Spacecraft Magnetic Cleanliness & Field Calibration

Why a boom is neededA magnetometer detects the local field, including spacecraft-generated fields. Sensor sensitivity may be excellent while absolute scientific accuracy is poor unless the host spacecraft is magnetically characterized.

1. Field composition

\[\mathbf B_\mathrm{measured}=\mathbf R\,\mathbf B_\mathrm{ambient}+\mathbf B_\mathrm{spacecraft}+\mathbf b_\mathrm{sensor}+\boldsymbol\epsilon.\]

Here $\mathbf R$ represents coordinate transformation, $\mathbf b$ sensor offsets, and $\boldsymbol\epsilon$ stochastic error. Real analyses also include scale factors, nonorthogonal sensing axes, temperature dependence and time-varying interference.

For a point magnetic dipole $\mathbf m$ observed at displacement $\mathbf r$ in free space,

\[\mathbf B(\mathbf r)=\frac{\mu_0}{4\pi r^3}[3(\mathbf m\cdot\hat{\mathbf r})\hat{\mathbf r}-\mathbf m].\]

The $r^{-3}$ law motivates boom placement, but a cable harness carrying current or distributed ferromagnetic structure cannot always be modeled as a single dipole.

2. Physical contamination pathways

Static: permanent magnetic moments, residual magnetization, torquers, mechanical components.
Time varying: switching power converters, heaters, reaction wheels, harness return loops, instrument scan motors.
Measurement itself: fluxgate drive harmonics, sensor cross-axis calibration and temperature-dependent offsets.
Geometry: spacecraft attitude changes the relation between fixed spacecraft fields and external field components.

Use an equipment magnetic-moment budget, interface requirements for current-loop areas, magnetic screening, controlled demagnetization and sensor placement studies.

3. Two-sensor boom analysis

Two calibrated magnetometers placed at different boom distances can help discriminate spatially varying spacecraft contamination from a slowly varying ambient field. However, subtraction is not automatically an unbiased ambient field estimator: gradients in the natural field, sensor offsets and mounting errors must be modeled.

Numerical exercise: use magnetic_boom_model.py to plot a spacecraft dipole’s interference magnitude along a boom; add a constant ambient vector and random noise, then study how uncertainty in dipole location limits subtraction.

4. Verification and references

Test at representative power modes, instrument duty cycles and wheel speeds. Measure calibrated sensor alignment, temperature dependence, remanent moments and spectral lines; record the field in the correct spacecraft coordinate frame.