IGS LEO tracking data

IGS LEO tracking data analysis

*IGS LEO Homepage*	*Associate Analysis Centers*	*LEO Orbit Campaigns*	*SP3 and RINEX acronyms*	*Links*
		*CHAMP campaign* *JASON-1 campaign*
	*Orbit comparisons*	*Tracking data analysis*	*Precision estimates*
		*CHAMP SLR analysis* *JASON SLR analysis* *JASON DORIS analysis*

Concept
The only physical measure of orbit precision is provided by the tracking data for that satellite. The relative precision of different orbit solutions in a set can be assessed by computing tracking data residuals to every orbit in exactly the same way, while keeping this orbit fixed to the input solution. Under the assumption that the dominant error in the tracking residuals is the orbit error, the tracking data residuals define a lower limit to the orbit pecision. This assumption is not always valid, though:

Orbit error may not be the main component in the tracking residuals. The tracking observations contain other errors like station coordinate errors, measurement noise, residual errors due to troposphere or ionosphere, timing errors, round-off effects due to limited numerical resolution.
The orbit may not be independent from the analysis tracking data set. If the orbit has been estimated on some or all of the tracking data that is subsequently used to assess the precision of that orbit, data errors may have been absorbed in the POD solution. The result is that the tracking data residuals appear lower than what they should be.
The analysis software may not be independent from the estimation softwrae. The effect will be similar to the above, namely, shortcomings of the POD software will be masked if the same algorithms are used to compute the tracking data residuals to the campaign orbits. This will mainly apply to the solutions contributed by ESOC, where the campaign analysis is being run. Other solutions may still show similar dependencies, in particular if different solutions use the same subset of the tracking data, the same Earth rotation parameters, etcetera.

The conclusion is that tracking data residuals provide a useful information, but that this information must still be interpreted with great care.

Available data sets
The satellites of interest to IGS LEO will be equipped with a GPS receiver, which may or may not be the main tracking system for that LEO satellite. The most relevant alternative tracking systems that can be used for orbit analysis are SLR, DORIS, altimetry (-crossovers).

GPS data can be used in different observable types, e.g. undifferenced code and phase observations, single differenced data, double- or triple differenced data. Even if GPS data is used for the POD process itself, it is unlikely that all possible GPS observables are used, so that one of the other observables may be used as - more or less - independent check. The coupling of orbit error and receiver clock erorr may require the combined analysis of orbit and GPS clock, though.

Of the other tracking data sets SLR is usually considered as the most reliable indicator of LEO orbit precision. This is partially because of the high precision of the SLR data itself, and partially because the data is in most cases too sparse to introduce major dependencies in the POD solution. The main objective of SLR retroreflectors on LEO satellites is just to provide an independent observation of the orbit. Both for CHAMP and JASON, the SLR data forms the main source of information on orbit precision.

DORIS data, if available on the LEO satellite, will in almost all orbit solutions form the main tracking data set to which the orbit has been fit. This means that independance between the orbit and the tracking data is questionable. Apart from that, the smoothness of the DORIS data in combination with the need to estimate certain pass-dependent parameters usually leads to residual levels that are identical under all circumstances. As an analysis data set for comparing different orbit solutions, the relevance of DORIS is limited.

Altimetry data can offer a more independent check, because it is typically not used as a tracking data set for orbit determination purposes, or perhaps just in the form of crossover differences.

Applied analysis
The set-up of tracking data analysis for the purpose of the orbit campaigns is straightforward. The input solutions are used to provide the satellite position at the time of observation, and tracking residuals are computed to a fixed set of data points. Because all data processing is exactly identical for all contributed solutions, the differences in the tracking residuals can only be caused by differences in the orbit solutions. The tracking residuals give a reasonably accurate absolute precision estimate of the orbits, but their value as relative precision reference is just about perfect.

The information that can be extracted from the tracking data analysis is complementary to the information from the orbit comparisons, and by combining these two sources of information a reasonably reliable absolute precision estimate can be obtained. Please follow the various links to see the results.

Go to the CHAMP tracking data analysis
Go to the JASON tracking data analysis