IGS LEO orbit campaigns

The reason for mounting a GPS receiver on a LEO platform is usually the precise navigation of this platform itself. By consequence, most of the groups that process LEO GPS data are only interested in Precise Orbit Determination for the LEO satellite, while using the GPS constellation as a known reference. The approach of the IGS LEO Pilot Project is the other way around: it considers the LEO platform as an orbiting IGS station with certain special properties that a terrestrial GPS tracking station can not offer.

For processing the flight receiver data in combination with ground based GPS data, an intuitive requirement will be that the precision at which the LEO receiver position is known is compatible with that of the ground based receivers. This limits the tolerable positioning errors of the LEO platforms to the level of a few centimeters, or even millimeters. None of the foreseen IGS LEO Pilot Project analysis is possible until the AACs control the topic of LEO POD to state-of-the-art precision levels.

Unfortunately, the centers that have traditionally concentrated on the generation of the IGS product family do not have extensive experience in LEO POD, and vice versa. To assist the IGS LEO Associate Analysis Centers in their analysis efforts, the LEO orbit campaigns turn out to be of great value. The campaigns provide the means to exchange information between the centers that are good at LEO POD - without necessarily being involved in GPS product generation - and the GPS centers that will in the end have to provide the principal IGS LEO analysis results.

The first IGS LEO orbit campaign was initiated in October 2001 for the CHAMP satellite. The second campaign, for JASON-1, started early 2003. In principle further campaigns will be organised for any GPS LEO satellite for which data becomes available during the course of the Pilot Project.

The main purposes of the orbit comparison campaigns are

• To exchange information on LEO POD, and thereby help to solve GPS-based POD problems for Low Earth Orbiters
• To enable analysis of a substantial set of different POD solutions in a consistent manner, so that differences between solutions can be identified
• To provide a set of reliable reference orbits for analysis of LEO GPS data
• To assess absolute orbit precision levels that can be achieved for the LEO GPS satellites
• To provide a basis for studying combination solutions for the LEO satellites

Various types of analysis can be run on the set of orbit solutions:

• Comparisons between two orbits: difference vectors are determined between each pair of solutions. Assuming that the errors in the two compared orbits are independent of each other (which is debatable) the error RMS in each orbit separately can never be larger than the RMS of differences between the two orbits. In other words, the differences provide an upper limit to the true orbit error in each solution.
• Tracking data analysis: measurement residuals are computed to an identical set of tracking observations. The orbit error will hardly ever be oriented exactly along the line of sight of the observation. If orbit error is assumed to be the dominant error component in the observation residuals, the actual orbit error must always be larger than the tracking data residuals.
• Reference frame collocation analysis: different tracking data types use different ground stations, and the Earth-fixed coordinates of these stations will usually be obtained from estimation processes that use the actual tracking data sets. To investigate possible offsets between the different sets of tracking station coodinates, orbits based on one data set can be compared to those based on another data set.

• CHAMP
• JASON-1