Near real time orbit detemination of BILSAT-1

Abstract

Reduced dynamic models combined with GPSObservations on board the satellites within an extended Kalman Filtering algorithm increases the accuracy of near real time orbit determination. Gravity model as the main force and numerical integration methods are the key concepts in the GPS based reduced dynamic orbit determination. EGM 2008 Earth Gravity Model represented by U.S. National Geospatial Intelligence Agency (NGA) EGM Development Group offers a new model for the determination of gravity accelerations. Some other Earth's Gravity Models which have been generated recently from the dedicated satellite gravity field missions such as GRACE without including terrestrial observations are also evaluated in the force model for near real time orbit determination. On the other hand, numerical integration methods preferred in GPS based reduced dynamic orbit determination mostly affect the performance of the processor. In this context, another alternative to rigorous implementation of variational equation is the determination of the state transition matrix by using Markley's method which in turn contains only a simplified force model.In this study, BILSAT 1 orbit is evaluated by using EGM2008 and Grace GGM03S Earth gravity models and Markley state transition matrix. GPS based reduced dynamic orbit determination are used as the algorithm. Effect of unmodeled and unknown forces acting on satellite are approximated by first order Gauss Markov Process. Single frequency pseudorange measurements are the observations acquired from SGR 20 spaceborne GPS receiver onboard the BILSAT 1.

Time Transfer From GPS for Designing a SmallSat-Based Lunar Navigation Satellite System

“…Furthermore, we utilize the LNSS satellite position and velocity from known/pre-computed ephemeris to aid our timing filter. Indeed, less frequent updates, depending on the orbit, are needed to correct the lunar orbit perturbations and are, thus, independently handled outside of our proposed time-transfer filter, such as via an onboard orbit determination module (Erdogan & Karslioglu, 2009) or via infrequent updates from Earth ground monitoring stations. Specifically, we utilize the known satellite position and velocity of the LNSS satellite to compute the expected range and range rate measurements (no clock effects incorporated here), which are later compared with received GPS measurements to formulate the input for the measurement update step.…”