Mission description

GOES 8 is the eleventh in a series of NASA-developed,NOAA operated, geosynchronous and operational spacecraft. The triaxis-stabilized spacecraft carries

1. an Imager and Sounder system to provide visible and infrared images of cloud cover, and to determine atmospheric temperature and water vapor content at various levels;
2. a meteorological data collection system to relay processed data from central weather facilities to regional stations equipped with APT (Automatic Picture Transmission) and to collect and retransmit data from remotely located Earth-based platforms;
3. a Space Environment Monitor (SEM) system to measure proton, electron and solar X-ray fluxes and magnetic fields;
4. a Search and Rescue (SAR) system to detect and relay distress calls from land and ocean; and
5. a WEFAX (weather facsimile) system to disseminate weather information to the user community via fax.

The cylindrically shaped spacecraft measures 190.5 cm in diameter and 230 cm in length, exclusive of a magnetometer that extends an additional 300 cm beyond the cylindrical shell. The imaging telescope is mounted on the equipment shelf and views the Earth through a special aperture in the side of the spacecraft. The solar array of 1,057 W supplies two nickel-cadmium batteries of 12 Ah each. The CCSDS (Consultative Committee for Space Data Systems)-compliant telemetry is in real-time at 2.0 kbs through S-bands. The eventual parking longitude of the spacecraft will be over 75 deg W.

Detector description

The implemented data set consists of averaged SEM measurements.

The energetic particle sensor consists of three independent detectors:

1. EPS telescope;
2. Dome Assembly; and
3. High Energy Proton and Alpha Detector (HEPAD)

The EPS telescope operated on the dE/dX - E mode, each of the detectors being a surface barrier semiconductor; pulse height analysers could identify a particle either as a proton or as an alpha, besides binning them into narrower energy ranges. The Dome detector carried three separate windows of differing thicknesses, behind which lay a pair of 1500 micron thick surface barrier silicon detectors. Outputs from this three pairs of detectors passed through pulse height analysers to provide counts in narrower bands. HEPAD is a Cerenkov counter, backed by pulse height analysers. Over all, there were 11 energy channels for protons, eight for alpha particles and one for electrons of energy > 2 MeV. However each such channel carried nontrivial contamination by other species. The counts from each of the 20 channels were accumulated for a few seconds (3 to 12 seconds, depending on the channel) before sampling the accumulated total for telemetry. There were also saturation limits to the level of accumulated countsm varying from 1,200 to 25,000 counts, depending upon the channel. The proton and alpha channels covered the energy range of several hundred keV to several hundred MeV.

The X-ray monitor consisted of two ion chambers, mounted behind a slim rectangular field-of-view (48 deg x 3 deg) collimator made of lead-lined aluminum. The chamber for the lower wavelength band of 0.05 to 0.40 nm was filled with Xe-He mixture with an entry aperture made of 20 mil Be sheet. For the other band, 0.1 to 0.8 nm, the gas was Ar-He mixture and the aperture was a 2 mil Be. The treshold sensitivities were 10^-12 J cm^-2 s^-1 for the lower wavelength band, and 10^-11 J cm^-2 s^-1 for the higher band; each had a dynamic range of four decades. Entry of charged particles was prevented by the strong magnetic field located at the chamber windows.

The magnitude and direction of the magnetic field are measured by two redundant Schonstedt triaxial magnetometers located on a boom 3 m, and 2.7 m away from the spacecraft body. The electronics are located inside the body. The X, Y, and Z component signals from the three axes are digitized by a 16-bit converter, at a sampling rate of 0.512 s. The sensitivity is 0.1 nT, and the range +/- 1000 nT. After temperature correction, and before (undescribed) stray-field correction, the accuracy is at about 1 nT level.