FLTSATCOM employed a single system for tracking, telemetry, and command (TT&C). This spacecraft uses only 977 commands for infrequent commanding. The telemetry requirements are 178 mainframe words (0.5 s frame rate) and fewer than 1,000 subcommutated words—typical of a minimum subsystem for TT&C. A wide-beam antenna mounted on the tip of the main payload's antenna mast ensures communications access to the satellite.

The communications requirements for HEAO-B were driven by HEAO's low-altitude and low-inclination orbit which allowed infrequent access to the spacecraft for commanding and data readout Table 10-40 summarizes characteristics of the communications subsystem and the subsystem for command and data handling. HEAO-B's spacecraft had to maneuver and take payload data while out of sight of a ground station. The commanding subsystem included a stored command programmer which could store and execute 256 commands. Data remained on a tape recorder until the ground station read it out at communication intervals.

TABLE 10-40. Characteristics of the Subsystem for Communications and Command and Data Handling on HEAO-B.



Communications Frequency Radiated power Antennas Weight

S band 1.0W

2 wide beam cross strapped 16.2 kg

Tape Recorder Capacity Data rate Weight

Power (communications and tape recorder)

84x10® bits

6.4 kbps and 128 kbps

13.8 kg

3.4 W cruise; 78.4 W ground pass

Command and Data Handling Command rate Stored commands Telemetry rate Weight Power

200 bps

256 30 bit commands 6.4 and 128 kbps 28.4 kg

33.8 W cruise; 41.6 W ground pass

The FLTSATCOM and HEAO-B components require temperatures of 5-50 °C —no challenge to the designers. Thermal balance is achieved by mounting the components on external panels of the equipment compartment and allowing the panels to radiate excess heat to space. Second-surface mirrors serve as radiators, taking advantage of their low solar absorptivity and high infrared emissivity. Areas not required for radiation are insulated. The spacecraft's interior surfaces are black to enhance internal heat transfer, and guard heaters prevent excessively low temperatures when equipment is off, or on sensitive assemblies such as propellant lines.

Table 10-35 presented power budgets for FLTSATCOM and HEAO-B. Table 10-41 shows the characteristics and components of their power subsystems. FLTSATCOM uses planar arrays oriented toward the Sun by solar-array drives and controls the body's attitude about the Z-axis (yaw control). The array produces 1,800 W at beginning of life. HEAO-B used planar solar panels which were body-mounted. They were oriented toward the Sun by X-axis (roll) attitude control which kept the Sun in the X-Z body plane. With this attitude, the Sun was up to 75 deg away from the array normal. To improve this poor illumination, designers sized the array for 1,500 W under full solar illumination.

Both FLTSATCOM and HEAO-B use NiCd batteries. The cycle life for FLTSATCOM was less than 1,000 cycles, and the three batteries operated at 70% depth of discharge. They also contain bypass electronics to allow removal of failed cells. The three HEAO-B batteries had a cycle life of over 5,000 cycles and operated at 20% depth of discharge.

Table 10-41 also shows the characteristics of the power control, switching, cabling, and conversion equipment on FLTSATCOM and HEAO-B. The FLTSATCOM spacecraft uses an unregulated bus and switches power to user subsystems in central power-control units. A central power converter provides secondary power for the spacecraft bus subsystems. HEAO-B used a regulated solar array and also switched power to users in a set of integration assemblies. These units also contained power converters.

TABLE 10-41. Characteristics and Components of the Electric-Power Subsystems.

Characteristics and Components

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