The magnetic sensitivity of a Ka-band isolator’s output phase is measured at 7 × 10–4 deg/G level. This high degree of precision is enabled by the sensitive phase measuring capabilities of a testbed built to mimic NASA’s Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. Its ground-based testbed was used to measure the magnetic sensitivity of a flight-spare Ka-band isolator, and the authors found it to be 0.0052 ±0.0007 deg/G along its most sensitive axis. The GRAIL mission was able to incorporate microwave isolators into its instrumentation because the spacecraft orbited the Moon and, thus, did not travel through a permanent magnetic field as it would in a mission around Earth. Understanding this magnetic sensitivity is key to evaluating the impact an isolator would have on data quality for future gravity missions such as GRACE-FO (Gravity Recover and Climate Experiment — Follow On), a scheduled follow-on mission to GRACE, which has been mapping out Earth’s gravity for over a decade.
An isolator’s sensitivity to magnetic field change was measured by inserting a GRAIL flight-spare microwave isolator into the Ka-band cable path from one GRAIL mock-instrument to the other in GRAIL’s ground testbed. The isolator was affixed inside a pair of Helmholtz coils generating a single-axis magnetic field. The field was varied, and the resultant phase shifts were monitored. The most sensitive orientation was for the isolator’s internal magnetic field pointing parallel to the externally applied field.
This will allow GRACE-FO to evaluate whether or not to incorporate these isolators, and therefore improve its phase detection sensitivity over that of the original GRACE mission, despite the fact that it will orbit the Earth and experience a time-varying magnetic field that is spatially correlated to the planet it is trying to map. It is confirmed that switching the direction of the magnetic field switches the direction of the phase shift, and that moving the isolator from the grail microwave assembly’s input to its output does not. Therefore, future gravity-mapping missions based on the GRACE/GRAIL twin-spacecraft technique can be confident that mounting the isolators with their internal magnetic fields oriented in opposite directions on the input versus output of the microwave assembly will lead to first-order cancellation of this magnetic effect.