Info

Ft

= Date rate - bps

85x10s

1.42X106

In many cases we do not need all data collected by the satellite's sensor. For example, our FireSat data is of no interest unless the sensor observes heat from a forest fire. Onboard data processing can be used to dramatically reduce the required data rates. For FireSat, onboard processing would consist of selecting and transmitting only those pixels receiving thermal energy above a specified temperature. The amount of data-rate reduction depends on the portion of the observed area that is burning. We must also insert extra bits to identify the position of the pixels or groups of pixels in the scan. Such data processing can reduce the data rate by a factor of 3 to 10 or more, depending on the nature of the data.

Another technique for reducing the data rate is to transmit only the changes in the amplitude of the data samples. For example, the amplitude of the first pixel in a frame of data could be transmitted by an 8-bit word. Changes in amplitude of subsequent pixels, relative to the previous pixel amplitude, are then transmitted as 3-bit words, thus reducing the data rate by 3/8.

Considerable effort has gone into reducing, or compressing, the data rate of a digitized voice channel [O'Shaughnessy, 1987]. One technique is Adaptive Differential Pulse Code Modulation, which transmits the difference between the actual voice sample and a predicted value based on several previous samples. Data rates have been reduced from 64 to 32 or 16 kbps using these techniques while maintaining commercial toll-quality voice. Even greater reduction in data rates have been achieved with Vocoders and Linear Predictive Coders. With this method, receivers use transmitted spectral or excitation parameters to control a voice synthesizer. It requires a data rate of only 600 bps to 2,400 bps, but the voice often sounds unnatural. Voice-excited vocoders combine the best features of the approaches described above, producing a reasonably natural-sounding voice channel with a data rate of 4,800 or 9,600 bps [Gerson, 1990].

Reducing the data rate by processing or compression on board the satellite decreases the required transmitter power, significantly reducing satellite mass. (Data processing or compression uses VLSI circuits, which add little to satellite mass.) Instead of reducing the satellite mass, the ground stations can be made smaller with increased mobility and lower cost. A lower rate link can also better survive jamming. However, a compressed signal is less tolerant to bit errors, thus negating some of the advantages listed above. We expect increased use of data compression in the future as performance improves and cost decreases.

13.2.3 Data Relay

Most communications satellites and data-relay satellites simply retransmit the data received through a receiver-transmitter combination called a transponder. The total bandwidth capacities of three communication satellites are in Table 13-8. (See Table 13-6 for the TDRS capacities.) Transponder bandwidths of commercial geostationary communication satellites are usually 36 MHz or 72 MHz. (These transponders are repeaters. See Sec. 13.5 for a description of the processing transponder.

TABLE 13-8. Relay Bandwidth Capabilities of Representative Communication Satellites.

The maximum data rate can be several times the bandwidth, depending on the modulation and ground station size. The first entry for Intelsat-V is read as 4 transponders at 36 MHz and 1 transponder at 41 MHz. The total relay bandwidth is calculated by multiplying the number of transponders by their bandwidth and adding them together.

TABLE 13-8. Relay Bandwidth Capabilities of Representative Communication Satellites.

The maximum data rate can be several times the bandwidth, depending on the modulation and ground station size. The first entry for Intelsat-V is read as 4 transponders at 36 MHz and 1 transponder at 41 MHz. The total relay bandwidth is calculated by multiplying the number of transponders by their bandwidth and adding them together.

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