Unified S-band
The previous programs, Mercury and Gemini, had separate radio systems for voice, telemetry, and tracking.
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.
Largely because of expediency, the following separate units were employed: 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.
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: 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.
[3] 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 held on January 17, 1963 for NASA (Manned Space Center and Headquarters) and North American Aviation, Inc.[3] 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 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.
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.
A single carrier frequency is utilized in each direction for the transmission of all tracking and communications data between the spacecraft and ground.
This allows measurements of the carrier doppler frequency by the ground station for determination of the radial velocity of the spacecraft.
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.
In addition to the primary mode of communications, the USB system has the capability of receiving data on two other frequencies.
The tracking data improved the analysis of the impact recorded by the seismometers left by earlier Apollo crews.
Microphone audio was keyed manually or by VOX, but unlike ordinary half duplex two-way radio both sides could talk at the same time without mutual interference.
This is why Apollo uses a low modulation index: to leave a strong carrier that can be used for highly accurate velocity tracking by measurement of its Doppler shift.
This "two-way" technique allowed velocity measurements with a precision on the order of centimeters/second, by observing the Doppler shift of the downlink carrier.
The passive laser retroreflectors left by the Apollo 11, 14 and 15 missions provide much greater accuracy, and have far outlived the active electronics in the other ALSEP experiments.
Sidebands generated by the information also carried by the system had to be kept away from the carriers to avoid perturbing the phase locked loops used to track them.
In the preferred NBFM subcarrier mode, as the link degrades, impulse or "popcorn" noise appears suddenly and grows until it covers the astronauts' voices.
The ground station generated a pseudorandom noise (PN) sequence at 994 kilobit/s and added it to the baseband signal going to the PM transmitter.
However, an orbit determination program can find the unique spacecraft state vector from range, range-rate (relative velocity) and antenna look angle observations made by one or more ground stations assuming purely ballistic spacecraft motion over the observation interval.
A wideband frequency modulation system provided improved signal-to-noise ratio owing to the capture effect.
[citation needed] In August 1971, radio amateurs Paul Wilson (W4HHK) and Richard T. Knadle, Jr. (K2RIW) heard voice signals from Apollo 15 while it circled the Moon.
The lasting engineering influence of the USB is that almost every human mission in space has had a unified microwave communications system of some kind.
