(Effective Isotropic Radiated Power) (Transmitter Req't)

For a constant EIRP: As antenna size (gain) increases, the transmitter power requirement decreases

EIRP (dB) = transmitter power + antenna gain - front end losses

Min EIRP required=space loss+atmo loss + antenna pointing loss - receiver antenna gain - receiver sensitivity

(Receiver antenna gain/receiver sys noise temp) (Receiver Reqt)

See Table 13-10 for various communication system temperatures and G/Ts

G/T is the sensitivity of the receiving station and a common Figure of Merit; for an existing satellite link, a ground station can only vary its antenna gain and system noise temp to Improve the system signal-to-nolse ratio

Classic trade studies include size of the antenna aperture vs. transmitter power, solid-state amplifiers vs. traveling-wave tube amplifiers (TWTAs), and spacecraft complexity vs. ground complexity. If we increase an antenna's aperture size, its gain increases; therefore, we may decrease the transmitter's RF output power and still maintain the received signal strength. Unfortunately, large antenna apertures are very heavy and have narrow beam widths (producing more stringent pointing requirements). As Chap. 13 (Eq. 13-17) shows, the beam width decreases with an increase in the antenna's aperture size. Depending on the frequency and gain needed, we commonly decide between solid-state and TWT amplifiers—a system-level trade. To do so, we must assess the effects of the spacecraft's total mass, solar-array size, system reliability, and antenna aperture size. Solid-state amplifiers tend to be more reliable, lighter, and smaller. TWT amplifiers have a lower technology risk (at higher gains) and a higher efficiency. We must use TWT As when the RF output power requirement is too high at a given frequency for solid-state amplifiers, or when the solid-state amplifier efficiency is too low for our application. At today's technology level, we can design solid-state amplifiers with power levels of 65 W at UHF frequencies, 40 W at S-band, and about 20 W at EHF frequencies.

The old rule-of-thumb was to keep the satellite as simple as possible by moving all the complexity and processing to the ground. With modern processors, however, we can now do a tremendous amount of processing on a satellite. Thus, we can design for lower downlink data rates and simpler ground stations or we can collect more data while not overburdening the TT&C subsystem. The new trend is to process as much information as possible on the satellite whenever the mission or science community do not need the raw data.

At the system level, the TT&C subsystem can interface with a fixed or a mobile ground station, as well as a relay satellite. Table 11-20 lists examples of these Systems. We usually select the system-level interfaces when establishing mission, satellite, and operational requirements.

TABLE 11-20. Options for System-Level Interfaces to the TT&C Subsystem. Shown below are several Interface possibilities for a TT&C subsystem. If the interface is an existing system, we also provide the system's document number. (Courtesy of TRW)


Example of Systems

Where to Find Subsystem Parameters

Fixed Ground Station

SGLS—S-band system SDLS—Secure 44/20 GHz GSTDN—S-band 9 and 26 m NASA DSN S-, X-band 26,34,70 m Mission-dedicated or unique

TOR-0059 (6110-01)-3, Reissue H

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