Spin-axis estimation of the Radiation Belt Storm Probes spacecraft using RF Doppler data

Abstract

A methodology for using RF Doppler data for spin-axis estimation is presented. The NASA Radiation Belt Storm Probes (RBSP) mission, a dual-spacecraft mission planned for a 2012 launch, will yield valuable insights into the physical dynamics and processes of the Earth's radiation belts, particularly those that produce hazardous space weather effects and those that can affect solar system exploration. Both spacecraft have on-board Sun sensors that, in conjunction with magnetometer data, provide the required data to determine the attitude. However, the quality of the magnetometer measurements is compromised at higher altitudes because of a weaker magnetic field and also during solar storms because of variations in the Earth's magnetic field. Each spacecraft's on-board S-band radio frequency (RF) system includes a JHU/APL Frontier Radio transceiver, which provides a coherent downlink signal used for Doppler navigation. The RF system also includes two low-gain antennas offset from the spin axis with boresights parallel to the spin and anti-spin axes. Due to the spin of the spacecraft, a Doppler-induced modulation is present on the downlink carrier. Once the orbit determination is performed and the Doppler signal is analyzed, this modulation, or spin signature, is apparent. This spin signature can be used to accurately determine the spin rate, spin phase, and, in conjunction with Sun sensor data, the spin axis orientation. This provides a level of system redundancy with the magnetometer for attitude determination. The Sun sensor provides the angle from the spin axis to the Sun vector; the amplitude of spin signature on the Doppler data provides the instantaneous aspect angle from the spin axis to the ground station. These two measurements establish the spin axis direction. This paper describes the established theory and a method for processing the spin signature and determining the spin axis as applicable to the RBSP spacecraft. Further, RF-related errors on the Doppler carrier frequency measurement are characterized and related to the error in the spin axis orientation determination. Finally, this paper introduces the concept of attitude determination by processing the Doppler spin signature data alone over a period of time that encompasses a sufficient change to the spin axis/ground station aspect angle. This provides a further level of redundancy in the event of a failed Sun sensor. Results from processing actual data from a current mission are given.