This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 10 M

Organization of earth station

Antenna subsystem:

It generally consists of a parabolic reflector with casssegrain feed system, waveguide transmission line; referred to as duplexer and related mounts.

• Antenna: The station antenna functions both in transmit and receive modes but at different frequencies. High gain, narrow beam width antennas are employed. A narrow beam is necessary to prevent interference between neighbour satellite links.
• Feed systems: The primary feed system used in existing earth stations performs a number of functions. Depending on the type of earth station, these functions may be:

1. To illuminate the main reflector
2. To separate the transmit and receive bands
3. To separate and combine polarisation in a dual polarisation in a dual polarised system.
4. To provide error signals for some types of satellite tracking systems.

The feed system typically includes polarisers and OMT which helps separate and combine polarisations and the diplexer which separates transmit and receive bands thus allowing the earth station to handle transmit and receive signals simultaneously. Other arrangements of the feed systems are possible, depending on the applications. For e.g.in $\frac{6}{4}$ GHz linear single polarised system, it is only necessary to isolate the received carriers from the transmitted carriers. This is achieved by using diplexer. Here polarisers and OMT are not required.

Transmitter operations:

Starting at the bottom of the block diagram, the first block shows the interconnections equipment required between satellite station and the terrestrial network. Let us assume telephone traffic at the input. This may consist of number of telephone channels in multiplexed format.

It may be that groupings different from those used in the terrestrial network are required for satellite transmission, and the next block shows the multiplexing equipment in which the reformatting is carried out.

Each multiplexed signal is modulated on to the carrier wave t the intermediate frequency, usually 70 MHz IF.

The modulated signal is then up converted to the required microwave carrier frequency. A number of carriers may be transmitted simultaneously and although these are at different frequencies, they are generally specified by their normal frequency, for e.g. 6 GHz or 14 GHz carriers.

After passing through the up converters the individual carriers are combined and the resulting wideband signal is amplified in the high power amplifier.

Receiver operations:

The antenna receives the downlink signal from the satellite it drives the feed system which allows the earth station to handle transmit and receive simultaneously.

In the receiver side the feed system ensures that only the received frequencies pass through the sensitive receiving circuits. It blocks the high power transmit signals, which can occur simultaneously with reception. This prevents overload and damage to the receiver. If the incoming signal is dual polarised, the feed system separates the polarisation and then feeds the signal to the receiver.

Redundancy:

Redundancy means certain units are duplicated. A duplicate or redundant unit is automatically switched into a circuit to replace a corresponding unit that has failed. In the block diagram above the HPA and LNA are shown to have redundant units because they are extremely critical to the operations of the earth station. If either of them fail the entire transmit chain/receive chain of the earth would be non-operational. Other redundant units could be down converters, up converters, modulators and demodulators.