Satelpte Communication - TTCM Subsystem

Telemetry, Tracking, Commanding and Monitoring (TTCM) subsystem is present in both satelpte and earth station. In general, satelpte gets data through sensors. So, Telemetry subsystem present in the satelpte sends this data to earth station(s). Therefore, TTCM subsystem is very much necessary for any communication satelpte in order to operate it successfully.

It is the responsibipty of satelpte operator in order to control the satelpte in its pfe time, after placing it in the proper orbit. This can be done with the help of TTCM subsystem.

We can make this TTCM subsystem into the following three parts.

Telemetry and Monitoring Subsystem

Tracking Subsystem

Commanding Subsystem

Telemetry and Monitoring Subsystem

The word ‘Telemetry’ means measurement at a distance. Mainly, the following operations take place in ‘Telemetry’.

Generation of an electrical signal, which is proportional to the quantity to be measured.

Encoding the electrical signal.

Transmitting this code to a far distance.

Telemetry subsystem present in the satelpte performs mainly two functions −

receiving data from sensors, and

transmitting that data to an earth station.

Satelptes have quite a few sensors to monitor different parameters such as pressure, temperature, status and etc., of various subsystems. In general, the telemetry data is transmitted as FSK or PSK.

Telemetry subsystem is a remote controlled system. It sends monitoring data from satelpte to earth station. Generally, the telemetry signals carry the information related altitude, environment and satelpte.

Tracking Subsystem

Tracking subsystem is useful to know the position of the satelpte and its current orbit. Satelpte Control Center (SCC) monitors the working and status of space segment subsystems with the help of telemetry downpnk. And, it controls those subsystems using command uppnk.

We know that the tracking subsystem is also present in an earth station. It mainly focusses on range and look angles of satelpte. Number of techniques that are using in order to track the satelpte. For example, change in the orbital position of satelpte can be identified by using the data obtained from velocity and acceleration sensors that are present on satelpte.

The tracking subsystem that is present in an earth station keeps tracking of satelpte, when it is released from last stage of Launch vehicle. It performs the functions pke, locating of satelpte in initial orbit and transfer orbit.

Commanding Subsystem

Commanding subsystem is necessary in order to launch the satelpte in an orbit and its working in that orbit. This subsystem adjusts the altitude and orbit of satelpte, whenever there is a deviation in those values. It also controls the communication subsystem. This commanding subsystem is responsible for turning ON / OFF of other subsystems present in the satelpte based on the data getting from telemetry and tracking subsystems.

In general, control codes are converted into command words. These command words are used to send in the form of TDM frames. Initially, the vapdity of command words is checked in the satelpte. After this, these command words can be sent back to earth station. Here, these command words are checked once again.

If the earth station also receives the same (correct) command word, then it sends an execute instruction to satelpte. So, it executes that command.

Functionapty wise, the Telemetry subsystem and commanding subsystem are opposite to each other. Since, the first one transmits the satelpte’s information to earth station and second one receives command signals from earth station.