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

Marks: 5M

In satellite communication Commands, depending on the particular mode selected, can be:

• executed immediately after reception;

• stored in memory and executed on reception of a specific command;

• stored in memory and executed when activated at a given time determined by the time management system on board the satellite or when activated by a signal produced by one of the subsystems on board the satellite.

Repetition enables the integrity of the received words to be verified before they are used. One of the important characteristics of the telecommand link is security. It is fundamental for the survival of the satellite that it really is the selected command which is executed.Various precautions are taken, such as error correcting coding of the data words, repetition for verification and detection of possible differences, deferred execution of commands, etc.

Withdeferred execution of commands, the command is detected on board the satellite, stored inmemory, retransmitted to the ground by telemetry for verification and executed only afterauthentication by an execution command sent on the telecommand link.

Finally, precautions are also taken to make the system insensitive to signals transmitted by intruders (anti-jamming); these include narrow band reception, input limiters, insensitivity to nonstandard signals and possible encryption on the link.