Attitude-Independent Magnetometer Calibration with Time-Varying Bias

Abstract

Adviser: James W. CutlerWe present a method for on-orbit, attitude-independent magnetometer calibration that includes the effect of time-varying bias due to electronics on-board a spacecraft. The calibration estimates magnetometer scale factors, mis-alignments, and constant as well as time-varying bias. Time-varying effects are mitigated by including spacecraft telemetry in the measurement model and estimating constant parameters that map the telemetry data to magnetometer bias. The calibration is demonstrated by application to flight data from the Radio Aurora Explorer satellite and significantly reduces the uncertainty of off-the-shelf magnetometers embedded within the satellite and subject to spacecraftgenerated fields. This method simplifies the satellite design process by reducing the need for booms and strict magnetic cleanliness requirements.

On-Orbit Calibration of Photodiodes for Attitude Determination

“…18 or the observability discussion in Ref. 19). RAX-2 utilizes a passive magnetic control system that aligns the spacecraft to the geomagnetic field and dampens rotational kinetic energy within the first few weeks after deployment from the launch vehicle.…”

“…The initial measurement covariance matrix R is composed of the individual sensor uncertainties. The standard deviation of the PNI magnetometer measurements after on-orbit calibration [19] is approximately 320 nT, and from pre-flight testing, the approximate standard deviation of the individual photodiode measurements is 0.015 V. Volts are used rather than amps because even though the measurement model for photodiodes was given in terms of current, the voltage across a resistor is measured in their implementation, which is directly proportional to the current. To account for uncertainty in the Earth albedo model, an additional uncertainty corresponding to 50 W/m 2 is added to the photodiode measurement uncertainty.…”

“…Another common scalar objective function is the expected magnitude of the measured vector. For example, a common magnetometer calibration technique is to minimize the difference between the expected and measured magnitude of Earth's magnetic field as a function of the calibration parameters [17][18][19]. Both of these attitudeindependent methods can be applied to various types of three-axis sensors.…”

“…The dynamic models of the calibration states are therefore given by Eq. (19), where w C , w α , and w are each mean zero Gaussian random vectors.…”

“…The initial attitude estimate is calculated from the measured magnetic and sun vectors using the q-method [24]. The magnetometers were first calibrated using an on-orbit, attitudeindependent method [19], and the sun vector measurement for the initial attitude calculation utilizes the initial calibration parameters. The initial rate gyroscope bias is assumed to be zero.…”