Moonport: A History of Apollo Launch Facilities and Operations|
Changes in the Telemetry System
The success of the Saturn V flights depended in large part on the performance of the telemetry system. A characteristic of all spacecraft programs, telemetry transmitted prelaunch and flight performance data from the vehicle to ground stations. From the start of the planning for Apollo, NASA realized that many more varied and sophisticated demands would likely be placed on the system. The development of launch operations at KSC was, in part, conditioned by those demands [see chapter 16-5].
For prelaunch operations, the ability of the launch vehicle to check itself out was limited by requirements for ground support. A digital computer in the Saturn V instrument unit was primarily intended for guidance and navigation. It had triple redundancy throughout, except in memory and power sources, and its self-check capability was limited primarily to flight. Support for prelaunch operations came from the digital data acquisition system located in each stage. Tailored to the specific needs of its stage, this system transmitted data either through the data link or by means of pulse-code-modulated radio transmissions. The radio link could be used either on the ground or in flight.22
The Saturn V launch vehicle had 22 telemetry links carrying more than 3.500 instrumentation measurements during flight. In prelaunch checkout each link and instrumentation channel was tested to assure operation within specified tolerances. Since the vehicle instrumentation system was used to acquire data during tests on other vehicle systems (such as pneumatics and control), frequent prelaunch checks of the instrumentation were required.23
The S-IC contained six very-high-frequency links, including the single-sideband-frequency-modulated telemetry system, one of the Saturn V components that had evolved throughout the launch vehicle development program. Because data during staging might be concealed or lost due to the effects of the engine exhaust, a tape recorder was included in the stage to collect that information, which was subsequently recovered by playback over radio. Range-rate data for the tracking of the vehicle was provided by the offset doppler transponder in the stage. Two other telemetry links used ultra-high-frequency receivers for range safety purposes. If the safety officer on the Cape issued a destruct command to these receivers, they would trigger the explosive network.24
The second stage had systems similar to those on the S-IC, but had one less single-sideband link. The S-IVB (third stage) carried no tracking transponders; otherwise, its telemetry equipment was identical to that of the first stage. The instrument unit carried an offset doppler, an Azusa (or Mistram) transponder, two C-band beacons, and a command and communication system. It had no range safety receivers.25
Long before the first Saturn V flew, the configuration of the vehicle allowed the use of either the Mistram or Azusa tracking systems, but not both at once. To reduce the complexity of the system, Phillips in 1965 directed that Azusa be used on future Saturn flights. Real-time support at the Cape would be required at least through the AS-503 mission. Experience in tracking early Saturn vehicles indicated a need for only one beacon, and some viewed even that as possibly unnecessary. It was later confirmed that the Saturn V was large enough to reflect enough radar energy to be visible on ground indicators to the limits of safety responsibility. Though a beacon might not be required for tracking purposes, range safety personnel considered it desirable.26
To receive telemetry from its vehicles, NASA maintained three ground networks. One of these, the Manned Space Flight Network, was under the operational control of Goddard Space Flight Center, Greenbelt, Maryland, during Apollo missions. In order to operate effectively for the lunar landing program, the system had to be able to control the spacecraft (both the command and lunar modules) at lunar distances. While the equipment had been adequate for earth-orbit missions, the greater distances, as well as the complexity of Apollo, led to the introduction of the unified S-band system.27
The term S-band derived from the period of the Second World War when letters were used to designate bands of frequencies. The band selected for Apollo lay between 1,550 and 5,200 megahertz. For use with its unmanned space probes, the Jet Propulsion Laboratory (JPL) had developed equipment that operated on these frequencies. A useful feature of the JPL equipment was the combination of several radio functions into a single transmission from only one transmitter to a given receiver. For Apollo, these functions included tracking and ranging; command, voice and television communications; and measurement telemetry. The versatility of the system was inherent in its structure.28
For the lunar mission the unified S-band offered the twin advantages of simplicity and versatility. The line-of-sight signal lost little of its strength when it passed through the atmosphere, and transceiver and power supply equipment could be relatively small. In providing direct communications between the spacecraft and ground stations, the unified S-band worked equally well in near-earth operations or circling the moon.
Apollo's tracking system required close, continuous communication among the major centers and the Manned Space Flight Network. This was accomplished by means of digital data, teletype, and voice links which were the responsibility of the NASA communications system centered at Goddard. A combination of land lines, undersea cables, high frequency radio, and satellites linked more than 100 locations throughout the world. For Apollo, the system had to be augmented. Major switching centers ensured maximum sharing of circuits, while giving Houston priority for real-time data during Apollo missions.29
During Apollo operations, the three manned spaceflight centers were connected outside the Goddard system by two links - the launch information exchange facility and the Apollo launch data system. Operated by Marshall during launch operations, the former was primarily an information transfer link between Huntsville and KSC with connections to Houston. It carried real-time telemetered data, closed-circuit television, facsimile, classified typewriter, voice, and countdown information. The Apollo launch data system was the primary information link from KSC to Houston. It had four independent subsystems that handled telemetry, television, countdown and status data, and launch trajectory data during prelaunch and launch operations. By using the Apollo launch data system, personnel in Houston could conduct closed-loop tests of the spacecraft while it was at KSC. During powered flight, the system transmitted trajectory data from the impact predictor for the information of the flight director at Houston.30
The Apollo program significantly increased the tracking and data acquisition requirements for KSC and the Air Force Eastern Test Range. To ensure uniformity, the Office of Tracking and Data Acquisition, NASA Headquarters, was designated in August 1964 the "single point of contact" with the Department of Defense for such coordination. Although heavily involved in the development of the unified S-band system for Apollo operations, the Jet Propulsion Laboratory and Goddard were directed to support the planning and operations.31 The agreement that resulted between NASA and the Defense Department emphasized colocation of KSC and Air Force Range facilities whenever possible "to achieve a maximum of mutual assistance, to avoid unwarranted duplication, and to realize economies where practical and consistent with mission requirements. . . ."32 To support Apollo, range facilities needed considerable modernization. During 1965 about 85% of the existing Air Force tracking equipment was modified. Over three years, the cost exceeded $50,000,000, including the updating of telemetry stations downrange as well as at the Cape.33
The entire Apollo tracking and data acquisition network, including ships, planes, and unified S-band ground stations, was integrated with the Manned Space Flight Network between November 1966 and June 1968. The AS-202 mission in August 1966 provided the first test under actual operating conditions. By the launch of Apollo 9 the new system was operational at stations in Texas, Mexico, Ascension Island, the Canary Islands, Bermuda, Spain, Hawaii, Australia, Wales, and California.34
There was no major change in tracking and data acquisition comparable to the introduction of the mobile concept. The primary alteration in tracking was the increasing sophistication of the hardware.35 From early Saturn I missions through Apollo 9, development of hardware had tended to proceed steadily, dependent largely upon launch vehicle requirements. At the same time, less and less direct control over telemetry was allowed to KSC. In this respect, the attempt of NASA to spread the R&D among several centers had led to an unexpected constraint upon launch operations at LC-39. In the end, the Saturn V was measured and tracked by a telemetry system largely outside the control of KSC.