Page 203 - Intro to Space Sciences Spacecraft Applications

190 Introduction to Space Sciences and Spacecraft Applications
using contemporary component values. The elements include transponder
mass, hemispheric and directive parabolic antenna mass, and wiring mass.
Typical values for a combined TT&C and data handling subsystem are
2-6% of spacecraft dry mass.

Onboard Computer Subsystem

The onboard computer subsystem asks the designer to specify what
function the computer will be used for. The functions include:
attitude control
navigation control
orbital maintenance
command processing
telemetry processing
payload management
power management

The onboard computer may control functions such as power and data
management, recovery from failure, and others. The onboard computer
system mass and power are estimated as a function of computer memory
storage size (K-words) and throughput (Kbps). The designer indicates
which subsystems are communicating with the onboard computer. Based
on this selection, a design estimates the storage word size and the through-
put and establishes the subsystem mass and power.
The designer can also specify whether the satellite is designed with sim-
plified or complex ‘‘stand-alone” autonomy. Complex autonomy provides
significantly more computer capability because it is associated with systems
that are required to make important operational and emergency decisions
without benefit of ground analysis or control and may operate for several
months without any ground interaction whatsoever. The default design deci-
sion is to use full redundancy in this important design area. A designer can
change this decision if application goals, i.e., special limited lifetime mis-
sions, so warrant. In many designs, mass and power requirements for this
subsystem are accounted for with the ‘IT&C and data handling subsystems.

Structure Subsystem

Spacecraft structural considerations must deal with the spacecraft in its
many physical configurations. It must be able to support itself under the