Calibration

Broadly speaking the full calibration data set is used to determine the following:

1)    energy of accepted electrons during each measurement
2)    conversion from number of measured electrons to number of arriving electrons (taking into account the < 100% efficiency of the detector, and the variation of that efficiency with the energy of the measured electrons, and also the differences between the 12 detectors per PEACE sensor)

The onboard calibration parameter set is a reduced set of the full calibration data set used in ground data analysis. This is necessary for reasons of onboard resources. Inevitably, some of the calibration knowledge available to us is not properly represented onboard, for example we can only use one energy dependent efficiency curve to represent all the 12 anodes, though we know the anodes generally have somewhat different behaviour in practice.

Part of the calibration data set has been well measured in pre-launch experiments at MSSL. Other information, such as a scaling factor for the detector efficiencies, is best determined using comparisons of electron density measured in-orbit with instruments using other techniques (e.g. WHISPER) in plasmas of electron densities that can be well measured by both PEACE and WHISPER. To guard against a change in the detector sensitivity with age, we need to make periodic assessments of the detector performance. Thus it is inevitable that new calibration data would be uplinked at stages during the mission (see below).

 Onboard Calibration Status

The accuracy of the onboard moments calculations depends on the reliability of the onboard calibration information.

The initial calibration set was based on analysis of ground testing data. Certain parameters cannot be well determined in ground tests and were improved following in-flight calibration work. Further refinements may follow.

PEACE Onboard Calibration Set 1:
Launch to 09 June 2001               Based on ground based calibration tests; errors present

PEACE Onboard Calibration Set 2:
09 June 2001 to 11 Sept 2001     Revised Geometric Factors

PEACE Onboard Calibration Set 3:.
11 Sept 2001 to present               Revised Geometric Factors and Energy Efficiencies, a significant improvement.

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By Andrew Fazakerley
Last updated on 14/01/02  by  Andrew Fazakerley