The newly published magnetic control plan for TRACERS, described in the journal Space Science Reviews, lays out in unusual detail how the team confronted this challenge. The stakes are concrete and quantifiable. The mission’s fluxgate magnetometer, MAG, must measure the local DC magnetic field with an accuracy of 100 nanotesla at a temporal resolution of 0.1 seconds, while the magnetic search coil, MSC, must capture AC fields from 1 to 1000 hertz. The overlapping frequency ranges were deliberately defined around the science performed by the two instruments, with the fluxgate covering zero to 50 hertz and the search coil extending to kilohertz frequencies. A third instrument, the MAGIC technology demonstration, also senses fields from zero to 50 hertz, but as a do-no-harm technology payload it imposes no requirements on the host mission.

Contamination comes in several distinct flavors. Static magnetic fields from magnetized or ferromagnetic materials directly corrupt the measured low-frequency and static field, while time-varying electrical currents contaminate the AC measurements. Perhaps less obviously, the ACE electron instrument is also a victim: stray magnetic fields near its aperture, at frequencies below the local electron gyrofrequency of roughly one megahertz, deflect incoming lower-energy electrons and blur the angular resolution of the measurement, altering the apparent direction at which particles arrive. The control plan therefore had to protect three separate classes of sensors, each with different sensitivities and different tolerances, mounted on a spacecraft packed with batteries, heaters, latch valves, reaction wheels, and electronics that all generate magnetic signatures of their own.

The first line of defense was distance. Magnetic fields from localized sources decay rapidly, roughly as the cube of the distance, so simply moving sensors away from noise sources buys enormous leverage. The baseline design placed the fluxgate and search coil sensors 100 centimeters from the spacecraft body on two-segment non-magnetic deployable booms. But in a twist that reshaped the entire magnetic strategy, a flight boom failed during environmental testing late in the mission, forcing engineers to adopt a 70-centimeter fixed rigid bracket instead. The change was made possible by the generous volume available when TRACERS was assigned the top, so-called cake-topper position on a SpaceX Falcon 9, but it meant the plan had to be re-evaluated around the reduced separation and the resulting amplification of apparent noise.

Source control came next. The team used a preferred and discouraged materials list, drawing on heritage guidance from NASA magnetic cleanliness studies, to steer part selections away from ferromagnetic components. Where magnetic parts were unavoidable, geometry became the tool: latching relays, which are inherently magnetic because of their internal permanent magnets, were arranged in mirrored pairs so their stray fields partially cancel. The batteries, whose asymmetric string-cell configuration generates a field dependent on charge and discharge current, were placed near the center of the spacecraft and arranged back-to-back to reduce the magnetic moment in two axes. Latch valve assemblies were oriented in opposite directions for the same reason, and heaters were fabricated with self-cancelling current trace paths. Notably, the team declined to use compensating permanent magnets, wary of creating strong local fields near the ACE electron instrument mounted on a spacecraft face sheet.

Electrical design rules targeted the time-varying half of the problem. All harnessing used balanced twisted pairs to provide dedicated return paths and minimize open current loops, and subsystems were forbidden from using chassis ground for current return, instead implementing local star grounding. The solar panels were back-wired, following techniques pioneered on the Magnetospheric Multiscale mission, so that interconnects ran anti-parallel and the bulk current flow in each panel largely cancelled itself. Special zones around the magnetometer sensors were built from non-ferromagnetic materials including aluminum, titanium, carbon fiber composites, and PEEK engineering plastic, with specialty titanium and aluminum connectors at field joints and careful grounding of the multilayer insulation to avoid open loops in ground wires.

Design alone, however, could not guarantee cleanliness, so the mission screened components and assemblies with four complementary techniques. The MAG team measured stray fields by moving test articles at fixed intervals past a magnetometer centered in a mumetal shield, repeating the test for each axis. The spacecraft provider used a much larger three-layer mumetal shield assembly for subsystems. The mission team built a semi-automated apparatus, documented in a 2024 paper, in which test articles rotate on a magnetically clean plate inside a one-meter cubic shield while three magnetometers at varying distances capture the modulated field, allowing a dipole moment to be fitted by spectral analysis. Complete instrument assemblies were simply waved by hand at a fixed distance from a stationary magnetometer as a quick clean-room check.

The screening campaign was governed by an elegant allocation scheme. Rather than predicting the exact stray field at the sensors, the plan ensured the total stayed below a maximum allowable 100 nanotesla at the MAG sensor. Each object was pro-rated against the spacecraft’s total mass, with half of the budget held in reserve for discretionary release by the magnetic control board, and random orientations assumed so sources add in quadrature, a deliberately conservative approach that overestimates the aggregate field of many point sources. A de-minimis threshold of 1 nanotesla at one meter exempted negligible parts, and objects that failed individual screening could pass as part of a higher assembly, exploiting opportunistic cancellation. In practice, the latch valves emerged as the largest stray-field offenders by a significant margin and were dispositioned as use-as-is, with their DC contribution to be removed by post-processed in-situ calibration rather than risky magnetic compensation near the electron instrument.

Perhaps the most intricate chapter concerns the two fluxgate magnetometers themselves. Mounted 50 centimeters apart on a common bracket, MAG and MAGIC form an ad-hoc gradiometer that can separate the common geophysical field from spacecraft-generated noise, but their proximity creates mutual interference. The dominant effect arises from the global negative magnetic feedback that both instruments use to null the field inside their sensors and linearize their response: the feedback windings generate a quasi-DC stray field proportional to the ambient field being cancelled. Using a Biot-Savart model originally developed to optimize the Tesseract sensor’s internal feedback field, the team simulated the external stray field of both MAGIC sensor variants and validated the predictions in a two-meter Merritt coil facility at the University of Iowa, finding a robustly linear gain error of roughly 5 nanotesla at 20,000 nanotesla that can be corrected through on-orbit calibration. A second interference path, in which the two sensors’ 8192-hertz core drive signals beat against each other, was fully eliminated by synchronizing MAGIC’s voltage-controlled oscillator to MAG’s drive signal with a pulse-width-modulated control loop.

The final safety net is software. Swing tests, adapted from the Van Allen Probes program, suspended each spacecraft from a crane and fitted a dipole model to the oscillating stray field measured by stationary magnetometers, while controlled rotations mapped the field’s angular dependence. Preliminary uncertainties of about 50 percent are being reduced by a new gradient-fitting analysis that suggests the 100-nanotesla threshold was met with margin. For the residual noise introduced by the shorter bracket, the team leaned on a discipline that has advanced dramatically in recent years: singular spectrum analysis and machine-learning techniques, proven on missions from e-POP to Parker Solar Probe, have mitigated dynamic noise sources by 89 to 95 percent, meaning TRACERS can tolerate roughly a tenfold increase in stray field at the sensors. At 70 centimeters, the predicted amplification is only about two to four times, a change assessed to leave the mission’s first two science objectives untouched, with only a low-likelihood but medium-consequence risk to the baseline closure of the third objective, which requires characterizing field-aligned currents near 100 nanotesla and Alfvén waves near 1 nanotesla. Whether the plan succeeds will be judged on orbit, where in-situ calibration offsets will validate the DC program and any unexpected AC sources will be hunted down against the catalog built during integration, closing the loop on one of the most thorough magnetic hygiene campaigns ever flown on a small-satellite mission.

Subject of Research: Magnetic cleanliness engineering for the TRACERS twin-satellite mission studying magnetic reconnection in Earth's magnetospheric cusp

Article Title: The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) Magnetic Control Plan

Article References: Miles, D. M., Lasko, A., Blandin, M., Bounds, S., Caron, R., Carton, A., Dolan, J., Dvorsky, R., Finley, M. G., Flores, A., Goss, C., Greene, K., Halekas, J., Hospodarsky, G. B., Kletzing, C. A., Mark, D., Miller, I., Nguyen, P., Omar, S., … Washington, A. (2026). The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) Magnetic Control Plan. Space Science Reviews, 222(6), Article 70. https://doi.org/10.1007/s11214-026-01326-2

Image Credits: AI Generated

DOI: 10.1007/s11214-026-01326-2

Keywords: TRACERS, magnetic cleanliness, magnetometry, fluxgate magnetometer, magnetic reconnection, magnetospheric cusp, gradiometer, stray magnetic field, MAGIC, signal processing, spacecraft screening, NASA