Making the data intelligible

The ground tracking and data acquisition stations did not possess equipment to convert the electrical signals into human-readable form. The first opportunity for examination of the data quality and content occurred at JPL for the Explorer I data and for the Explorer III low-power data. The Explorer III high-power data were first examined at NRL. Those activities, as explained earlier, were limited to extracting engineering data and making a cursory check on the operation of the scientific instruments. All further processing and scientific analysis for the cosmic ray data were done at the University of Iowa.

The continuously transmitted data The data arriving at our Iowa City laboratory were processed and displayed as paper strip-charts, from which our data clerks could calculate the GM counter rates. Although those arrangements were archaic when ex­amined after the passage of 50 years, they were standard practice then. The equipment and the procedures were a direct outgrowth of our experience with the balloon, rocket, and rockoon data during the early 1950s.

The ground processing equipment for the continuously transmitted data from Explorers I and III began with an Ampex tape recorder that read the data from the tapes received from JPL. Its output fed a bank of filters and discriminators that provided outputs that mimicked the signals that had been generated by the sensors on the satellite. Those outputs were converted to inked traces on the continuously moving paper charts.

Figure 11.4 shows the equipment setup as it existed in late 1958. The seven-track Ampex tape playback unit in the second rack from the left had been added by that time. The camera recorder extending from the panel on the far right was installed to handle the data from the onboard recorder in Deal II. The full equipment lineup

310

shown here was also used during that summer to process the Explorer IV data, as detailed in Chapter 14.

An example of one of the charts made to show the Explorer I high-power data is shown in Figure 11.5. Channel 4, carrying our cosmic ray data, displayed a complete cycle (positive, to negative, and back to positive) for every 32 particles that had been detected by the GM counter.

This figure also shows the cylindrical shell and transmitter temperature data from channels 1 and 2, respectively, and the micrometeorite microphone data on channel 3. The engineers at JPL read a similar chart to determine that the shell temperature (in this sample) changed from 22.5 to 21.0 degrees centigrade, while the transmitter temperature remained steady at 34.5 degrees. Also from the comparable JPL chart, the AFCRC scientists determined that the microphone did not register more than three hits during that pass, since no transition occurred in the output of the factor-of-four scaler that followed the microphone.

Similar charts were produced for the data from the low-power transmitters on both Explorers I and III. On those charts, channels 1,2, and 3 displayed the front cone skirt temperature, front cone tip temperature, and number of severed micrometeorite grids,

FIGURE 11.5 A portion of a SUI paper strip-chart displaying data from the high-power transmitter in Explorer I. These data were recorded on 4 February 1958 at the Microlock station at Patrick Air Force Base, Florida. The time trace at the bottom of the chart indicates that this segment started 14 seconds before 0241 LIT and covered a total period of 111 seconds. The two vertical lines represent the approximate beginning and end of usable cosmic ray data from that pass. (Courtesy of the University Archives, Papers of James A. Van Allen, Department of Special Collections, University of Iowa Fibraries.)

OPENING SPACE RESEARCH

respectively. The low-power system cosmic ray data and time markers were identical in form to those shown in Figure 11.5 for the high-power systems.

Since we used the temperature measurements read by JPL, and had no responsibility for the micrometeorite data, our focus was fully on the channel 4 cosmic ray data and time markers. As a rule, we processed the data from only one of the transmitters for each station pass. In the few cases where both signals were recorded (primarily at the JPL and PAFB stations), we used the better of the two.

The Explorer III onboard recorded data Handling the data from the recorder in the Explorer III satellite instrument package presented a completely different challenge. For a typical operational sequence, the ground station operators prepared for a pass by pretuning their receivers, pointing the antennas in the direction where the satellite was expected to appear, and starting the ground recorder for the low-power signal ahead of time. Arrival of the satellite on the horizon was announced by the appearance of an initially noisy signal from the low-power transmitter. Of course, there was no signal from the high-power transmitter, as it had yet to be turned on.

As the satellite rose above the horizon, the signal from the low-power transmitter became stronger and clearer. The antennas for both the low – and high-power signals tracked the satellite as it progressed across the sky. When the antennas reached a reasonable height above the horizon, and as the low-power signal became sufficiently clear, the operator started the ground recorder for the high-power signal and then transmitted a command to the satellite to turn on the high-power transmitter. If all had been set up properly, the command resulted in the immediate appearance of a signal from the high-power transmitter. After two seconds, the onboard tape recorder began its playback. For occasions when the onboard recorder had stored a full orbit’s data since its last interrogation, its readout took about six seconds. When the tape readout was complete, the transmitter turned off, and the onboard system reset itself to record the next orbit. Thus, the entire readout operation occurred typically within a brief eight-second interval.

The ground station tapes were annotated during recording with voice announce­ments and timing markers, and handwritten comments were entered by the operators in the logs.

The pulses during the brief burst of data appeared at a rate of about 1000 per second. The task in the Cosmic Ray Laboratory’s processing facility was to pick out the burst of information for each pass and to display that information in usable form. Two techniques were employed.

The first, valuable for a quick look at the general form of the data, was to record the signal on another moving pen strip-chart recorder, similar to that being used for the low-power data. Since the pulse rate was somewhat beyond the frequency response of the chart recorder, the traces were distorted, and it was not possible to count the

CHAPTER 11 • OPERATIONS AND DATA HANDLING 313 FIGURE 11.6 A sample of the data from the Explorer III onboard tape recorder, as produced by the film recorder in the data-processing equipment at Iowa. This portion of a continuous 70 mil­limeter filmstrip contains a one minute segment of the satellite recorder’s data. Since the satellite’s orbital period was about 116 minutes, the filmstrip for a data dump from a full orbit was about 116 times this length. This example is completely noise-free, a rare occurrence—most readouts contained varying amounts of noise superimposed on the traces. The occurrence of only a sin­gle transition of the instrument’s scaler during this one minute period indicates that the raw GM counter rate was very low at that time. The normal in-orbit cosmic ray counting rate produced a missing pulse about once every seven of these one second pulses, thus, this example was probably made during ground testing. The author was unable to locate any still-existing filmstrips of original Explorer III flight data.

individual pulses from that source. The charts did convey, however, a very distinctive pattern to trained data readers. As it turned out, once the data blanking due to the high-intensity radiation was understood, those quick-look charts were invaluable in delineating the extent and location of the radiation belt, as described in detail in the next chapter.

The second method for reading the Explorer III onboard tape recorder data used a special camera that had been constructed for the purpose. That camera is shown on the far-right rack of equipment in Figure 11.4. It displayed the received signal on a small cathode ray tube, which had a frequency response far beyond that needed to follow the data traces. Seventy millimeter film moved vertically past the horizontal trace on the cathode ray tube. Thus, the pulses were arrayed along the length of the film, as illustrated in Figure 11.6.