Automating Telemetry Operations

The move from the Redstone to the Saturn era brought a pressing need to automate data reduction.* The Saturn I's telemetry, the primary source of postflight test data, represented a three-fold increase over previous rockets and the Saturn V would be an even greater jump [see table 1, chapter 2-1]. The increase posed a problem of space. Continued reliance on analog strip charts would have forced Saturn V engineers to review thousands of feet of chart after each launch. Time was a second consideration. In September 1961 Fridtjof Speer, chairman of Marshall's Saturn System Evaluation Working Group, expressed concern about possible delays in the delivery of postflight data. Although LOD had agreed to submit telemetry data within 12 hours, Speer wanted backup blockhouse records (strip charts and event recorders) within 24 hours. He asked Debus to "exploit every possible course of action to satisfy this requirement."21

A month after Speer's letter, Dr. Rudolf Bruns's data reduction team pioneered the use of computers in Saturn I launch operations. Digital computers offered two advantages. During a launch the computer could record incoming telemetry data on a magnetic tape and subsequently process compact printouts in a relatively short time. The computer, supported by peripheral equipment such as cathode ray tube consoles, could display critical information in real time.22

The Flight Instrumentation Planning and Analysis Group set up a Burroughs 205 computer alongside the telemetry station in hangar D prior to the SA-1 launch. A tarpaper shack housed the computer - a far cry from facilities the team would enjoy in a few years. Despite its primitive surroundings, the 205 provided guidance data and some measurement reduction in real time. A General Electric computer replaced the Burroughs machine on SA-3; the GE computer's solid state circuitry and core memory provided a faster sampling of Saturn telemetry.23

During the last hours of the SA-3 countdown, the launch team periodically tested telemetry transmitters and the data reduction computer. The team "dumped" the telemetry data from the GE computer at several predetermined times, taking a quick look at the printout to see if the measurements were within calibration. At T - 30 minutes the computer began processing data from the Saturn's ten commutators. The GE 225 took approximately ten seconds to complete one sample of the Saturn's ten telemetry links; most of that time was taken up by the telemetry station switching device, switching from one commutator to the next. As the 225 printer could not match the computer's calculating speed, the real-time printout listed only the out-of-tolerance values. The GE 225 processed telemetry data until the signal faded a few minutes after launch. During postlaunch activities, the telemetry station rewound its analog tapes and played them back for the computer. The digital magnetic tape data, produced by the playback, were then converted in another computer process to specific engineering units (e.g., degrees Celsius), which were displayed on a printout or a Stromberg-Carlson 4020 plotter.24

Efficient data reduction depended on parallel advances in the telemetry station's digitizing system, the equipment that converted the Saturn's analog telemetry into a digital message for the GE computer. The earliest digitizer employed one analog-to-digital converter and one synchronizer. Each of the ten commutators on SA-3 fed its measurements (27 each for the low-speed commutators and 216 for the high-speed commutator) into a subcarrier oscillator. The oscillators sent the signals out over the launch vehicle's six telemetry transmitters as a pulse-amplitude-modulated wave. The receiver in the ground telemetry station removed the subcarrier and directed it to one of three discriminators where it was demodulated. The analog-to-digital converter changed each signal voltage from a magnitude to a pulse. A 0.6-second delay in synchronizing the converter's switch from one commutator to another limited the digitizer's output to the GE 225. Due to the switching delay, the computer received approximately one measurement of every 120 that came from the Saturn's high-speed commutator.25

Work on a faster digitizing system began in mid-1962. An analog-to-digital converter was added for each vehicle commutator; a parallel programmer-addresser generated a 12-bit address word to identify the data. A digital scanner, essentially an electronic 16-position switch, scanned the outputs of the data channels (convertor and addressor), transferring new data to a core memory. The memory stored each measurement according to its digital address and provided the computer with random access to any data. Although the telemetry team experienced problems interfacing the scanner and core memory, the specifications were ready by November.26

The telemetry scanning and digitizing system was added to a GE 235 computer for the block II series of the Saturn I program. The GE computer, given immediate access to all data through the digital scanner and core memory, recorded data in real time on a magnetic tape. Since the 235's printing lagged behind its computations, real-time display was still limited. The 235's other functions included: separating data by program (S-I, S-IV, instrumentation unit, and spacecraft) and measurement, converting measurements to engineering units, arranging data for use on the 4020 plotter, and comparing engineering units versus time on printouts and 4020 plots (microfilm or hard-copy graphs). Within two hours of the SA-7 launch, all the 4,020 plots had been processed. The total data reduction program was completed eight hours later.27

Measuring techniques needed to be improved to keep up with the advances in digitizing and telemetry reduction. Automation of measurements during checkout of the Saturn vehicle was begun in March 1962, using an IBM card system. Punched cards, placed in a card reader, selected the appropriate channel (and relay if calibration was necessary). The card reader compared the signal returning from the launch vehicle's measuring device with data prepunched on the card and gave a "go, no-go" evaluation.** Following experimentation during a checkout on LC-34, the system was installed in LC-37's measuring station for the block II launches.28

As the automation plans gained momentum, Debus expressed concern about their impact on LOC's relations with the Air Force. On 21 February 1962 Debus penned a brief note to Gruene's weekly report:

Hans: One day we have to start an analysis of what this entire automatic checkout with computers will mean in our countdown-and-test interfaces with the range! For instance: timing, on-off commanded to the Range, TM [telemetry] receiving . . . and a host of other interactions. . . . Does it still make sense to plan a "joint" TM station. . . ?29

* Data reduction means the transformation of observed values into useful, ordered, or simplified information. With telemetry it could involve transferring an analog electrical signal onto a brush recording or eliminating unnecessary portions of a message (e.g., the address), restructuring the data, and directing it to various users.

** A go, no-go indication told the operator whether a device was functioning properly, without indicating how far out of tolerance it might be or what was wrong.

Email darren@apolloexplorer.co.uk