In situ calibration of offsetting magnetometer feedback transients on the Cassiope spacecraft

Abstract

Abstract. We present an in situ calibration process to derive the transient behavior of the offsetting fluxgate magnetometer (MGF) instruments on the Cassiope spacecraft. The dynamic behavior of the MGF changed on orbit following a software update. Characterizing the new instrument dynamics during normal spacecraft operations and then removing the transients was confounded by significant magnetic interference from the reaction wheels used to orient the spacecraft. Special operations were performed where data were taken in a safehold mode, with the reaction wheels stopped, following a single-event upset of the spacecraft bus flight computer after transiting the South Atlantic Anomaly. The slow single-axis rotation of the safehold mode was used to characterize the fluxgate's new feedback dynamics. This characterization process was then adapted for routine operation intervals with slow reaction wheel rates to allow the transient behavior to be characterized over long intervals of data spanning a wide range of temperatures. Subtracting these characterized transients from the flight data improves the instrument's noise floor and allows the instrument to accurately track rapidly changing local fields without loss of measurement fidelity. More generally, this characterization process should apply to other situations where the dynamics of an offsetting instrument must be calibrated in situ.

Identification and Removal of Reaction Wheel Interference From In‐Situ Magnetic Field Data Using Multichannel Singular Spectrum Analysis

“…The first experiment shown highlights the ability of M‐SSA to successfully decompose real magnetometer data into physically meaningful components that can be automatically grouped using the technique described in Section 2.3. Figure 6 illustrates the results of M‐SSA applied to a pair of magnetometer measurements containing previously identified field‐aligned currents and Alfvénic activity (Miles et al., 2019). The inboard and outboard magnetometer measurements are illustrated in Figure 6a.…”

“…During this time, the CASSIOPE/Swarm-Echo spacecraft crossed a dynamic dual discrete arc aurora. Previous analysis of this event (Miles et al, 2019) demonstrated that, as the spacecraft transits magnetically conjugate to the aurora, it observes in situ magnetic signatures of both a static field-aligned current (with ∼10 to 20 s period) and the nonstationary electrodynamics of reflected and interfering Alfvèn waves (at ∼1 to 10 Hz) associated with the small-scale dynamic features observed in the auroral imaging.…”

“…While the wave speeds are typically fast compared to the speed of the spacecraft, the apparent field of the quasi‐static Earth field and quasi‐static current systems are strongly doppler‐shifted by the spacecraft motion, causing them to appear strongly time‐varying to an in situ sensor. In practice, these measurements are often contaminated by stray magnetic fields generated by the host spacecraft, its subsystems, and other instruments (Miles et al., 2019). These stray fields can have magnitude as large—or larger—than the geophysical signals of interest and have significant overlap in the frequency domain, making them problematic to isolate.…”

Separation of Spacecraft Noise From Geomagnetic Field Observations Through Density‐Based Cluster Analysis and Compressive Sensing

“…The use of a single magnetometer requires that spacecraft noise be carefully characterized before launch. Otherwise, a change in spacecraft behavior may require special maneuvers to re-characterize noise signatures in situ (Miles et al, 2019). The use of multiple magnetometers allows for the discovery of noise signals through the comparison of magnetometer data.…”

“…In the case of the spacecraft Cassiope, a software update changed the behavior of the spacecraft's fluxgate magnetometer (MGF). Special spacecraft maneuvers to decrease the spacecraft's noise signature were required in order to recalibrate the MGF (Miles et al, 2019). Algorithms to autonomously identify spacecraft noise would allow Cassiopie to do in situ MGF calibration without special spacecraft maneuvers.…”