The Unified S-band (USB) system is a tracking and communication system developed for the Apollo program by NASA and the Jet Propulsion Laboratory (JPL). It operated in the S band portion of the microwave spectrum, unifying voice communications, television, telemetry, command, tracking and ranging into a single system to save size and weight and simplify operations. The USB ground network was managed by the Goddard Space Flight Center (GSFC). Commercial contractors included Collins Radio, Blaw-Knox, Motorola and Energy Systems.
Basis The previous programs, Mercury and Gemini, had separate radio systems for voice, telemetry, and tracking. Uplink voice and command, and downlink voice and telemetry data were sent via ultra high frequency (UHF) and very high frequency (VHF) systems. The tracking capability was a C band beacon interrogated by a ground-based radar. With the much greater distance of Apollo, passive ranging was not feasible, so a new active ranging system was required. Apollo also planned to use television transmissions, which were not supported by the existing systems. Finally, the use of three different frequencies complicated the spacecraft systems and ground support. The Unified S-band (USB) system was developed to address these concerns. The USB system did not completely replace all other radio transmitters on Apollo. Apollo still used VHF between astronauts and the Lunar Module (LM) and Lunar Roving Vehicle during extra-vehicular activity; between the lander and the command module, and between the spacecraft and Earth stations in the orbital and recovery phases. As a backup the CM could measure range to the LM over the VHF voice link. The spacecraft radar systems operated on frequencies separate from those of the USB.
Development The S-Band communications and ranging system was developed by the MIT Lincoln Laboratory in Lexington, Massachusetts, under task A of the Lincoln Laboratory Apollo contract. The design approach was the development of an alternative integrated communication system functionally compatible with the spacecraft design. The concept was presented by Lincoln Laboratory in an initial report on July 16, 1962 titled Interim Report on Development of an Internal On-Board RF Communications System for the Apollo Spacecraft. In this report, it was shown that many on-board electronic functions could be performed very effectively by a single system that was a suitable adaptation of the transponder developed by Jet Propulsion Laboratory for use with the DSIF tracking stations. This was the origin of the Goal System for Apollo, later called the Integrated (or Integral) RF system, then later known as the Unified Carrier System. The idea behind the unified S-Band communications system was to reduce the number of systems previously used in the Mercury space program, which provided a multiplicity of electromagnetic transmitting and receiving equipment. In early flights, these operated at seven discrete frequencies within five widely separated frequency bands. Largely because of expediency, the following separate units were employed:
HF voice transmitter and receiver UHF voice transmitter and receiver Command receiver Telemetry transmitter No. 1 Telemetry transmitter No. 2 C-band transponder beacon S-band transponder beacon Ground facilities matching this capsule equipment were included in many of the Mercury network stations. When the Apollo project was initiated, NASA stipulated that as much as possible of the existing Mercury ground network equipment should be utilized. In addition, the spacecraft was to include a transponder compatible with the Deep Space Instrumentation Facility (DSIF) ground stations established by the Jet Propulsion Laboratory. This transponder would be used for the communications and tracking in cis-lunar space between earth and the moon. In the preliminary research of the Unified S-Band, North American Aviation, Inc., (the company that developed Apollo's command and service modules) indicated the following four pieces of equipment would be installed in Apollo for ground-to-spacecraft use:
DSIF transponder (S-band) (for cis-lunar distances) for transmission of TV, voice, telemetry data, and ranging signals VHF FM transmitter (for near-Earth distances) for transmission of telemetry data VHF AM transceiver (for near-Earth distances) for transmission and reception of voice and guidance of rescue aircraft C-band transponder (for near-Earth distances) for radar tracking The DSIF transponder had a basic capability to perform the functions of the VHF FM transmitter, the VHF AM transceiver, and the C-band transponder at near-earth distances. Significant features of the transponder and its ground equipment were all-coherent, phase-locked operation and the use of a pseudo-random (noiselike) binary code for unambiguous range measurements at long distances. The choice of optimum modulation methods and waveforms for the upward and downward RF links was a key factor in the adaptation of the unified carrier system to Apollo requirements. Additional electronic apparatus was to be deployed for rendezvous guidance, for lunar (and Earth) altimetry, and for lunar landing control. The requirements for this additional equipment had not been firmly established when Lincoln Laboratory began its research. From experience with the Mercury space program, it was apparent to Lincoln Laboratory that considerable on-board simplification would result if a single integrated communications and tracking system were used in Apollo instead of the four systems listed above.
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