A GPS-FREE NAVIGATION SYSTEM FOR EARTH-ORBITING SMALL SATELLITES

Onboard orbit determination can be challenging in GPS-denied environments. For these situations a navigation architecture is proposed and numerically investigated. This architecture involves two satellites, a Chief satellite and a Deputy satellite, orbiting Earth. The Chief satellite is capable of obtaining precise attitude measurements and measuring the body-relative range and direction to the Deputy satellite. These measurements are then used with an Extended Kalman Filter (EKF) equipped with a second-order zonal harmonic gravity model to estimate the inertial position and velocity of both satellites. The numerical investigation is comprised of multiple Monte Carlo analyses and the effects of several factors on the steady-state accuracy such as filter tuning parameters and the six Keplerian orbital elements are quantified to provide insight into mission capabilities. The best performance is achieved when the satellites are in a high-Earth orbit (HEO) and have a separation varying from 100 km to 200 km. This scenario yields a minimum 3σ steady-state position estimation accuracy of 22 m per axis and minimum 3σ steady-state velocity estimation accuracy of 0.17 cm/s per axis.