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.
Unified S-Band demonstration Early in 1962, a small group of Lincoln Laboratory staff members was asked to provide a demonstration of the Unified Carrier concept to NASA by December 31, 1962. The demonstration was aimed at providing experimental evidence that the unified carrier concept was feasible. Since manpower was limited, it was decided to concentrate on the space-vehicle-to-Earth link, the critical link in the system. The demonstration was available by December 17, 1962. The demonstration was held on January 17, 1963 for NASA (Manned Space Center and Headquarters) and North American Aviation, Inc. The demonstration of the unified carrier concept for the space vehicle-to-Earth link was limited to transmitting a ranging code and wideband telemetry signal on a 47.5-mc carrier by hard wire via a noisy and attenuating medium. The simulated ground receiver used a
phase-locked loop. The carrier reference generated by the
VCO of the carrier phased-locked loop was used to heterodyne the received signal to video, a process of
synchronous demodulation. A correlation method was used to process the transmitted and received codes for ranging. The demonstration simulated the Doppler effect and signal-to-noise ratio expected for an Apollo mission. The phase-locked loops in the receiver acquired the transmitted carrier, telemetry subcarrier, and code clock components almost instantaneously for the signal-to-noise ratios predicted to exist at maximum Apollo range and for a radial space-vehicle velocity of 36,000 ft/sec. Range code correlation generally took only a few seconds. In the beginning, it was suggested that the DSIF transponder could be modified and augmented so as to be used for lunar altimetry and rendezvous ranging. However, as increased emphasis was placed on lunar landing and lunar orbital rendezvous techniques, it became apparent that specialized radar and optical equipment would be preferable for those applications. Accordingly, most of the effort at M.I.T Lincoln Laboratory was directed toward the communication and tracking link between the Apollo spacecraft and earth. ==Technical summary==