M

' After 1 bit per sample added for signalling and supervision.

' After 1 bit per sample added for signalling and supervision.

new codes it is possible to achieve a bit error rate (BER) of about 10~10 at a bit energy to incremental noise (Ef,/N0 ) ratio of only 5 dB.

Digital communication techniques are used instead of analog for a number of reasons. First, digital signals can more precisely transmit the data because they are less susceptible to distortion and interference. Second, digital signals can be easily regenerated so that noise and disturbances do not accumulate in transmission through communication relays. Third, digital links can have extremely low error rates and high fidelity through error detection and correction. Also, multiple streams of digital signals can be easily multiplexed as a single serial-bit stream onto a single RF carrier. Other advantages are easier communication-link security and implementation by drift-free miniature, low-power hardware, including microprocessors, digital switching, and large scale integrated circuit chips. In this chapter we will consider only digital communications.

Using the formulas developed in Chap. 5, we can easily determine the relationship between the quantity of data, D, the data rate, R, and the parameters for a single ground station pass from Sec. 5.3.1. Specifically,

where is the maximum time in view (i.e., the pass duration when the satellite passes directly overhead) from Eq. (5-52), F is the fractional reduction in viewing time due to passing at an Earth central angle away from the ground station, X^ is the maximum Earth central angle from Eq. (5-36), Tinitiale is the time required to initiate a communications pass, and M is the margin needed to account for missed passes due to ground station down time, sharing of ground resources, transmission of other data, or conflicts on board the satellite or within the communications process. A reasonable value for Tinitiate is about 2 minutes. M is conservatively estimated at a value of 2 to 3 unless it is a dedicated ground station with a specified value for the percentage of pass time that will be used for collecting data. For the fraction of time in view, we may wish to use mean values rather than one for a specific ground station pass in Eq. (13-3). As discussed in Sec. 5.3.1, the average value of F is about 80% for satellites in a circular low-Earth orbit, and 86% or more of all passes will have F greater than 0.5.

With this background on digital techniques, we now consider the data rate requirements for the three types of architectures discussed in the previous section: telemetry, tracking, and command (TT&Q; data collection; and data relay.

The number and accuracy of functions being monitored in the satellite determines the telemetry data rate. Several hundred functions such as voltages, temperatures, and accelerations may require monitoring to determine if all satellite subsystems are operating correctly, and, if not, to determine where a failure occurred. Sampling each telemetry sensor in sequence with a multiplexer combines all telemetry data into a single bit stream. The sampling rate is usually low, perhaps once every second or once every 10 sec, because the monitored parameters vary slowly. For example, suppose we want to monitor 50 temperature sensors and 50 voltages once every 10 sec with an accuracy of 1-5%. The data rate required is 100 samples per 10 sec times 5 bits per sample, or 50 bps. Some applications require precise time or amplitude resolution of the data. In these cases, the data may be transmitted in analog form by frequency modulation of one or more subcarriers [Morgan and Gordon, 1989].

The rate needed to transmit commands to a satellite is usually quite low—perhaps only one per second. A command message may be 48 to 64 bits long, consisting of a synchronizing preamble (a set series of bits), an address word that routes the command to its satellite destination, the command itself (often a single on-off digit), and some error detection bits to make sure the command was correctly received. Some commands can cause irreversible functions or damage the satellite if performed at the wrong time. These commands are usually first transmitted and stored in the satellite. Correct reception by the satellite is verified by telemetry, after which a second command is transmitted to execute the function. If the command is to be executed later when the satellite is out of the ground station's view, a time of execution is added to the command word and stored in the satellite. The command is executed later when the time contained in the command word coincides with the satellite's clock time.

To track a satellite, the ground station measures range or range rate for computing and updating the orbit ephemeris. For example, the Air Force adds a one Mbps pseudorandom (PN) code to the command link. The satellite command receiver extracts this code. It is then retransmitted as part of the telemetry downlink signal. The ground station measures the arrival time of the code relative to its uplink transmission time to determine the round-trip delay, from which the range is computed. NASA's. Goddard Range and Range Rate system operates the same way except it uses several harmonically related sinusoidal tones plus a pseudorandom code. Intelsat uses only four ranging tones.

In most cases we would want to use an existing TT&C ground station network. Table 13-6 summarizes the key parameters of four networks (see Chaps. 11 and 15 for additional details). The ratio of downlink-to-uplink frequencies listed in the table applies when the satellite transmitter is phased-Iocked to the received uplink carrier. TTus mode allows the Doppler frequency shift of the RF carrier to be accurately measured at the ground station to determine the range rate. The United States has NASA's Deep Space Network and the Air Force's Satellite Control Network. Intelsat and other communications satellite operators use their own TT&C system, which eliminates the need to pay for the services of a larger network. The TT&C requirements for FireSat are quite modest and can easily be handled by its own system, except during the launch phase.

TABLE 13-6. Parameters of Existing Satellite TT&C Systems.

Network

Command (Uplink)

Telemetry (Downlink)

DUUL Carrier Freq. Ratio

Range Measurement

Freq. (GHz)

Data Rate (bps)

Freq. (GHz)

Data Rate (bps)

Air Force SCN (SGLS)

0 0

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