An Assessment of GPS-based precise point positioning of the low earth-orbiting satellite CHAMP

Precise point positioning (PPP) with the international GNSS service (IGS) products, which consist of precise orbits and clock correction information, has been demonstrated by several investigators to achieve a centimeter-decimeter level positioning accuracy in real-time for land and aerial vehicular navigation. The purpose of this paper is to present one phase of study conducted at National Defense Academy (NDA) on performance evaluation when such a PPP technique is extended for orbit determination of the low Earth orbiters (LEO) in post-flight processing or even in real-time. In the present study the satellite CHAMP was selected as an LEO since high accuracy orbit is prerequisite to that mission. A PPP filter with a simple white noise plant model was designed, and a basic PPP software which was originally developed at NDA for land and aerial vehicles was intensively modified to process GPS data taken by the dual-frequency receiver ldquoBlackJackrdquo onboard the CHAMP satellite. To generate a reference orbit of CHAMP, we proposed a hybrid orbit determination (OD) method consisting of classical least-squares and Hill-Clohessy-Wildshire (HCW) solution, where a least-squares point positioning solution of 3D coordinates from pseudoranges of CHAMP was employed as the pseudo-observations. It was shown that our PPP filter produced an overall positioning accuracy of better than a sub-meter for transverse and normal components and 90 cm for radial component for 3-hour data arc, and in particular of a few decimeters for each component over the last one-hour segment of good-quality data (no data gap), the implication of the precision real-time on-orbit satellite navigation using only a single, dual-frequency GPS receiver, along with the prediction portion of IGS rapid or ultra-rapid products.