The payload 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.

Communication 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.

Detached payloads 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.

The communication 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.

The S-band 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.

The payload 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.

Communication 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.

The payload 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 data interleaver.

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.

The communication 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 payloads.

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.

The payload 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.