The pyramid sun sensor basically is a null sensor, i.e. it is designed to provide the position of the sun with respect to the sensor boresight axis, with an accuracy better than 1 degree of arc (3 ó) throughout all operating environmental conditions and during the whole mission duration. When the sun is acquired on the boresight, the outputs of all detectors on the four faces of the pyramid will be equal and differential signals from devices at opposite faces of the pyramid yield zero output for the sun on axis (see bottom-left part of figure above).
This is why the sensor is named Sun Acquisition Sensor (SAS). While optimized for GEO missions, this CSS sensor can operate in virtually any type of mission (LEO, SSP, MEO, HEO, GEO, interplanetary), though for near Earth missions, the Earth albedo would adversely affects the sensor performance. Summation of detector outputs yield information about sun presence (to prevent earth lock).
The sensor can either deliver differential- and sum- outputs from detector combinations or individual detector outputs. The latter allow the AOCS to input the sensor data to algorithms with which offset sun positions can also be measured with coarse (+/- 2 degrees of arc) accuracy in a large part of the sensor FOV. The algorithms itself are not a deliverable; the sensor hardware comes with calibrated cosine response data from individual detectors.
The sensor main characteristics are summarized in the table below.
||Performance / interfaces budget
|Mass per unit
||approx 210 gram (without strap-on baffles, with rear cover)
|Dimension per unit
||122x129x30 mm3 (inclusive all protrusions from connectors, bonding stud)
||Near hemispherical (precise dimensions depend on baffle geometry)
||Analog, either current mode (0-35 mA) or voltage output (0 to 100 mV).
||nil: CSS is passive
|Accuracy on sensor boresight
||Better than +/- 1° (3 ó) on boresight (throughout mission lifetime), based on simple "balancing outputs" (differentials from detectors at opposite faces of the detector pyramid).
|Accuracy in central portion of the FOV (after correction with ground-cal data)
||Better than +/-2° (3 ó). Data must follow from individual outputs from detectors, applied in algorithms in the S/C AOCS computer. Figures apply for conditions without Earth (or planet) albedo.
|Accuracy beyond central portion of the FOV
||Depends on the solar aspect angle (in steep slope or flat part of cosine response of detector). Errors can be large up to several degrees of arc, particularly for large solar aspect angles.
|Noise equivalent angle
|Impact of albedo
||Sensor is analog and detectors are sensitive to albedo. For orbit altitudes below 10000 km, summed detector output shall be used to discriminate between sunlit and solely earthlit sensor.
|Redundancy and reliability
||Depends on choice for electrical interfacing with the S/C AOCS electronics; maximum flexibility achieved with individual detector outputs. Failure rate 12FIT at 60°C Interface temperature.
||Better than 0.1°, established with fixation holes (no alignment kit required)
||-80°C to +100°C at temperature reference point of sensor
||Detectors are radiation hard (EPI technology) with 300 microns thick cover glass. Tested up to 1x10 to the power of 16 1 MeV electrons normal incidence fluence (equivalent to more than 20 years in GEO).