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At the subsystem level, the TT&C subsystem interfaces directly with every subsystem except for propulsion. Hie interface with guidance, navigation, and control deals primarily with antenna pointing. It uses gimbals, or motorized rotary joints, that move in 1 or 2 axes to steer die spacecraft's narrow-beam antennas. Nongimbaled or fixed directional antennas rely on feed arrays for small movements of the antenna beam or on spacecraft attitude maneuvers for large movements. The World Administrative Radio Conference established antenna-pointing requirements for geostationary satellites. The pointing error must be the smaller value of 10% of the half-power (-3dB) beamwidth or 0.3 deg. Table 11-21 summarizes the TT&C subsystem's constraints and requirements on other subsystems.

TABLE 11-21. TT&C Subsystem Constraints and Requirements on Other Subsystems.

The TT&C subsystem Interfaces with all these subsystems and must reliably pass data back and forth or receive support

TABLE 11-21. TT&C Subsystem Constraints and Requirements on Other Subsystems.

The TT&C subsystem Interfaces with all these subsystems and must reliably pass data back and forth or receive support

Subsystem

Requirement

Constraint

Attitude Determination and Control

• Antenna pointing requirements for gtmbaled antennas (glmbal degrees of freedom, amount of rotation)

• Pointing requirements of the lesser of 1/10 of antenna beamwidth or 0.3 deg

• Closed-loop pointing requirements (Le., cross-links requiring autotracking)

• Spacecraft pointing and attitude knowledge for fixed antennas may impact antenna beamwidth requirements

• Uncertainty in attitude and pointing estimates for the pointing loss in the link budget

Command and Data Handling

• Command and telemetry data rates

• Clock, bit sync, and timing requirements

• 2-way comm requirements

• Autonomous fault detection and recovery requirements (ROM stored command sequence that automatically selects the backup receiver and omnl-antenna)

• Command & telemetry electrical interface

• Onboard storage and processing

Electrical

Power

Subsystem

• Distribution requirements

• Amount and quality of power, including requirements for duty cycle, average, and peak power

Structure/ Thermal

• Heat sinks for traveling wave tube amplifiers

• Heat dissipation of all active boxes

• Location of TT&C subsystem electronics and antennas (locate comm electronics as close to the antennas as possible to minimize RF cable loss)

• A clear field of view and movement for all glmbaled antennas

• Temperature uncertainty on non-oven-controlled frequency sources resulting In frequency uncertainty

Paytoad

• Requirements for storing mission data

• RF and EMC interface requirements (conducted emissions, conducted susceptibility, radiated emissions, radiated susceptibility)

• Special requirements for modulation, coding, and decoding

• Maximum data rates for mission or science telemetry

• Maximum data volume for mission or science telemetry

Propulsion

• None

• None

The interface with the subsystem for command and data handling passes spacecraft commands and telemetry, as well as the TT&C subsystem's control and reporting of health and status. The interface must allow the system to receive spacecraft commands while transmitting real-time telemetry. It also must permit safing of the subsystem and autonomous fault detection and correction.

The interface with the electrical-power subsystem controls the amount and quality of spacecraft power to the TT&C subsystem. One common design has the electrical-power subsystem deliver +28 Vdc unregulated to the transponders and other active boxes in the TT&C subsystem and +28 Vdc regulated to the TWTAs. This design requires dc-to-dc converters at each piece of equipment to provide the correct voltage changes. Another interface design centralizes the power conversion and conditioning, for the TT&C subsystem's active elements. However, because TWTAs require specific voltage levels (-1,000 Vdc, +1,000 Vdc, and +4,000 Vdc,), centralized power conversion and conditioning is not very common with those types of amplifiers.

The payload interface mainly transfers mission or science telemetry data to either the ground station or a relay satellite. To characterize this interface we must know the data rate, data volume, and any data storage requirements. This interface may have to couple signals between the payload and the TT&C subsystem and to modulate the payload telemetry.

Table 11-22 gives a design process for the TT&C subsystem. Once we state the performance parameters for TT&C and identify the ground and spaceborne equipment, we use the methods in Chap. 13 to determine overall performance of the communication links. We must iterate this process many times within the design team to attain an acceptable spacecraft weight, configuration, and performance level.

TABLE 11-22. Preliminary Design Process lor the TT&C Subsystem.

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