ISEE 1

Telecommunications Description

Two solid state S-band transmitters radiating continuously and on separate frequencies were used. Transponder A downlinked scientific analog data on a frequency of 2264.8 MHz at 2.5 watts and transponder B downlinked pulse coded modulation (PCM) data on a frequency of 2215.5 MHz at 2.5 watts. PCM data could be switched to transponder A or B, but analog data could not be switched to transponder B. Ranging was accomplished through either unit. Transponder B had an ``auto turn-on after eclipse'' feature. The choice of S-band was primarily dictated by the need for very accurate ranging and coherent Doppler techniques that required an accuracy better than 1 degree and 1 km. Uplink frequencies were 2085.5 MHz for receiver A and 2040.1 MHz for receiver B. Receiver A was the prime receiver.

The data handling system gathered the scientific and engineering data from all systems in the spacecraft and formatted these into a PCM stream for transmission. The system provided all timing and control signals required for this task. It consisted of a Data Handling Unit (DHU), an analog subplexer with two analog subcoms, and a digital subplexer with one digital subcom. The DHU and subplexer were similar to units flown on the International Ultraviolet Explorer (IUE) spacecraft. The DHU had two fixed formats and two programmable formats which could be altered by ground command. There were two DHU's flown on the spacecraft which could operate at variable rates, selectable by ground command. The DHU's on ISEE 1 operated at 4096 and 16384 information bits per second. (Since the data were normally convolutional coded PCM, the telemetry bit rates were twice the information bit rates). The high bit rate was used 20% of the time - one in five orbits.

Twelve Spacecraft Tracking and Data Network (STDN) stations provided launch and early orbit support. From launch until February 1, 1985, all support of the ISEE 1 and ISEE 2 spacecraft was accomplished through the STDN facilities. From February 1, 1985, through the life of the missions, this support was shared by the Deep Space Network (DSN). Telemetry, command, tracking, orbit data processing, and scheduling functions were provided by DSN stations at Madrid, Canberra, and Goldtone using 26-meter antenna subsystems. (These stations replaced the STDN stations at Madrid, Orroral, and Goldstone). All PCM data from both ISEE 1 and ISEE 2 acquired by the stations were transmitted via Nascom to the Information Processing Division (IPD) at the Goddard Space Flight Center (GSFC) within 24 hours after the data were received.