6.12 Satellite Data Transmission and Reception

6.12 Satellite Data Transmission and Reception

Transmitted data from remote sensing satellites involve not only image data but also telemetry data including temperature, electric voltage and electric current of various onboard equipment. Such data are usually transmitted as a digital signal in the form of PCM (pulse code modulation ) with a binary pulse because the digital signal has the advantages of being noise proof, requiring less electric power and having available narrow radio bands. As the data volume or rate of transmission is very high, high frequency bands, such as S band or X band ranging from several GHz to several tens of GHz, are used to achieve the high rate of transmission.

These data are generally received by direct reception at a ground station. However this direct method is limited to reception only when the satellite is in view, nominally several degrees over the horizon, but usually above the horizons will suffice.

There are two methods used to record the satellite data at other areas outside the look angle; these are MDR (mission data recorder) and TDRS (tracking and data relay satellite).

MDR can record other data from areas other than the covering area of the ground station, and replay the data when the satellite flies over the ground station. For examples, NOAA, SPOT, JERS-1 etc. have the MDR system.

TDRS's have been launched by NASA over the equator at 41 W and 171 W , which can cover the whole of the earth tracking a lower altitude satellite and relaying the data to the ground station, located at White Sands, United States, as shown in Figure 6.12.1.

Landsat 4 and 5 are linked to TDRS. Table 6.12.1 shows the reception method for areas outside those covered by receiving stations.