The Advanced Geosynchronous Studies Imager (AGSI) system design combines the latest available technologies into an instrument design concept which could deliver the improved performance sought by the National Weather Service at NOAA and meet NASA earth system science goals in a joint program. The instrument could cover the Earth disk eveiy 15 minutes with subsatellite point resolution from 1/2 kilometer in the visible to 2 kilometers in the long wave infrared. Simultaneously, it could provide coverage of a 3000 x 5000 kilometer region in 5 minute intervals and 30 second updates of a 1000 kilometer square region containing a weather system of interest. We found that performance margins could be improved even as we drove the design iterations with emphasis on reducing the mass. Scan speed was chosen by maximizing perfonnance while trading off the acceptable impact on the total system. The resulting 18-channel design could deliver vastly improved performance over the present GOES without great increases in mass or volume, while still paying close attention to control of development costs and impact on the host spacecraft. The design could be adapted to changed requirements or descoped to have lower data rates and fewer channels.
The GOES Sounder is similar in appearance, size, and design to the GOES Imager. It provides independent radiomen-ic sounding of the atmosphere from GOES-East and GOES-West (GOES 8 and 9), which was accomplished on previous GOES by a single instrument with responsibility split between the functions of imaging and sounding. With 19 channels ranging from 0.70 m to 14.71 m, the Sounder probes the atmosphere to measure radiance at different depths (altitudes) while also monitoring surface and atmospheric temperatures, ozone, water vapor content, and cloud cover by means of the calibration of radiance data at selected wavelengths.Like the Imager, the Sounder scans the full earth, and can be commanded to scan local regions of interest. The scan mirror steps across the disk of the earth in synchronization with a dead zone on a filter wheel. The filters defme 18 of the channels as they rotate on the wheel in the path of light split into three beams traveling to detectors divided into longwave, midwave, and shortwave arrays. For visible (0.70 m) sounding of cloud cover, energy is split off ahead of the filter wheel and passed to the 19th channel and also to star sensing detectors used for instrument navigation.
In any earth orbiting system designed for the sensing of inputs from remote sources, it is of prime importance to identify the location of the remote source. This implies a priori knowledge of the pointing direction of the instrument. In some applications, at the time of input sensing, accuracy must be within a few seconds of arc. Real-time sensing of perturbations to pointing of an instrument in orbit may be needed for attitude determination or control. These perturbations may occur over a large bandwidth.
The low -flying MAGSAT spacecraft, launched October 30, 1979, included a Vector Magnetometer to accurately map the magnitude and direction of the magnetic field of the Earth. Calibration of the magnetometer included arc -second precision determination of the relative orientations of the three sensor axes in a coordinate system defined by optical references. This determination began with laboratory measurements of the relative alignments of optical components mounted with the magnetometer. The actual calibration procedure then consisted basically of accurate and repeatable positioning of the Vector Magnetometer within a unique magnetic test facility which nulls the earth's magnetic field, then generates magnetic fields of various orientations and strengths. Analysis of the magnetometer sensor outputs together with the position and alignment data then gave the axes orientations. We used precision theodolites and methods related to surveying techniques to achieve the accurate positioning and optical component alignment measurements. The final calibration accuracy exceeded results previously achieved in the facility.
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