The TRAC magnetometer was provided as a standard facility used for attitude restitution on all AOUS satellites and was under the responsability of the Soviet project team.
It is classical three axis fluxgate magnetometer with a drive frequency of ~ 7 kHz. The cut-off frequency is ~ 10 Hz on all three axis.
The theory and practice of this type of magnetometers have been extensively worked out (e.g. Primdhal, 1970) and a more detailed description of the actual magnetometer used on board AUREOL-3 may be found in Afanasiev (1969).
The magnetometer was calibrated separately in laboratory before mounting on the satellite and scalar calibration factors were provided for each axis by the Soviet project team.
The S/C system provides the values of the three components of the Earth's magnetic field in the magnetometer frame digitized on 8 bits.
The resulting resolution of +/- 200 nT is obviously not relevant for the measurement of field aligned currents since one aims at a resolution of a few nT compatible with typical magnetic perturbations of ~ 100nT due to auroral or cusp field aligned currents during periods of low auroral activity. To achieve such a performance while taking account of the constraints imposed by the S/C and telemetry systems, a special electronics was developed to process the three analog signal outputs of the magnetometer. This electronics acts as a magnifying lens over successive periods of time during which the largest expected overall temporal variations of the magnetic components are such that the desired resolution of +/- 5nT is achieved using the 8 bit A/D converter available in the TM system.
The principle of operation of this electronics is identical for all three axis and explained for the X axis in the following lines.
The X output signal is sampled and digitized with a 12 bits A/D converter every 43.2 seconds. The resulting digital word obtained at time t0, X0 is sent through telemetry over the form of two 8 bits words, the first one representing bits 0 to 5 and the second one bits 6 to 11. X0 is simultaneously converted to analog with a 12 bits D/A converter and feeds the negative input of a difference amplifier, the positive input of which being the X signal output. During the remaining period of 43.2 seconds the output of difference amplifier provides the variation in time DeltaX(t)=X(t)-X0(t) of the X component relative to the value X0 sampled at time t0. The range of the difference signal is +/- 1320 nT which corresponds to the resolution of +/- 5nT after a 8 bits A/D conversion. This value is compatible both with the desired resolution indicated above and with the amplitude of the quasi DC magnetic noise of the S/C which was observed in flight to be less than 10nT.
Performance specifications and calibration factors produced by the constructor are expected to provide an overall absolute accuracy of +/- 300nT when one takes into account the long term drifts, temperature effects, non linearity of the sensors, errors in positionning of the magnetometer, and cross coupling between the three sensors, which is the most adverse effect. For a typical telemetry pass with duration < 30 minutes, the temperature variations are quite small and the variations of the geomagnetic field components do not exceed typically 150 to 200 mg. Under these conditions, the stability of the error on the measurement on each axis is certainly better than +/- 100nT. In practice the change in time of this error is rather slow, with a typical time constant on the order of ~ 30 minutes and thus totally different from the temporal variations of the field aligned current induced perturbations which are characterized by typical variations of several tens of nT over time scales of 10s or less.
Version:2.4.0
The TRAC magnetometer was provided as a standard facility used for attitude restitution on all AOUS satellites and was under the responsability of the Soviet project team.
It is classical three axis fluxgate magnetometer with a drive frequency of ~ 7 kHz. The cut-off frequency is ~ 10 Hz on all three axis.
The theory and practice of this type of magnetometers have been extensively worked out (e.g. Primdhal, 1970) and a more detailed description of the actual magnetometer used on board AUREOL-3 may be found in Afanasiev (1969).
The magnetometer was calibrated separately in laboratory before mounting on the satellite and scalar calibration factors were provided for each axis by the Soviet project team.
The S/C system provides the values of the three components of the Earth's magnetic field in the magnetometer frame digitized on 8 bits.
The resulting resolution of +/- 200 nT is obviously not relevant for the measurement of field aligned currents since one aims at a resolution of a few nT compatible with typical magnetic perturbations of ~ 100nT due to auroral or cusp field aligned currents during periods of low auroral activity. To achieve such a performance while taking account of the constraints imposed by the S/C and telemetry systems, a special electronics was developed to process the three analog signal outputs of the magnetometer. This electronics acts as a magnifying lens over successive periods of time during which the largest expected overall temporal variations of the magnetic components are such that the desired resolution of +/- 5nT is achieved using the 8 bit A/D converter available in the TM system.
The principle of operation of this electronics is identical for all three axis and explained for the X axis in the following lines.
The X output signal is sampled and digitized with a 12 bits A/D converter every 43.2 seconds. The resulting digital word obtained at time t0, X0 is sent through telemetry over the form of two 8 bits words, the first one representing bits 0 to 5 and the second one bits 6 to 11. X0 is simultaneously converted to analog with a 12 bits D/A converter and feeds the negative input of a difference amplifier, the positive input of which being the X signal output. During the remaining period of 43.2 seconds the output of difference amplifier provides the variation in time DeltaX(t)=X(t)-X0(t) of the X component relative to the value X0 sampled at time t0. The range of the difference signal is +/- 1320 nT which corresponds to the resolution of +/- 5nT after a 8 bits A/D conversion. This value is compatible both with the desired resolution indicated above and with the amplitude of the quasi DC magnetic noise of the S/C which was observed in flight to be less than 10nT.
Performance specifications and calibration factors produced by the constructor are expected to provide an overall absolute accuracy of +/- 300nT when one takes into account the long term drifts, temperature effects, non linearity of the sensors, errors in positionning of the magnetometer, and cross coupling between the three sensors, which is the most adverse effect. For a typical telemetry pass with duration < 30 minutes, the temperature variations are quite small and the variations of the geomagnetic field components do not exceed typically 150 to 200 mg. Under these conditions, the stability of the error on the measurement on each axis is certainly better than +/- 100nT. In practice the change in time of this error is rather slow, with a typical time constant on the order of ~ 30 minutes and thus totally different from the temporal variations of the field aligned current induced perturbations which are characterized by typical variations of several tens of nT over time scales of 10s or less.
| Role | Person | StartDate | StopDate | Note | |
|---|---|---|---|---|---|
| 1. | GeneralContact | spase://CNES/Person/CDPP-Archive/Dominique.Lagoutte |