communication system is used to transfer information between the
orbiter and its payload or payloads. It supports hardline and radio
frequency communications with a variety of payloads. The system
is used to activate, check out and deactivate attached and detached
payloads. The two main users of this system are NASA and the DOD.
Other users include various satellite consortiums, private industry
and foreign countries.
with an attached payload takes place through the payload patch panel
at the crew compartment flight deck aft station, which is connected
to external equipment with internal systems. From this panel, the
attached payload telemetry can take different paths. All standard
command and telemetry signals are processed on board. Non-standard
signals are sent to the ground through Ku-band. This method of transmission,
referred to as bent-pipe telemetry, means that no onboard signal
processing occurs before the telemetry is sent to the Ku-band system.
Payload telemetry can go directly to the S-band, FM or Ku-band systems
for transmission to the ground, payload analog or payload digital;
to the payload recorder for later transmission; or to the payload
data interleaver to be interleaved with other payload data in a
selected format called the decommutator format load. The system
also processes commands and tracks the status of various payload-related
controls and displays.
communicate with the orbiter on an RF signal through the payload
antenna by the payload interrogator. The main frequency carrier
of the detached payload telemetry is demodulated by the PI. The
telemetry is sent directly to the Ku-band system for transmission
to the ground (bent-pipe telemetry) or to the payload signal processors.
The PSP demodulates the subcarrier from the telemetry and sends
the telemetry to the payload data interleaver to be interleaved
with other payload data. Attached payload commands are routed to
the PSP and then to the payload patch panel, which is also referred
to as the payload station distribution panel. Detached payload commands
are routed to the PSP and then to the PI for transmission through
the payload antenna.
interface unit replaces the PSP during DOD missions and uses space-ground-link-system-compatible
payloads and ground stations to provide communication security.
The CIU interfaces indirectly with the payload data interleaver
through the patch panel because the CIU is treated as an attached
payload accessed through a patch panel input of the payload data
interleaver. The payload signal processor is hard-wired directly
to the payload data interleaver.
payload antenna is located on the top of the outer skin of the orbiter's
forward fuselage, just aft of the upper hemispherical antenna. The
payload antenna is covered with reusable thermal protection system.
This antenna is used as the radiating element for S-band transmission
and reception to and from the orbiter to detached payloads through
the forward link and return link.
The basic elements
in the payload communication system are the payload interrogator,
payload signal processor, communication interface unit, payload
data interleaver, pulse code modulation master unit, payload patch
panel, payload recorder and payload MDMs 1 and 2. These elements
are in the forward avionics bay and are controlled by switches on
panels A1 and L10.
interrogator is a transmitter/receiver unit that provides full duplex
RF communications between the orbiter and a detached payload. It
transmits commands to and receives telemetry from NASA- or DOD-compatible
payloads through the payload antenna.
problems involving antenna position relative to payload position
are not evident while the payload is within a half mile of the orbiter.
However, to maintain good communication with the orbiter from distances
of several miles, the payload must be within an 80-degree beamwidth
(with reference to the minus Z axis) of the orbiter's PI antenna.
The boundary of the 80-degree beamwidth is the 3-decibel point (or
half-power point), which must be considered during communication
with deployed payloads. This constraint is normally satisfied by
the payload and retrieval process.
interrogator receiver automatically acquires and tracks an unmodulated
or modulated RF signal. PI telemetry is available through the operational
instrumentation MDM to verify signal strength and frequency lock.
When the payload
outputs a data rate that is not compatible with the payload signal
processor or communication interface unit, all data received by
the PI is throughput (bent-pipe) directly to the K-band signal processor
through a dedicated channel that operates independently of, but
parallel to, the NASA and DOD channels. Standard payload telemetry
is sent to the PSP for processing before being routed to the payload
signal processor is the command interface between the ground or
flight crew and five attached/detached payload services. It is also
a detached payload telemetry interface to the payload data interleaver.
interface unit is used in place of the PSP whenever an SGLS-compatible
payload is flown. This provides a command and telemetry path between
the orbiter guidance, navigation and control GPC and an SGLS-compatible
payload or between the flight crew and an SGLS-compatible payload.
The CIU passes commands and telemetry to either attached or detached
In the NASA
mission configuration, the payload patch panel interfaces attached
payloads to the PDI. Attached payloads are wired to specific input
channels in the PPP during prelaunch activities. When the PDI is
reconfigured by the flight crew, programming procedures include
assigning inputs from the PPP to the desired decommutator.
In the DOD
configuration, the PPP is the command and telemetry interface between
the CIU and attached payloads as well as the telemetry interface
for detached payloads from the CIU to the PDI.
data interleaver allows the payload communication system to interface
with the rest of the orbiter communication system and computers.
It receives up to six different inputs from attached or detached
payloads and one test input. For missions using the PSP, a maximum
of five attached payloads can be accommodated on inputs 1 through
5. Input 6 is reserved for detached payload commands and telemetry
using the RF link through the PSP. For missions using the CIU, all
data, attached or detached, are routed through input 5. The PDI
routes four of the six available inputs to the PCMMU for downlink
to the SM GPC for display purposes.