Explorer III operation

The network of ground stations was the same for Explorer III as it had been for Explorer I. The major difference in operation was that the high-power system was dedicated to telemetering the stored data from the in-orbit data recorder. Since the high-power transmitter had to be turned on from the ground for that purpose, and since only the Minitrack stations possessed command transmitters, high-power system data reception was limited to those stations.

The low-power signal was used in the same manner as for Explorer I, for tracking and for recovering the continuously transmitted data. Although both the Minitrack and Microlock stations had the technical capability for receiving and recording the

OPENING SPACE RESEARCH

low-power signals, it was decided to depend primarily upon the Microlock stations for collecting those data, thus permitting the Minitrack stations to concentrate on commanding and receiving the high-power data. Although the Minitrack stations used the low-power signal to assist in pointing their antennas, they did not record the low-power data.

The Explorer III operation is summarized as follows10:

• As in the case of Explorer I, there were no problems with the Explorer III primary mechanical structure or with its provisions for controlling its internal temperatures.

• Since the type of whip antenna used in Explorer I had been replaced for Ex­plorer III, there was no recurrence of the antenna abnormalities seen in that satellite.

• The low-power continuously transmitting system operated perfectly from launch on 26 March 1958 until 8 May. During 8 and 9 May, the operation of one of the telemetry channels was erratic, and on 10 May, the radio frequency carrier disappeared. Although the carrier reappeared on 14 May, it was without the subcarrier tones. It disappeared for the last time on 21 May 1958. Thus, operation was normal for six weeks of operation, somewhat shy of its two-month design lifetime. The pattern of that cessation is not completely understood but may have been due to the depletion of batteries.

• Operation of the entire scientific instrument for the high-power system, with its onboard data storage recorder, command receiver, and associated electronics, was also perfect during the time that it was operational. It operated perfectly for 44 days, until 9 May 1958, when responses to interrogations became intermittent. Response to interrogations ceased completely on 12 May but reappeared briefly on 21 May. The final response was received on 24 May. The 44 days of normal operation again fell short of the design lifetime by about two weeks but were long enough to satisfy all of the mission requirements. The behavior pattern for the high-power system around the end of life was also not completely understood. It is partly explainable by the large number of command attempts that were made during the satellite’s early life, since energy from the telemetry system batteries was consumed each time a command was received by the satellite, whether the data were observed on the ground or not.

• The fact that erratic operation of the entirely independent low – and high-power telemetry systems began at about the same time led to speculation that some external event may have been involved. However, no abnormalities in instrument temperature or radiation intensity were observed during the days leading up to 9 May, so such an event was probably not a factor.

CHAPTER 11 • OPERATIONS AND DATA HANDLING