ISEE-3 Telemetry Systems

Source: NASA

The data handling system gathers the scientific and engineering data from all systems in the spacecraft and formats these into a PCM serial stream for transmission. The system provides all timing and control signals required for this task. It consists of a data multiplexer unit (DMU), one or two subplexer units, and a mass storage unit. The first two items are identical to units flown on the International Ultraviolet Explorer (IUE) spacecraft, while the last, the mass storage unit, is unique to ISEE.

The DMU has two fixed and two programmable formats which can be altered by ground command. There are two DMUs on the spacecraft which can operate at variable bit rates, selectable by ground command, of 2048, 1024, 512, 256, 128, and 64 information bits per second. (The telemetry bit rate is twice the information bit rate). The subplexer is used to handle data input from the scientific or spacecraft instruments and for generating additional timing signals. It acts as a buffer to the DMU and is used when the number of inputs from the spacecraft exceeds the DMU's capacity. The mass storage unit is used for a number of scientific instruments that require rapid sampling over short intervals of time.

The data are temporarily stored in the mass memory until it is telemetered. A mass memory is employed in place of individual memories in each instrument to reduce cost. The functions of telemetry, command and ranging are handled by two identical S-band transponders. On ISEE-3 one transponder was designed to operate continuously, transmitting PCM telemetry. The second transponder is used only for ranging; however, the PCM telemetry can be switched to the second transponder in the event of failure of the first transponder. The choice of S-band over VHF for ISEE-3 was dictated primarily by stringent telemetry downlink requirements such as the long range and a view angle near the solar direction.

The uplink frequencies are 2041.95 and 2090.66 MHz; the downlink frequencies are 2217.50 and 2270.40 MHz. The transmitter output power is 5 watts. Both transponder transmitters can transmit through the medium gain antenna simultaneously, although they will radiate with opposite circular polarizations. This antenna has a 9-dB gain and a vertical beam width of 18 degrees. From initial launch, the ISEE-3 mission was supported by the Ground Spaceflight Tracking and Data Network (GSTDN) with orbit determination (OD) performed at the Goddard Space Flight Center (GSFC).

At the comet encounter distance of 70 million km from earth, the spacecraft was beyond the range of the GSTDN. This necessitated the transfer of support, in early January 1984, to the Deep Space Network (DSN) which is operated by the Jet Propulsion Laboratory (JPL). Responsibility for OD was given to the institution providing the tracking support. However, during December 1983, GSFC and JPL carried out parallel tracking and OD support. Responsibility for maneuver analysis and design, along with trajectory product generation for investigator support, remained at GSFC for the duration of the mission. After January 6, 1984, tracking and data acquisition of ICE was limited to the DSN 64-meter subnet. However, the first year of interplanetary cruise was supported by only one antenna, DSS 63 in Spain.

Beginning in mid-December 1984, occasional passes were provided by DSS 14 at Goldstone. In mid-January 1985, DSS 63 went down for modifications, leaving only DSS 14 to provide tracking support. After June 1985, both stations supported the mission through the comet encounter. The ISEE-3/ICE bit rate was nominally 2048 bps during the early part of the mission, and 1024 bps during the Giacobini-Zinner comet encounter. The bit rate then successively dropped to 512 bps (on 9/12/85), 256 bps (on 5/1/87), 128 bps (on 1/24/89) and finally to 64 bps (on 12/27/91).

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