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., NASA TM-X55492. 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.Interim Report on Development of an Internal On-Board RF Communications System for the Apollo Spacecraft Massachusetts Institute of Technology - Lincoln Laboratory, July 16, 1962Final Report: The Unified Carrier System, Massachusetts Institute of Technology - Lincoln Laboratory, August 9, 1963.
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 From a NASA technical summary: The design of the USB system is based on a coherent doppler and the pseudo-random range system which has been developed by JPL. The S-band system utilizes the same techniques as the existing systems, with the major changes being the inclusion of the voice and data channels. A single carrier frequency is utilized in each direction for the transmission of all tracking and communications data between the spacecraft and ground. The voice and update data are modulated onto subcarriers and then combined with the ranging data [...]. This composite information is used to phase-modulate the transmitted carrier frequency. The received and transmitted carrier frequencies are coherently related. This allows measurements of the carrier doppler frequency by the ground station for determination of the radial velocity of the spacecraft. In the transponder the subcarriers are extracted from the RF carrier and detected to produce the voice and command information. The binary ranging signals, modulated directly onto the carrier, are detected by the wide-band phase detector and translated to a video signal. The voice and telemetry data to be transmitted from the spacecraft are modulated onto subcarriers, combined with the video ranging signals, and used to phase-modulate the downlink carrier frequency. The transponder transmitter can also be frequency modulated for the transmission of television information or recorded data instead of ranging signals. The basic USB system has the ability to provide tracking and communications data for two spacecraft simultaneously, provided they are within the beamwidth of the single antenna. The primary mode of tracking and communications is through the use of the PM mode of operation. Two sets of frequencies separated by approximately 5 megacycles are used for this purpose [...]. In addition to the primary mode of communications, the USB system has the capability of receiving data on two other frequencies. These are used primarily for the transmission of FM data from the spacecraft.
Frequencies
The Unified S-Band System used the 2025–2120 MHz band for transmission to the spacecraft (uplinks), and used the 2200–2290 MHz band for transmissions from the spacecraft (downlinks). These bands are allocated internationally for space research and operations, though by 2014 standards the ALSEP uplink was in the wrong part of the band (deep space instead of near earth). Apollo frequency assignments Spacecraft To Earth (MHz) To Space (MHz) Coherent ratio Command Module PM 2287.5 2106.40625 221/240 Command Module FM 2272.5 Lunar Module 2282.5 2101.802083 221/240 S-IVB PM 2282.5 2101.802083 221/240 S-IVB FM 2277.5 Lunar Rover 2265.5 2101.802083 Apollo 11 Early ALSEP 2276.5 2119 Apollo 12 ALSEP 2278.5 2119 Apollo 14 ALSEP 2279.5 2119 Apollo 15 ALSEP 2278.0 2119 Apollo 15 subsatellite 2282.5 2101.802083 221/240 Apollo 16 ALSEP 2276.0 2119 Apollo 17 ALSEP 2275.5 2119
The Lunar Communications Relay Unit (LCRU) on the Lunar Rover (Apollo 15, 16, 17) had its own downlink frequency (to avoid interference with the LM) but shared the LM's uplink frequency as it did not implement a coherent transponder. Separate voice subcarriers were used on the common S-band uplink, 30 kHz for the LM and 124 kHz for the LCRU, so that the LM and LCRU would not both relay uplink voice and interfere with each other.
The S-IVB had a USB tracking transponder for use after separation of the CSM. The tracking data improved the analysis of the impact recorded by the seismometers left by earlier Apollo crews. The S-IVB used the same frequency pair as the LM. Normally the LM was inactive during flight, however, this was a problem when during the Apollo 13 flight as the LM had to be powered up early to be used as a life boat.
The LM frequencies were also used by subsatellites deployed in lunar orbit after the LM had departed the Moon, as part of the later J-missions.