A Case Study in NASA-DoD - The Black Vault

-131-
chiefly for storing commands for later processing and for verifying
received commands with those stored in memory.
DESCRIPTION OF STPSS AND COMPARISON WITH AEM
The STPSS spacecraft is designed for Air Force missions. It has
an S-band communication system which can handle a maximum command rate
of 2 kbps and telemetry rates of 256 kbps. It is SGLS-compatible and
uses ternary frequency-shift keying (FSK) coding. An on-board computer
can handle stored commands, telemetry storage, format control, and
memory dumps. Data and commands can be encrypted if necessary.
The C&DH for the STPSS spacecraft is far more sophisticated and
has a much greater capacity than that on the AEM (see Table C-1). It
is doubtful if experiments of the size that would be carried on the AEM
would require as sophisticated a system as presently envisioned for the
STPSS. However, currently planned AEM telemetry and control equipment
probably could not be used because of the basic incompatibility of the
NASA-STDN and AF-SGLS systems.
To make the AEM compatible with the SGLS system requires replacing
the S-band transmitter and the S-band transponder, the command demodulator,
and modifying or replacing the PCM encoder and the command
decoder/processor. Personnel at Boeing indicate that the "black boxes"
can be replaced one-for-one with SGLS-compatible equipment without
causing major spacecraft redesign. It appears that SGLS-compatible
equipment exists that could be used on the AEM. Encryption and decryption
units can be added to SGLS equipment if required, but not to
STDN. There is some question whether the AEM can meet the signal isolation
requirements of encrypted missions. However, Boeing personnel
state that an SGLS-compatible AEM can have encryption capability. Items
such as the sequencer timer and remote command processor are one-time
programmable, with the programming dependent on the spacecraft and
mission, and could be used with the proper programming. The STPSS's
bus controller, computer, and data interface units are more sophisticated
than anything currently on the AEM. The functions that these
would handle on the AEM are done as part of the PCM encoder and the
command decoder/processor, although those done on the AEM are simpler.