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This work was carried out in the frame of development and qualification of key equipments for application in payloads or platform (sub) systems to enhance their capabilities or to match specific opportunities in the multimedia domain. It aimed at the development and qualification of a Coarse Sun Sensor (CSS), suitable for the Attitude and Orbit Control Subsystem of telecom-satellite platforms for multi-media applications, like the Spacebus-4000 platform. The sensor is a European product, built with components free of export restrictions imposed by US ITAR legislation.
The Coarse Sun Sensor (CSS) from TNO - sometimes called Sun Acquisition Sensor (SAS) - is a pyramid sun sensor, which provides 2-axis coarse sun position information in the form of analogue voltage signals in a near hemispherical FOV.
The sensor consists of an aluminium structure with a truncated pyramid in the centre of shadowing rims. On the four faces of the pyramid, dual chip detector devices are installed, such that their lines of sight are inclined with respect to the boresight viewing direction and at 90 degrees apart (see pictures). In this way the four dual-detectors span a hemispherical FOV, constrained by the shadow from the outer rims of the structure. These rims can be shaped to specific mission requirements by strap-on baffles, attached to the main structure of the sensor. The detectors act as optical sensing elements, sensitive to sunlight. The dual construction leads to built-in redundancy of the sensor outputs.
The key issue of the CSS project was to produce a fully European Coarse Sun Sensor, based on an European detector. Furthermore, the qualification program of the European detector included a 10,000 thermal cycles test between extreme temperatures of -90°C to +125°C, which makes the Coarse Sun Sensor equipped with these detectors compliant with the requirements of the long-duration major European telecommunication and multi-media satellite platforms. This is further supported by the use of epitaxial silicon material for the detector, which makes the sensor radiation hardened.
From technical point of view, the extension of the detector thermal qualification -both in terms of temperature range and number of cycles (several thousands)- and its radiation hardness makes this new CSS product readily suitable for most demanding missions in the telecom market.
From strategic point of view, this fully European version of the Coarse Sun Sensor helps European commercial (telecommunication, navigation, earth observation) satellite manufacturers to build platforms to be operated and launched without the risk of incurring in non-EU exports regulations.
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).
The development and qualification of the European dual chip detector has been successfully performed by OSI Norway in 2008 under TNO specification. The newly developed detectors have then been integrated by TNO in the Coarse Sun Sensor EQM for equipment level qualification testing to comply with requirements obtained from Spacebus-4000 and Alphabus mission types. This test program was successfully completed by TNO early in 2009 and the test results have been successfully reviewed at system level by Thales in spring the same year, confirming the compliance of this CSS with both the SB-4000 and Alphabus multimedia platforms.
The program has been successfully completed early 2009. The Final Presentation was organised at Estec in October 2009.