Page 202 - Intro to Space Sciences Spacecraft Applications

Spacecraj? Systems 189
tions (commands) for future actions. Modem sophisticated spacecraft are
equipped with solid-state control devices on which mission logical
instructions can be placed and through which the spacecraft will perform
those instructions autonomously. As part of this overall spacecraft coordi-
nation, the controller may interrogate its separate systems for their status
and perform corrections in cases of malfunction or failure.
The telernetrj, trucking, and control (TT&C) subsystem usually has a
low data rate (1-2 Kbps) and can do double duty serving as a downlink for
low-rate payload data. If the composite payload data rate is low, it can be
accommodated by the TT&C subsystem. If the composite data rate is high,
a payload specific channel is required in the data handling subsystem.
Like the data handling subsystem, a TT&C design calculates the sub-
system mass and power requirements from available systems assuming
two transponders and omnidirectional antennas for redundancy.

Data Handling and TT&C As a Combined Subsystem

The data handling and telemetry, tracking, and command (TT&C) sub-
systems, while separate in the subsystems listing, are associated in much
the same way as the propulsion and orbital maintenance subsystems. To
conserve weight and space, a design may take advantage of the shared use
of telecommunications resources. If an S-band TT&C link can also be
shared to collect the payload information being collected, then satellite
complexity and cost is reduced.
The data handling requirements are based on the payload input values of
data rate, duty cycle, and whether or not onboard recording is specified. The
data rate will determine whether a low-frequency or high-frequency channel
is required for transmission (see the information bandwidth description in
Chapter 5) and, hence, whether the low-frequency (S-band) 'IT&C channel
can be shared for remote payload data collection. If the data rates warrant, a
wider bandwidth channel (X-band or even K-band) will be added as a sepa-
rate data handling subsystem. A designer could amend this by taking advan-
tage of a low-percentage duty cycle (instrument on time of a few minutes per
orbit) and use a recorder playback at a lower rate to return to a shared data
handling TT&C design. If the payload input has onboard recording as a
requirement, the designer can take this into account in determining the mass
of the data handling subsystem. The designer is still responsible for selecting
the type of recorder desired (magnetic tape or solid state).
Once the component suite is complete, the designer tallies the mass and
power requirements for the combined data handling and TT&C subsystem