Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 10M

Telemetry, tracking and command (TT&C) systems support the function of spacecraft management. These functions are vital for successful operation of all tracking and command satellites and are treated separately from communication management. The main functions of a TT&C system are to:

a) monitor the performance of all satellite sub-systems and transmit the monitored data to the satellite control centre;

b) support the determination of orbital parameters;

c) provide a source to earth stations for tracking;

d) receive commands from the control centre for performing various functions of the satellite.

In effect these functions are performed by different sub-systems on a satellite but since all are related to the management of the satellite, the sub-systems are categorized together.

The function of the telemetry sub-system is to monitor various spacecraft parameters such as voltage, current, temperature and equipment status, and to transmit the measured values to the satellite control centre. The telemetred data are analysed at the control centre and used for routine operational and failure diagnostic purposes. For example, the data can be used to provide information about the amount of fuel remaining on the satellite, a need to switch to a redundant chain or an HPA overload.

The parameters most commonly monitored are:

a) voltage, current and temperature of all major sub-systems;

b) switch status of communication transponders;

c) pressure of propulsion tanks;

d) outputs from attitude sensors;

e) reaction wheel speed.

Figure shows the main elements of a telemetry sub-system. The monitored signals are all multiplexed and transmitted as a continuous digital stream. Several sensors provide analog signals whereas others give digital signals. Analog signals are digitally encoded and multiplexed with other digital signals. Typical telemetry data rates are in the range 150—100bps. For low-bit rate telemetry a sub-carrier modulated with PSK or FSK is used before RF modulation. PSK is the most commonly used at RF. It is vital that telemetry information always be available at the satellite control centre, therefore a redundant chain is always available to improve reliability. There is also a provision to crossstrap the transmitters.

The telemetry signal is commonly used as a beacon by ground stations for tracking. During orbit-raising when the main communication antennas are stowed, or under conditions when attitude control is lost, an antenna with near omnidirectional pattern is used. During the operational phase, the TT&C system uses the antenna system deployed for communications. Distributed telemetry systems are increasingly being favoured. In this configuration, digital encoders are located in each sub-system of the satellite and data from each encoder are sent to a central encoder via a common, time-shared bus. This scheme reduces the number of wire connections considerably. This type of modular design also permits easy expansion of the initial design and facilitates testing during assembly of the satellite.

Tracking

Tracking of the satellite is accomplished by having the satellite is accomplished by having the satellite transmit beacon signals which are received at the TT&C earth stations. Tracking is obviously important during the transmitter and drift orbital phases of the satellite launch.

When on-station, a geo-stationary satellite will tend to shifted as a result of the various distributing forces, as described previously. Therefore it is necessary to be able to track the satellites movements and send correction signals as required. Satellite range is also required for time to time. This can be determined by measurement of propagation delay of signals specially transmitted for ranging purposes.

The command system receives commands transmitted from the satellite control centre, verifies reception and executes these commands. Example of common commands are:

a) transponder switching

b) switch matrix reconfiguration

c) antenna pointing control

d) controlling direction and speed of solar array drive

e) battery reconditioning

f) beacon switching

g) thruster firing

h) switching heaters of the various sub-systems.

Typically, over 300 different commands could be used on a communication satellite. From the example listed above, it can be noted that it is vital that commands be decoded and executed correctly. Consider the situation where a command for switching off an active thruster is mis-interpreted and the thruster remains activated — the consequence would be depletion of station-keeping fuel and possibly loss of the satellite as the satellite drifts away from its nominal position. A fail-safe operation has to be achieved under low carrier-to-noise conditions (typically 7—8 dB). A commonly used safety feature demands verification of each command by the satellite control centre before execution. To reduce the impact of high-bit error rate, coding and repetition of data are employed. Further improvements can be obtained by combining the outputs of two receive chains. The message is accepted only when both outputs are identical.

Use of multi-tone frequency in tracking system

Distance (Range) measurement is performed by means of specific sub-carriers which modulate the telecommand carrier, are coherently demodulated in the receiver and are then used to modulate the telemetry carrier The tone system is currently used. In this case, the sub-carrier is a sinusoidal wave of fixed frequency f. Measurement of the phase shift between the transmitted and received tones, which is a function of the distance R from the station to the satellite (a round-trip trajectory of 2R), enables this distance to be determined:

where c is the velocity of light.

A high frequency (100 kHz) is necessary to ensure accuracy of phase measurement; conversely the frequency must be low enough for the wavelength to be long enough with respect to the distance to be measured for there to be no ambiguity. The difficulty is avoided by transmitting two tones simultaneously; these are a major tone at 100 kHz which permits good measurement accuracy and a minor tone, obtained by division of the major tone (and hence in phase with it), which enables the ambiguity to be resolved.