Info

3.9-4.2 12.2 or17.7

1,000-4,800 1,000-4,B(K>

Not applicable

4 Ranging Tones: 27.777 kHz, 3,968.25 Hz, 283.477 Hz, 35.431 Hz

TDRS"' (user satellite altitude below 12,000 km)

2.1064 "SA S-Band 2.025-2.120 "SA K-Band 13.775

10 kbps max 300k max

2.2875 SA S-Band

2.2-2.3 SA K-Band 15.0034

1kto1.5M 1kto12M 1 k to 300M

(S) 240/221 (S) 240/221 (K)1,600/1.469

3 Mbps PNCode

"MA—Multiple Access, up to 20 users simultaneously

"SA—Single Access '"Frequencies to and from user satellite

Also shown in Table 13-6 is NASA's Tracking & Data Relay Satellite (TDRS), which provides an alternative to ground stations for supporting the TT&C link [Yuen, 1983]. The orbit of the user satellite must be below synchronous altitude to be in view of the TDRS antennas.

13.2.2 Data Collection

In the second type of network, a satellite sensor, such as an optical or radar scanner, collects data. This data is transmitted to the ground station for processing and viewing by the user. The pictures of global cloud cover on the evening television news come from a satellite sensor. Although we could include the sensor data as part of the telemetry data discussed above, we usually consider the sensor data separately when the sensor data rate is greater than 100 kbps or so. Data rate requirements for pay load sensors are discussed in Sec. 9.5.5.

Table 13-7 lists the data rates for two satellite-sensor configurations. One example is a geostationary satellite with a radiometer which scans the entire Earth in 20 min with 1 km resolution. Here the data rate is 1.42 Mbps, similar to a meteorological satellite such as GOES.

On the other hand, using Eq. (9-23), we calculate the FireSat data rate to be 85 Mbps—too high for any practical, cost-effective system. Let us review the FireSat mission requirements. First, we need to scan 150,000 acres (about 25 km by 25 km) in4 min with 30 mresolution. By limiting the sensor coverage to the 150,000-acre area under surveillance as the satellite passes overhead, the sensor output rate would be only 2,9CK) pixels per second,* for a data rate of 39 kbps (for 1.6 samples per pixel, 8 bits per sample and q = 0.95). This rate is far more attractive for practical designs. We could also use some form of data processing on board the satellite, as discussed below.

TABLE 13-7. Data Requirement for Two Example Sensor-Satellite Systems. For FlreSat the satellite ground track velocity provides scanning of the ground.

Parameter

FireSat

Geostationary

h

= Orbit altitude (km)

0 0

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