Improved Attitude Modeling for GPS III Satellites in the Eclipse Season

Kurzfassung

GNSS satellites traditionally apply a yaw steering attitude to keep their antenna pointing to the Earth while maintaining the solar panel rotation axis perpendicular to the Sun-spacecraft-Earth plane. During the eclipse season, i.e., in periods with low Sun elevation ($\beta$-angle) above the orbital plane, the idealized, nominal yaw steering typically results in angular rotation rates that exceed the capabilities of the momentum wheels for attitude changes. To cope with this issue, different types of rate-limited yaw steering laws are applied by GNSS satellite designers during noon and midnight turns in the eclipse season. Knowledge of the yaw steering profile is essential for a proper modeling of the antenna phase center offset from the center of mass as well as the phase wind-up. This makes it a prerequisite for precise orbit determination and time synchronization (ODTS) as well as precise point positioning (PPP). For GPS III satellites, however, no detailed attitude model has been published by the manufacturer so far. Instead, different substitutes are presently in use by the various analysis centers (ACs) of the International GNSS Service (IGS). Making use of the fact that attitude modeling errors are largely absorbed in the estimated clock offsets of the highly-stable GPS Rubidium Atomic Frequency Standards (RAFS), the performance of individual models is analyzed using precise clock products and associated attitude quaternions of individual IGS ACs. While partly masked by stochastic clock variations, use of the Jet Propulsion Laboratory (JPL) attitude quaternions yields a clearly reduced clock variance in the vicinity of noon and midnight turns when compared to alternatives such as the GPS IIR and IIF models considered by other ACs. Since JPL attitude quaternions are only published with delays and no specification of the underlying model has been released, we show that the JPL model can well be represented by a yaw steering law with an orbit-angle-dependent $\beta$-angle modification. This enables an analytical description of the GPS III attitude that can readily be used for ODTS and PPP applications and is suggested as a common GPS III attitude model for future IGS processing.