between C/N and S/N, but it merely permit the use of lower C/N ratios than would otherwise be possible. It is mandatory requirement of the INTELSAT that threshold extension demodulators should be used for all baseband configurations up to and including 252 channels. T.E.D.s could also be used for baseband configurations higher than 252 channels if found to be advantageous.

3.0 Link Calculations

The figure below shows the satellite link with major sources of noise, i.e., Uplink thermal noise, Downlink thermal noise and Intermodualtion noise generated by the spacecraft at the output of the TWT. The uplink is defined as the link from the ground transmitter to the spacecraft receiver input. The downlink is the link from the TWT output to the ground station receiver input.

Fig.

Pt=Transmitter power in the carrier of interest.

CUP=Carrier power at the spacecraft receiver input.

CO=Carrier power at the spacecraft TWT output.

CDN=Carrier power at the ground receiver input.

GSR=Gain of spacecraft antenna in receive direction.

GST=Gain of spacecraft antenna in trans direction.

GER=Gain of earth station antenna in receive direction.

GET=Gain of earth station antenna in trans direction.

PL(DN)=Path loss in the down direction.

PL(UP)=Path loss in the up direction.

B=Noise bandwidth of the carrier.

TSC=Equivalent noise temperature of the spacecraft.

TES=Equivalent noise temperature of the earth station.

CUP=PT . GET . PL(UP) . GSR

[1]

CDN=CO . GST . PL(DN) . GER

[2a]

=CUP . GSC . GST . PL(DN) . GER

[2b]

NUP=K . TSC . B

[3]

NDN=K . TES . B

[4]

The uplink noise may be referred to the ground receiver input by using the following relation :

NUP,G=NUP . GSC . GST . PL(DN) . GER

[5]

Similarly, for the IM noise

NIM,G=NIM . GST . PL(DN) . GER

[6]

The overall C/N is computed at the ground receiver input as

From [2a], [2b], [5] and [6]

Therefore,

3.1FM EQUATION

The basic FM equation states that

where,

[S/N]=The ratio of test tone signal power to unweighted thermal noise power in the top channel of the baseband referred to zero transmission level point.

[C/N]=The ratio of carrier power to noise power in the radio bandwidth.

fd=Peak deviation due to 0 dBm test tone expressed in Hz.

f2=The upper frequency bound of the pass band of the highest telephone channel in the baseband (Hz).

f1=The lower frequency bound of the pass band of the highest telephone channel in the baseband (Hz).

B=Carson's radio bandwidth (Hz).

=2 [F + fm], where F is the peak deviation of the FM transmission due to multichannel composite signal.

b =the effective telephone channel bandwidth = (f2f1)

When the value of fm is more than about four times the value of 'b' the above equation can be replaced with negligible error by following relationship.

Taking into account the S/N improvement due to preemphasis and the psophometrically weighted thermal noise power, the S/N equation can be written as

where,

S/N=ratio of the test tone signal power to the psophometrically weighted thermal noise power in the top channel of the telephony baseband referred to the zero transmission level point.

W=Psophometric weighting gain, i.e. 2.5 dB (1.8) for an effective telephone channel bandwidth of 3100 Hz.

fd=Peak deviation = 2 f, where f is the RMS deviation due to zero dBm0 test tone. The equation can now be written as

In order to obtain the RF bandwidth, it is necessary to find a relationship between the RMS test tone deviation 'f' and the peak multichannel deviation of the FM transmitter 'F' due to the peak amplitude of the multichannel composites signal. A relation in general use for FDM/FM systems is f x g x l = F, where

g=multichannel peak factor adopted in satellite links expressed as numerical ratio = 3.16.

G=is the multichannel peak factor in dB (20 log g).

l=multichannel load factor, i.e., the mean power of the multichannel composite signal expressed as a numerical ratio.

l = antilog [(15+10 logN)/20]for N > 240

l = antilog [(1+4 logN)/20]

for N < 240

S/N equation can now be written as

Expressing both sides of the equation in dB

[S/N]dB= [C/N]dB+10 log B10 log b+ 20log (F/fm)+10 log p+ 10 log w

20 log g 20 log l

[S/N]dB= [C/N]dB+10 log B10 log b+ 20log (F/fm)+P + W G L

20 log g 20 log l

Also,

[C/N]=C/KTB

[C/N]dB= [C/T]dB10 log k10 log B

where,

k=Boltzmann's constant = 1.38 x 1023 Joules/oK = 228.6 dBW/Hz.

The S/N equation can now be written as

[S/N]dB= [C/T]dB10 log k10 log b+ 20log (F/fm)+W + P G L (1)

Also,

[C/T]dB= [C/N]dB+10 log k+10 log B

At threshold the [C/N] is about 10 dB and if the operating margin of the system is assumed to be 'M' dB above threshold, then

[C/T]dB= 10+ M + 10 log k+10 log B

Therefore,

M= [C/T]dB 10 10 log k10 log B

(2)

From equation (1) it can be seen that for a given S/N, the higher the deviation (hence the RF bandwidth), lesser will be the carrier power required to achieve that S/N. This in fact is the trading of power for bandwidth. But, however, form equation (2) it can be seen that larger the ratio bandwidth (hence the deviation) lesser is the operating margin that is available. Therefore, a compromise is required and the deviation must be so chosen that there is adequate margin above threshold. Once the [S/N] and the operating [C/N] are given the optimum deviation can be calculated.

4.0 Satellite Link Design

In the satellite link design the earth station engineer has no control over certain vital parameters like the satellite EIRP, Pathloss, Satellite receive G/T, etc. All that he can do as far as the uplink is concerned is to determine the tradeoff between HPA output power and the earth station transmit antenna gain for obtaining the requisite uplink EIRP to meet the C/N required at the satellite receiver. As far as down link is concerned, he can determine the required C/N of the received signal at the earth station demodulator which will give the required quality.

4.1Uplink [C/N] Equation

where,

TS=satellite receive system noise temperature.

[EIRP]E = PE LF + GTEwhere,

PE=Power output of earth station HPA in dBW.

LF=Feeder loss between HPA and the antenna in dB.

GTE=Transmit gain of earth station antenna dBi.

LU=Uplink free space path loss at 6 GHz (37100 KM) = 199.4 dB

Carrier power at the input of the satellite receive antenna = [EIRP]E LU dBW.

Let the single carrier saturation flux density (the carrier power impinging on each square meter of the satellite antenna which will drive the TWT amplifier of the satellite transponder to saturation) be WS.

WS=Carrier power at the input of satellite receiver antenna + gain of one square meter of satellite antenna.

WS=[EIRP] LU + Gm2where, Gm2 =gain of one square meter of antenna (satellite).

=4A/2For=0.05 meter (6 GHz), Gm2 = 37 dBi.

Let G be gain of the satellite receive antenna. Then the carrier power at satellite receive antenna output is

C=WS Gm2 + GSwhere, GS=the gain of the satellite antenna.

[C/N]U

=WS Gm2 + [G/T]S 10 log k 10 log B

[EIRP]

=WS + LU Gm24.2 Down Link [C/N] Equation

where,

TE=earth station receive system noise temperature

[EIRP]S = PS LF + GSTwhere,

PS=Satellite TWTA output in dBW.

LF=Feeder loss between TWTA and satellite transmit antenna.

GST=Trans gain of the satellite antenna.

Therefore, the carrier power reaching the input of the earth station antenna = [EIRP]S LD,

where,

LD=Downlink free space path loss = [4 d/ ]2=0.075 meter (4 GHz), and

d=37100 kms.

LD=195.87 dB

GRE=Receive gain of the earth station (dBi).

Therefore, the carrier power at the input of the earth station receiver = [EIRP]S LD + GRE.

The noise power at the input of the receiver = KTB, where T is the earth station receiver noise temperature.

[G/T] = Earth station figure of merit in dB/oK.

Therefore, [C/N]D = [EIRP]S LP + [G/T] 10 log B 10 log k . For a single across carrier [FDM/FM]. The carrier to the total noise power ratio at the input of the earth station receiver can be expressed as :

[C/N]T1 = [C/N]U1 + [C/N]D1In a satellite transponder amplifying more than one carrier, there is a further impairment from the intermodulation noise due to the non linearity and the AM/FM conversion in the satellite transponder TWT amplifier. Denoting this as [C/N]IM and assuming that the noise contributions are additive, as is usually the case, the ratio of the carrier power to total noise power at the input of the earth station receiver for multiple access FM/FDM carriers is given by the following relation.

[C/N]T1 = [C/N]U1 + [C/N]D1 + [C/N]M15.0 Typical Link Design for Single Carrier/Transponder FDM/FM Link

It is required to transmit 972 channels (FDM/FM) from an Earth station to another Earth station each having a G/T of 31.7 dB/oK. Determine whether the Bandwidth and power available in the Satellite transponder will support these many channels. Also, determine the Earth station HPA rating.

5.1Satellite Parameters

Satellite EIRP at saturation by single carrier input

+32 dBW

Nominal input power flux density for saturation

by single carrier input

80 dBW/m2Satellite G/T

6.0 dB/oK

Transponder bandwidth

36 MHz

5.2 Earth Station Parameters (Trans and Receive)

Maximum permissible uplink EIRP+85 dBW

Receiver G/T31.7 dB/oK

Feeder Loss between HPA and antenna1.5 dB

Transmit antenna gain54.5 dBi

Receive antenna gain51.0 dBi

Uplink Free Space Path Loss

Path Length 37100 kms199.4 dB

Downlink Path Loss Path Length 37100 kms195.87 dB

Signal to Noise Ratio Objective10,000 PWOP

(50 dB)

Noise budget for uplink, downlink and satellite

intermodulation noise5,500 PWOP

S/N=90 (10 log 5500)

=90 37.4

=52.6 dB

S/N=C/N + 10 log B + 20 log (F/fm) 10 log b + W + P L G

G=Multichannel peak factor = 10 dB.

L=15 + 10 log N = 15 + 10 log 972 = 14.8766 dBm

Baseband bandwith for 972 channels = 12 4028 KHz

fm =4.028 MHz

Radio bandwidth=36 MHz

36=2 ( F + 4.028)

F=13.972 MHz

20 log (F/fm) = 20 log (13.972/4.028) = 10.80 dB

10 log B = 10 log 36 x 106 = 75.56 dB

10 log b = 10 log 3.1 x 103 = 34.9 dB.

Therefore,

52.6=C/N + 75.56 + 10.80 + 6.5 34.9 10 14.8766

52.6=C/N + 92.86 59.77

C/N=19.51 dB

This indicates that a C/N of 19.51 dB will be REQUIRED at the input of the Earth Station receiver to support 972 channels.

C/N required = 19.51 dB.

5.3 Up Link C/N

[C/N]U=WS Gm2 + [G/T]S 10 log k 10 log B

[C/N]U=80 37 + [6] [228.6] 75.56

=80 37 6 + 228.6 75.56 = 30.04 dB

Earth station EIRP required = [EIRP] = 80 + 199.4 37 = 82.40 dBW

5.4 Down link C/N

[C/N]D = Satellite EIRP Downlink path loss + [G/T] 10 log k 10 log B

=32 195.87 + 31.7 + 228.6 75.56

[C/N]D=20.87 dB

[C/N]A1 =[C/N]U1 + [C/N]D1[C/N]A1=[30.04]1 + [20.87]1

=20.37 dB

Since the available C/N is greater than the required C/N, it is possible to support 972 channels with a S/N of 52.6 dB. Also, there is a margin of 0.85 dB for propagation anmalies etc.

5.5Multichannel peak deviation= 13.972 MHz

Multichannel RMS deviation = 13.972/g = 13.972/3.16 = 4.4215 MHz.

Test tone RMS deviation= (4.4215/l) = 4.4215/5.5441 MHz

= 796 KHz.

Test tone peak deviation= 796 x 1.414 = 1127 KHz.

5.5 Transmitter power required :

EIRP=PE + GTR LF82.4=PE + 54.5 1.4

PE=29.4 dBW = 870.96 Watts

Say a 1 KW transmitter will do.

6.0 Link Design for Multicarrier per Transponder Operation FDM/FM/FDMA

FDMA is the most commonly used multipleaccess method. In this technique several carriers separated in frequency share a common satellite transponder. In FDMA, where a number of carriers are simultaneously amplified by a common nonlinear TWT amplifier, there is severe impairment due to the intermodualtion noise. In order to keep the intermodulation noise to an acceptable level, the TWT amplifier of the satellite transponder is required to be operated at a power which is much lower than its saturation output power. This cutback in output power of a TWT amplifier amplifying multicarriers is called the OUTPUT BACK OFF. The output backoff is defined as the reduction or cutback in the output power at the operating point relative to the single carrier saturation power.

In addition to the reduction of the available power, there is also a reduction in the usable bandwidth in a transponder operating in the multicarrier mode. This is because a certain amount of guard band is required between the adjacent carriers in a transponder. The present systems are designed to keep the occupied bandwidth of each carrier to the middle 90% of the allotted bandwidth and to provide bandlimiting filters at each transmitting earth station. Generally, in a Satellite transponder amplifying more than one carrier, 10% of the bandwidth is kept apart as GUARD BAND.

In a 36 MHz bandwidth transponder, the available bandwidth is, thus, reduced from 36 MHz to 32.4 MHz. The Uplink and Downlink C/N equation for multiple access can be written as

[C/N]U = [WS BO1] Gm2 + [G/T]S 10 log k 10 log B

[C/N]D = [EIRP]S BOO LD + [G/T]E 10 log k 10 log B

Earth station EIRP = [WSBO1] Gm2 + LUwhere,

BOI=Input backoff

BOO=Output backoff

6.1An Example

Satellite Parameters

Satellite EIRP at saturation by single carrier input+32 dBW

Nominal input flux density for saturation by single carrier80 dBW/m2Satellite G/T6.0 dB/oK

Transponder Bandwidth36 MHz

Usable Bandwidth36 x 0.9 = 32.4 MHz

Output Backoff for multicarrier operation4.5 dB

Input Backoff10.0 dB

Satellite Carrier to Intermodulation Noise Ratio for

4.5 dB Output backoff= 18.0 dB

Earth Station Parameters (Trans & Receive)

Receive G/T= 31.7 dB/oK

Transmit antenna gain= 54.5 dBi

Receive antenna gain= 51.0 dBi

Free space uplink pathloss= 199.4 dB

Free space downlink pathloss= 195.87 dB

Feeder loss between HPA and Antenna= 1.5 dB

6.1Single carrier saturation flux density= 80 dBW/m2

Input backoff for multicarrier operation= 10 dB

Operation flux density for multicarriers= 90 dBW/m26.2 Calculation Available [C/N]AUplink C/N

[C/N]U=[WS BOI] Gm2 + [G/T]S 10 log k 10 log BU

[BU = 32.4 MHz]

= 90 37 6 + 228.6 75.10

=20.5 dB

Downlink C/N

[C/N]D=[(EIRP)S BOO] LD + [G/T]E 10 log k 10 log BU

=[32 4.5] 195.87 + 31.7 + 228.6 + 75.10

=16.83 dB

[C/N]IM

=18 dB

[C/N]A1=[C/N]U1 + [C/N]D1+ [C/N]IM1

=[20.5]U1 + [16.83]D1 + [18]M1

=13.4188 dB

On account of IM impairment and output backoff, available C/N has come down from 20.87 dB in single access to 13.42 dB in multiple access.

6.1Calculation of CHANNEL CAPACITY

S/N=C/N + 10 log BU 10 log b + 20 log (F/fm) + P + W

52.6=13.42 + 75.10 34.9 + 20 log (F/fm) + 6.5

20 log (F/fm) = 7.52

(F/fm) = 0.42

F=f x l x g = 3.16 x f x l

B=2 [F + fm]

32.4=2 [3.16 x l x f + fm]

16.2=3.16 x l x 0.42 fm + fm

=1.3272 fm l + fmBy substituting different values for 'N', the value of fm which satisfy the above equation can be determined. The value of 'N' which closely satisfies this equation is 564.

L=15 + 10 log N = 12.51 dB

l=antilog(L/20) = 4.2219

fm=4.2N = 2.3688 MHz

B=2 x [(1.3272 x 2.3688 x 4.2219) + 2.3688]

i.e. B =31.292 MHz.

F=15.646 2.3188 = 13.3272 MHz.

We know that,

[20 log f = 20 log F 20 log l 20 log (peak factor)]

Therefore,

S/N = 13.42 + 10 log 31.29 34.9 + 20 log(13.3272/2.3688) + 6.51012.51

= 13.42 + 74.95 34.9 + 15.004 + 6.5 10 12.51

S/N = 52.464 dB

The channel capacity has come down from 972 to 564 (i.e. by 408) on account of the reduction of satellite power and usable bandwidth due to multicarrier operations.

6.4Calculations of [C/N] for Four Carrier Working

Let us assume that 4 stations want to access this transponder and share the transponder power and bandwidth equally,

Number of channels per carrier = 564/4 = 141 say 132.

Occupied bandwidth of the transponder per carrier = 34.4/4 = 8.10 MHz.

Baseband width for 132 channels = 12552 KHz.

Top channel frequency fm = 552 KHz

Operating flux density = 90 dBW/m2So the operating flux density for each carrier for power sharing equally is

=90 dBW/m2 10 log 4

=96.02 dBW/m2[C/N]U

=96.02 37 6 + 228.6 10 log (8.10 x 106)

= 20.5 dB

[C/N]D

=[27.5 10 log 4] 195.87 + 31.7 + 228.6 69.08

=16.83 dB

[C/N]IM

=18 dB

[C/N]A1=[20.5]1 + [16.83]1 + [18]1[C/N]A

=13.45 dB

L

=1 + 4 log N = 1 + 4 log 132 = 1 + 8.482 = 7.482

G

=10 dB

Occupied bandwidth=8.1 MHz

Therefore,

8.1

=2 [F + 0.552]

F

=4.05 0.552 = 3.498 MHz

20 log (F/fm)=16.038

For the S/N objective of 52.6 dB, the required C/N may be calculated by using the following relation

526.

=C/N + 10 log 8.10 34.9 + 16.037 + 6.5 10 7.482

[C/N]required=13.365 dB

This confirms that the available C/N is greater than the required C/N and so the operation is possible.

7.0 Design of Single Channel per Carrier FM/FDM/FDMA Links

SCPC (Single Channel Per Carrier) as its name implies, produces a discrete RF carrier for each of the voice channels in use. It does not mean that the voice channel is directly modulating an RF carrier. It is more convenient to realize this system by having each voice channel modulate an IF carrier. Several such modulated IF carriers in the frequency band 70 MHz + 18 MHz band are combined and then upconverted from IF to RF by heterodyning. This is in direct contrast to what happened in a FM/FDM situation where a number of telephone channels are frequency division multiplexed to obtain a baseband which extends from 12 KHz upwards and this baseband, frequency modulates a single IF carrier. But in single channel per carrier case, the baseband is a single voice channel having an effective bandwidth of 3.1 KHz. The advantage that could be derived from the use of single channel per carrier approach is that it enables the introduction of a feature known as the VOICE ACTIVATION which leads to substantial saving in satellite power. Since there is a discrete RF carrier for each of the voice carrier, the SCPC afford the facility for SWITCHING OFF the carrier during instants when speech is actually not present such as pauses, listening periods, etc. A further advantage of the SCPC systems using voice activation is that there is a reduction in the IM noise. This is because of the fact that the voice activation of the carriers is a random phenomenon, and, therefore, instantaneous distribution of IM products across the baseband is also random. This results in an IM noise reduction of about 3 dB.

7.1Single Channel per Carrier Calculations

where

[S/N]I=Ratio of the signal power to unweighted noise power in the voice channel without VF signal processing.

[C/N]=Ratio of the carrier power in the RF bandwidth 'B' Hz to the noise power in the same bandwidth.

fd=peak voice channel deviation for 0 dBm0 test tone expressed in Hz.

f2=upper frequency of the voice channel bandwidth (3400 Hz).

f1=lower frequency of the voice channel bandwidth (300 Hz).

With f2 = 3400 Hz and f1 = 300 Hz, the S/N is given by

[S/N]I=[C/N] 104.2 + 20 log fd + 10 log B

Considering 6.3 dB improvement due to preemphasis and 2.5 dB psophometric weighting advantage, the uncompanded S/N is given by

[S/N]A=[C/N] 95.4 + 20 log fd + 10 log B

If the 17 dB companding advantage is included, the companded

[S/N]C=[C/N] 78.4 + 20 log fd + 10 log B

Uplink C/N

[C/N]U=[WS BOI 10 log N] Gm2 10 log k 10 log B + [G/T]Swhere, WS=Single carrier input saturation flux density,

=Voice activity ratio (0.4)

BOI

=Input backoff

EIRP per

channel=[WS BOI 10 log N] + LU Gm2[C/N]D

=[(EIRP)S BOO 10 log N] LD + [G/T]E

10 log k 10 log B

[C/N]T1=[C/N]U1 + [C/N]D1 + [C/N]IM17.2 Design a FM SCPC Link for the Following Parameters

SCPC uses FM modulation. It is voice activated and uses syllablic companding and a 6 dB per octave pre emphasis.

Pre emphasis advantage=6.3 dB

Companding advantage=17 dB

Peak voice channel

0 dBm0 test tone deviation=6.505

Psophometric weighting advantage=2.5 dB

Highest voice channel frequency=3.4 KHz

Lowest voice channel frequency=0.3 KHz

RF bandwidth (noise bandwidth)=25.6 KHz

(44 dB)

Adjacent channel carrier separation=45 KHz

Satellite Parameters

Single carrier saturated power output=+32 dBW

Single carrier saturation flux density=80 dBW/m2Satellite G/T=6 dB/oK

Multi carrier input backoff=10.5 dB

Multi carrier output backoff=4.5 dB

Satellite carrier to IM ratio=18 dB

Transponder Bandwidth=36 MHz

Earth Station Receive Parameters

Receive G/T=31.7 dB/oK

Trans antenna gain=54.5 dBi

Receive antenna gain=51.0 dBi

Feeder loss between HPA and antenna=1.5 dB

Noise objective is 10,000 pWOp including interference noise (intersatellite system and LOS to satellite) and Earth station equipment noise.

7.2.1 Carrier spacing = 45 KHz

Transponder bandwidth = 36 MHz.

No. of SCPC channels on a bandwidth limited case

= (36 x 106)/(45 x 103) = 800 channels.

7.2.2 [S/N]=[C/N] 78.4 + 20 log fd + 10 log B

52.6 =[C/N] 78.4 + 20 log 6.505 x 103 + 10 log 25.7 x 10352.6=[C/N] 78.4 + 76.26 + 44

[C/N]=10.74 dB

7.2.3[C/N]U=[WS BOI 10 log N] Gm2 10 log k 10 log B + [G/T]S

=[80 10.5 10 log 320] 37 + 228.6 44 6

=[80 10.5 25.5] 37 + 228.6 44 6

[C/N]U=26.1 dB

7.2.4Uplink EIRP=[WS BOI 10 log N] + LU Gm2

per channel

=[80 10.5 25.05] + 199.4 37

=46.9 dBW

7.2.5[C/N]D =[EIRP PER CHANNEL]S LD + [G/T] 10 log k 10 log B

=[32 4.5 25.05] 195.87 + 31.7 + 228.6 44

=22.88 dB

7.2.6 EIRP per channel of Satellite = 32 4.5 25.05 = +2.45 dBW

[C/N]IM = 18 dB

[C/N]T1 = [C/N]U1 + [C/N]D1 + [C/N]IM1

=[26.1]1 + [22.86]1 + [18]1

=16.3 dB

Therefore, operating margin available for propagation anomalies etc. is 16.3 10.74 = 5.6 dB. This would mean that the operating S/N for the link would be 58.2 dB [52.6 + 5.6] for most of the time and the S/N would degrade to 52.6 dB if only the margin is lost.

7.2.7 Assume that the 800 channels are used between two Earth stations say Bombay and Delhi and that each station transmits and receives 400 channels each. The HPA rating of Earth Stations can be calculated as follows :

[EIRP per channel]

=46.9 dBW

Earth station antenna gain=54.5 dBi

Feeder loss between

antenna and HPA

=1.5 dB

Power delivered by HPA

to the antenna per channel=6.1 dBW

No. of channels transmitted=400

Transmit output power

required

=6.1 + 10 log (400/2.5)

=6.1 + 22.04

=15.94 dBW

Transmitter output backoff=10.5 dBW

Power amplifier rating

=15.94 + 10.5 = 26.44 dBW

Say, a power rating of 300 watts will do.

Fig.

GROUND SEGMENT

1. Introduction

Satellite earth stations are the transmission and reception terminals of telecommunication links through a satellite. Configuration of an earth station is different from that of a terrestrial radio system due to the following special features of satellite communications :

High free space loss which is of the order of 200 dB from earth station to the satellite.

Continuous drift in the location of the satellite.

Long propagation delay.

Broadest mode of operation.

The equipment configuration in an earth station is also dependent on the multiple access technique adopted. In this handout, we shall discuss earth station equipment for FDMFMFDMA scheme of multiple access as this is the most widely used scheme in India and abroad. Two new concepts, echo suppression and energy dispersal which are applicable to satellite communication are introduced first.

2. Echo Suppression

A satellite link is 72000 Km long from end to end. Signals travelling at the speed of 3 x 105 km/sec take about 240 msec to go from one end to the other. Due to inevitable signal leakage at 4W2W hybrid point, speech signal received at 4W REC of the hybrid is transmitted back through 4W TRANS. This signal travels back to the transmitting end and is heard as an echo after a total delay of the order of 480 msec. Since echo is undesirable, the satellite links are equipped with echo suppressors.

3. Energy Dispersal

In a satellite transponder, if the carrier levels are not within the specified limits, intermodulation noise can be generated. Level of a carrier is dependent on the depth of modulation in FM. As the baseband level is raised, the carrier corresponding number of accesses per transponder of 36 MHz bandwidth.

:

Fig. 1

4. Earth Station Subsystems

The earth station equipment consists of the following main subsystems :

Multiplexing Equipment

Transit Chain

Power Amplifiers

Antenna System

Low Noise Amplifier

Receive Chain

Basic functions of these subystems are summarised in the following sections.

5. Multiplexing Equipment

Prior to transmission, the multiplexed telephone channels received from the terrestrial link, called end link, are rearranged to form the basebands which are to modulate multidestination carriers (Fig.2). The echo suppressor bay contains individual units for each telephony channel. This bay is usually installed just after 2W4W bay at the end of the communication link.

Fig. 2

On the receive side, carriers from various earth stations are received. The basebands received after demodulation of the carriers are demultiplexed, only those channels which are meant for the earth station are picked up and then the channels are multiplexed again to form the baseband for the terrestrial system (Fig.3). Telephony channels again go to the echo suppressor bay for echo suppression before conversion to 2W at the end of the link.

Fig. 3

6. Transmit Chain

The transmit chain comprises of the following units (Fig.4) :

Baseband Processing Unit

IF Modulator and Equalizers

UP converter

The transmit chain is in 1+1 redundant configuration for each baseband signal.

Fig. 4

6.1 Baseband Processing Unit

The baseband processing unit

combines the engineering supervisory circuits,

adds the energy dispersal signal and pilot to the baseband,

carries out preemphasis and signal limiting required for frequency modulation.

6.2 IF Modulator and Equalizers

The output of baseband processing unit is applied to frequency modulator which generates IF at 70 MHz. It is followed by IF filter, fixed and adjustable group delay equalizers.

6.3 Upconverter

The upconverter translates the 70 MHz IF output to the required RF band for transmission. The frequency of the output adjustable to any frequency within the full RF band from 5.925 to 6.425 GHz.

7. Power Amplifiers

To compensate for the loss in the signal strength from an earth station to the satellite, it becomes necessary to transmit sufficiently high level of the carrier. Therefore, the output of each upconverter is amplified using a high power amplifier (HPA) having power rating from a few units to 3 KW. The power rating is determined by a number of earth station parameters including the bandwidth of the carrier being transmitted.

There is a separate HPA for each carrier (Fig.5). In an earth station HPAs are provided in n+m configuration (in active and m standby) to increase reliability. The outputs of the HPAs are combined using waveguide hybrids or filters. The combined output is sent on a waveguide to the duplexer for emission.

Fig. 5

Waveguide switches are provided at the HPA inputs to connect the upconverter output to the main or standby (S/B) HPA.

8. Antenna System

The antenna system consists of the following components :

Mechanical Subsystem

Feed

Automatic Tracking Subsystem

8.1 Mechanical Subsystem

Mechanical system comprises of main reflector, subreflector, pedestal, driving gears and servo system (Fig.6). The main and sub reflectors direct the transmitted beam towards the satellite. The reflecting surfaces also focus the received signals from the satellite at the antenna feed.

The pedestal provides the support to the antenna structure. The servo system along with driving gears and motors is used to move the antenna in the direction of the satellite.

Fig. 6

8.2 Feed

The antenna feed illustrates the sub reflector with the transmitted RF signal. It also receives the satellite signals focussed at it by the sub reflector. It comprises of a horn, polarizer and a diplexer (Fig.7). The polarizer changes the type of polarization (linear or circular) and direction of polarization (horizontal/vertical, clockwise/anticlockwise) as required in both transmit and receive directions. The diplexer combines the transmit and receive RF signal paths.

Fig. 7

8.3 Auto Tracking Subsystem

To point the antenna accurately towards the satellite, the beacon signals transmitted by the satellite are amplified by the LNA (described below) and the beacon receiver detects the level of the received signal. By observing the changes in beacon level, the auto tracking subsystem generates control signals for pointing the antenna accurately to the direction of the satellite

Fig. 8

9. Low Noise Amplifier (LNA)

To receive very weak signals from the satellite, the output of diplexer is connected to a low noise amplifier through a 6 GHz transmit reject filter. It is mounted at the output of the diplexer just behind the main antenna reflector. The low noise amplifier is in 1+1 redundant configuration (Fig.9). It has 500 MHz bandwidth and it is a common amplifier for all the carriers transmitted by the satellite.

Fig. 9

The output of the LNA is applied to a RF divider so that the received signals from the satellite may be distributed to different receive chains.

10. Receive Chain

The receive chain consists of the following units (Fig.10) :

Down Converter,

Demodulator

Base Band Processing Unit

Fig. 10

For each carrier there is a separate receive chain. The input to the receive chain is from the RF divider. The redundancy for the receive chains is n+m (m standby for n chains in service), where m must be calculated for the desired reliability.

10.1 Down Converter

Down converter changes the radiofrequency signals (e.g. a carrier in 4 GHz band) received from the RF divider into intermediate frequency which is usually 70 MHz. By proper setting of the local oscillator frequency of the down converter, the desired RF carrier can be selected.

10.2 Demodulator

The basic purpose of the demodulator is to recover the baseband signal from the frequency modulated IF received from the down converter. Since carrier to noise ratio is not so good, specially designed FM demodulator called threshold extension demodulator (TED) is used in satellite communications.

10.3 Baseband Processing Unit

Deemphasis after frequency demodulation, removal of the energy dispersal signal and separating the ESC circuits are the basic functions carried out on the demodulated baseband by baseband processing unit. The baseband is then sent to DEMUX/MUX equipment for further transmission over the end link.

11. Overall Configuration We have discussed the functions of various subsystems installed in an earth station. Fig.11 shows overall block schematic of the earth station equipment in general. Actual configuration of an earth station is determined by :

Fig. 11

Number of carriers uplinked and their capacities,

Number of carriers received and their capacities,

Types of carriers telephony, TV

Size of antenna, etc.

For example, a small earth station antenna does not require the automatic tracking subsystem. To transmit or receive a TV carrier, the transmit and receive chains are different.

Besides these subsystems, an earth station is equipped with power plant to provide uninterrupted power supply, air conditioning and other support services.

SPACE SEGMENT

1.0Introduction

1.1A Satellite system consists of a space segment and a ground segment. Space Segment comprises satellite, launch vehicle which carries the satellite into its orbit and a telemetry and telecommand ground station which monitors and controls the satellite in orbit. Communication satellites could be either passive or active. A passive satellite could be natural, e.g. the moon or artificial e.g. the ECHO satellite. The ECHO satellite was launched in 1960 and consisted of a big metalcoated baloon used as a reflector of microwave signals. However, relay via passive satellites is not technically an advantageous proposition due to very high path losses encountered and only active satellites are now in use. Satellites could be low orbiting or could be located in the geostationary orbit 36,000 kms. above the Equator. Most of the communication satellites are located in the geostationary orbit due to the advantages of large coverage, 24 hour availability and simple tracking requirements for ground stations.

1.3A satellite consists of essentially two parts :

(i) Communication payload

(ii) Support subysstems

Communication or mission payload for communication satellites comprises communication transponders and transmit/receive antennas. Major support subsystems include :

Structure

Attitude and orbit control systems

Electrical power system

Thermal control system

Telemetry and Telecommand system

Propulsion

Basic description of communication payload and support systems is given in the following section.

2.0 Communication Payload

2.1Communication Payload Antennas

Radiation pattern of an antenna is determined mainly by two considerations :

(i) to provide maximum gain over the coverage area, and

(ii) to have minimum radiation towards areas outside the coverage area for reducing interference.

Spacecarft antennas usually consist of parabolic reflectors of aluminium or fibre glass with honeycomb structure. Offset feeds are used to obtain required beam shaping for the coverage area.

2.2 Communication transponder

Communication transponder receives the signals from ground station through spacecraft antenna (Fig.1). Signal from antenna is amplified in the common receiver, down converted to transmit frequency band and then separated in number of channels through an input multiplexer. Signals in each channel are amplified in a travelling wave tube amplifier (TWTA) or a solid state amplifier (SSPA) and after amplification are combined through an output multiplexer to feed to the antenna for transmission towards the earth.

2.3 Complete redundancy is provided for subsystems whose failure could result in failure of all communication channels such as common receiver unit. For front end amplifier field effect transistors are used. Noise figure of the front amplifier determines the spacecraft noise temperature and G/T (figure of merit). Solid state devices are used for all stages except for the final output stage where travelling wave tube amplifier is used. Transistor amplifiers for output stage also have now been developed and are likely to be used increasingly in future. Local oscillator in the mixer is crystal controlled and generally operates at frequency of 2225 MHz in case of conversion of incoming 6 GHz signals to 4 GHz downlink frequency. Input and output multiplexers are made of invar or graphite epoxy. Amplitude and group delay equalisers are provided to compensate for input and output multiplexer characteristics. Step attenuators are inserted prior to output amplifier for adjusting transponder gain setttings by telecommand signals in accordance with transmission plan requirements.

3.0 Support Subsystems

3.1Structure

A spacecraft requires a mechanical subsystem consisting of a structure to physically integrate and support all equipment. Structure also includes the mechanism for deployment positioning and separation functions and cabling for power and signal distribution. A thorough structural study of mechanical subsystems is required to ensure that the spacecraft withstand stresses, shocks and vibrations during launch and in orbit operations. Primary structure of the body of the satellite is made of light aluminium alloys. Solar panels and antenna reflectors are made of composite materials (e.g. carbon fibre, epoxy resin) with low thermal expansion coefficient.

3.2 Electrical Power Subsystem

3.2.1 Primary Source Solar Cells

Primary source of electric power in the spacecraft is provided through conversion of sunlight energy into electrical energy through solar cells with photovoltatic effect. A large number of such cells are used in series, parallel matrix to obtain required voltage/current levels as indicated in Fig.5.

3.2.2 Secondary Source Batteries

For a geostationary sattellite solar power is not available all the time since satellite goes through the shadow of the earth, i.e. eclipse periodically. Eclipse would occur at midnight for locations on the subsatellite longitude. The period of the eclipse is maximum during equinoxes (72 min.) as indicated in the Fig.6.

A simplified block diagram of electrical power subsystem is given in Fig.7.

a. .

3.3 Telemetry and Command Subsystem

Telemetry : Various housekeeping data indicating the overall status of each of equipment in the spacecraft, sensor information (e.g., voltage, current, pressure and temperature etc.) and verification to confirm that commands sent to the spacecraft have been correctly stored in the decoder.

4.0 Future Trends

With congestion in the conventional 6/5 GHz band, there is a trend to use Ku (14/11 GHz) and higher frequency bands (30/18 GHz). Higher gain and higher EIRP can be obtained using several spot beams at higher frequencies. Channel capacities can be increased further using satellite switched TDMA (SSTDMA) techniques. With on board regenerative repeaters even higher capacities can be realized to meet growing demands for digital satellite communications.

Fig. 1

Communication Subsystem Simplified Block Diagram

Fig. 2

Satellite Launch in Geostationary Orbit

Fig. 3

3Axis Stabilization

Fig. 4

Altitude Control Subsystem Block Diagram

Body Stabilized Spacecraft

Fig. 5

Solar Panel Power Vs Time in Orbit

Fig. 6

Satellite Eclipse Period

Fig. 7

Electrical Power Subsystem Block Diagram

Fig. 8

Command Subsystem and Telemetry Subsystem Block Diagram

VSAT SYSTEM COMPONENTS AND SPECIFICATIONS

How the System Works ?

The C200 Series Micro Earth Station is a part of two way data communication system. The network consists of :

(a) Master Earth Station

(b) Micro Earth Station

(c) a Geosynchronous Satellite

It operates at Cband Microwave frequency which travel in straight line. To cover large areas, modern satellite communication systems use geosynchronous communication satellite as a repeater. The satellite is positioned 36,000 km above the earth equator. The satellite can 'see' a very large portion of the earth's suface and hence the signal transmitted from it reach the Micro Earth Stations located at far off places. The technology used is spread spectrum multiple access (SSMA) method by which the size of the antenna is considerably reduced. This permits C200 series to transmit and receive signals regardless of its location as long as the antenna physically points at the satellite and is not blocked by any obstructions. Fig.1 shows a typical data communication network.

The LED displays consists of seven groups of LEDs, designated as group A through G. Each group consists of four discrete displays, with the LEDs within each group numbered 1 to 4 from left to right.

Fig. 1

Status Indicators

Error Messages

The error messages are displayed with the help of first four banks (A through D) of LEDs. If an error occurs, the B and C banks will be completely off. Then the hexadecimal reading on the four LED banks gives the error code. If the error occurs once, it is displayed only once. If the error persists then the display will cycle between the regular display and the error display.

Fig. 2

Simplified Data Communication Network

Establishment of Link

Switch on the power switch at the rear end of controller and look for the following LED sequence

(1) All the LEDs will be ON for about 2 seconds.

(2) A1, E4 and F1 LEDs start blinking indicating that the Space, Network and I/O processor are functioning normally. LED G4 will glow indicating that 5 volt supply in the controller is O.K.

(3) LED A2 glows till LEDs B2, B3 and B4 come ON, after which A2 goes OFF.

B2 ON indicates the upconverter has phase locked with the crystal in the controller.

B3 ON indicates that upconverter operating temperature is within the limits.

B4 ON indicates that power is applied to the upconverter.

(4) Simultaneously, the Micro will acquire the signal during which the following sequence can be observed in "D" blank.

(a) LED D4 will glow followed by LED D3.

(b) LED D2 will glow indicating the Micro has successfully recognised its PN sequence.

(c) LED D1 will glow indicating the synchronization of Master oscillator with PN frequency.

(5) Once "D" bank is fully ON, LED A4 comes ON indicating that the receiver has locked and is receiving data.

NOTE

A4 does not glow until D3 is ON along with other LEDs in D bank.

D3 OFF indicates that frequency is being adjusted

(6) About 10 seconds after A4 glows, "C" bank LEDs will glow which indicates the SNR value which depends on the antenna peaking.

(7) LED A3 comes ON indicating synchronization to the Master Earth Station.

(8) LEDs A3 and A4 will be OFF momentarily before being restored indicating the acceptance of ONLINE command from the Master Earth Station.

(9) LEDs A2 and B1 will come ON and glow till the Micro comes ONLINE, i.e., E1 and E4 LEDs blink simultaneously.

Care and Precautions for the Usage of C200 Series

The C200 series needs very little care. The controller unit will function perfectly well in a regular office environment. If the temperature (below 40oC), humidity and dust levels are pleasant for people, then C200 will like it too.

Here are some notes on caring of your C200 series.

1. As far as possible do not switch OFF the controller. It should be left ON at all times (even overnight when not sending data) to ensure uninterrupted services.

2. The controller should have at least 6 inches (150 mm) of free space around it, particularly around the fan and ventilation holes. Do no allow books, papers, or other units to block the flow of air around the unit.

3. The controller should not be exposed to dirt or dust.

Keep it off the floor and away from the machines that produce dirt, dust or moisture. It should be preferably in A/C room with temperature and humidity properly controlled.

4. All cords and cables should be out of the way so that they will not be kicked or otherwise damaged.

5. Make sure that the antenna has an unobstructed view towards the satellite. If there are bushes, trees, or any new structures in front of the antenna, it will either degrade the received signal or lose it altogether.

The antenna should be kept reasonably clean and free of any foreign matter. Antenna must be stabilised by putting 100 Kgs weight on each ballast pad.

6. Do not switch ON the controller unless the power to it is from a stabilised power supply. The output from the stabiliser should be checked before connecting the controller. The voltage between line point and neutral should be 220+5 volts and that between neutral and earth should be less than 5 volts.

In case the controller has to be switched ON or OFF, then the following procedure should be adopted.

(a) Switch ON the controller 3 minutes after the stabiliser is switched ON.

(b) Switch OFF the controller before stabiliser is switched OFF.

(c) In case of power failure, the controller should be immediately switched OFF.

7. The controller must be switched ON everyday even if it is not being used. This will help in troubleshooting your Micro from the Master Earth Station.

8. The coaxial cable should be protected and it should be properly laid. It should enter the room through a hole in the wall and not through the doors and windows. If at any time the coaxial cable connector has to be removed at any end, then before doing this, the controller must be switched OFF. If controller is not switched OFF and cable is removed then the upconverter and low noise amplifier are likely to fail.

9. Before connecting the user end equipment to the two output ports of the controller verify the RS232 voltages and check the interconnecting RS232C cable.

Following these simple guidelines will help to ensure that the C200 series provides reliable service.

User's Log

NICNET users are requested to keep a daily log on the usage of Micro and a monthly log on parameter values and send the same to Manager Product Support, IESL. The format for keeping daily and monthly logs are given in Annexure 2A and 2B, respectively. This is essential to determine nature of faults occurring under various climatic conditions and corrective actions to be taken to make your equipment trouble free. User's of other networks should contact their Networks Manager for the above purpose.

Troubleshooting/Check List

If your Micro Earth Station is not working, then some simple troubleshooting of C200 series can be done by you. All that is required is that you observe the system and notice the state of some simple indicators. Troubleshooting can be narrowed down to three main areas, in the following sequence.

1. Is the Antenna Secure and Clear ?

Check for the following points :

(a) Is the antenna pointed in the proper direction ?

(b) Is the antenna and attached cable secure and undamaged and free of foreign matter ?

Check for cable cuts.

(c) Finally, if all is secure at the antenna, are antenna cables connected securely to the controller unit ?

2. Is the Controller Working ?

It is easy to see if the controller has power. Power is good if any of the LED indicators on the front panel is illuminated or if the fan is running. If there is no indication of power, check the power switch on the back of the unit. Is it turned ON ? Check that the unit is plugged into a good power point. Check the fuse in the power input module at the rear of the unit. It should be rated at 2.5 amp. for C201 and 4 amp. for C250 controller.

The fuse may be checked by first removing the power cord from power module on the rear of the unit. Slide the power module plastic guard to the left to provide access to the fuse. Verify the fuse with a ohm meter, or by simply replacing it with a known good one. Make sure that the external voltage regulator is ON and operating correctly.

If the power is good, then check status display indicators in the front of the controller. The status display has seven groups of four LED indicators labelled A through G. The indicators in each group are labelled 1, 2, 3 and 4.

(a) Check LED G4, if it is ON (it may be dim) then power supply is OK.

(b) Another way of checking the power supply is to see if the fant at the rear panel is rotating or not when controller is switched ON.

(c) Switch OFF the controller momentarily and switch ON. All LEDs should glow for a short time immediately after switching ON.

(d) Check the D bank after 10 minutes of switching ON the controller. All D blank LEDs should be ON.

(e) E1 and E4 should blink simultaneously. IF E1 alone is blinking, then it means that either the master earth station has not sent an ONLINE command to your Micro or there is some other problem.

(f) LED A1 and F1 should be blinking.

(g) LED A3, A4, B2, B3, B4 should be ON.

(h) Check the C bank. Some of the C bank LEDs should be ON. Keep a record of C bank LEDs. This will indicate the S/N ratio and any variations can easily be detected.

3. Is Customer Terminal Equipment Connected and Working ?

Check your terminal equipment

(a) Is power connected and turned ON ?

(b) Is it initialised properly ?

(c) Is the terminal equipment connecting cable secure at both ends ?

(d) If the controller is ONLINE, then a prompt should come up on your end equipment screen. If the prompt does not come, then it means that your RS232C connectors are not OK or parameters in the controller are not set properly by Master Earth Station.

If you have any problem with Micro, then you must contact the master earth station and report the status of Micro in the format given in AnnexureI. You are requested to maintain a daily log on usage of Micro earth station and send the same in the format given in Annexure2A and 2B to your Networks Manager or any other person nominated by the Networks Manager.

NOTE

1. Do not disconnect any cable at any time after it is installed by the engineers.

2. Voltage stabiliser with output 220 + 5 volt must be used preferably with cut off system.

CAUTION

The microwave radiation permeates up to about 3.5 inches from feedhorn. At this close range, there are possibilities of hazard to personnel, especially discomfort to the eye. Do not stand in front of the antenna during Micro Earth Station operation. Do not look directly into the feedhorn during transmitter operation.

WARNING

There are no customer serviceable parts inside the controller, LNC or Upconverter modules. Maintenance is done by replacement of modules. The customer should not try to open the modules otherwise the WARRANTY will be VOID.

In case of problems contact your Master Earth Station.

NOTE

The customer shall ensure that the lightning arrester is installed close to the Micro Earth Station Antenna. The height of the lightning Arrester should be sufficiently higher than the highest point of the vast antenna. This is to avoid any possible damage to LNC. Upconverter modules from lightning strikes.

MULTI CHANNEL PER CARRIER

VERY SMALL APERTURE TERMINAL (MCPCVSAT)

1. Introduction

Department of Telecommunications has decided to provide reliable telecommunication services to sub divisional H.Q./Tahsil/Tourist Centre/Pilgrimage places as one of the major objective of its eighth five year plan, viz. 19921997. Most of such places are planned to be provided with CDOT 126 RAX or 64 lines MLT Local Telephon