Fundamental Technologies Ulysses HISCALE Pages

4.7.2 Sun Sensor Electronics

The sun sensors are a development of existing sun/Earth albedo detectors, but changes were required to incorporate the following specifications of Ulysses:

• Incorporation of a peak percentage threshold detector to cater to a S/C sun distance varying between 1 and 6 AU.
• High accuracy required at unusually small solar aspect angle, 1.25 degrees.

The electronics are located on both the sensors and on the Attitude Measurement Electronics (AME) unit.

Sensors Electrical Operation: Figure 4.50 gives a simplified schematic block diagram of the sensor electrical operation.

Figure 4.50 Sun sensor electronics

The output of the photocell is passed through a preamplifier with two gain settings. The "high gain" provides an increase of a factor of 5 over the "low gain" setting.

The output of the preamplifier is passed through a peak detector and threshold circuit. This detects the peak signal and sets a threshold at 50% of the peak. A small constant voltage with sign dependent on the output level (high/low) is added to the preamplifier output to provide a hysterisis effect. This prevents improper switching in the case of low rate of change of the light input. The modified preamplifier output is compared to the 50% level and a logic 1 or 0 generated.

The "peak step down control" logic slightly discharges the peak detector at the leading edge of each sun pulse, insuring that the peak detector follows slow changes in the input signal.

The "delayed peak reset" logic discharges completely the peak detector if a fixed period elapses without sun pulses. This is used in the event of failures or significant changes to the input signal as when changing gain.

Next: 4.7.3 AME Sun Sensor Conditioning Electronics

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