Impact of gravity modeling error on integrated GNSS/INS coasting performance

RTCA SC-159 Working Group 2C is developing requirements and test procedures for the integration of GNSS with a variety of grades of inertial measurement units (IMUs). The testing and evaluation of the gravity compensation mechanism is an important consideration in this effort. Models used for estimating and compensating for gravity range from simple models based on latitude and altitude only to complex models such as the spherical harmonics-based EGM2008 model. The residual errors that remain after gravity compensation impact the velocity determination and couple into the position and attitude estimates of the navigation solution. In this work several gravity models are studied and compared, including an analysis of the EGM2008 model using various degrees/orders. The EGM2008 model with a degree/order of 2159 is used as a truth model to test and evaluate several lower fidelity models. In addition, this paper presents an analysis of the impact of gravity compensation on the integrated GNSS/INS solution while coasting without GNSS. A representative extreme-case gravity anomaly region was identified and an aircraft trajectory traversing it was simulated for high, mid, and low-grade IMUs. The results show that the fidelity of the gravity model has a non-trivial impact on the navigation solution and thus must be considered during safety-certification.