MicroCosm® and GPS
At the core of the MicroCosm® Software are highly efficient precision satellite orbit determination and geodetic parameter estimation algorithms which utilize Cowell numerical integration and thorough physical modeling of the Earth and of the motions of the spacecraft. Extensive tracking measurement models are available, including 16 different models suitable for the processing of GPS tracking data and 6 models for the processing of GPS Earth-fixed C/A-code or P-code navigation ephemeris data. Iterative Bayesian least squares is used for parameter estimation. MicroCosm® does not use the GPS navigation ephemeris for the generation of measurement residuals. It integrates the trajectories of the GPS spacecraft and performs corrections to the initial state parameters. The GPS solution may be constrained when there is insufficient tracking data to perform a meaningful adjustment of the GPS orbits. By discarding the GPS navigation ephemeris MicroCosm® eliminates one of the two error components introduced by Selective Availability. This is in addition to performing a more accurate (25-75 cm accuracies are routinely achievable with a global network of receivers) determination of the GPS trajectories than is available from the GPS navigation ephemeris. The 16 MicroCosm® measurement models applicable to the processing of GPS data are based upon the following four models:
Each of these four models are available either with the receivers as stationary ground based receivers or with one receiver as spaceborne (e.g. TOPEX/POSEIDON). Each of the above eight measurement models are also available as average range rates, where successive range measurements are differenced and divided by the intervening time interval. When processing carrier phase data as average range rates, the need to estimate range ambiguities is eliminated and cycle slips are readily detected and eliminated by the automatic residual editor. The observation models in MicroCosm® do not explicitly differentiate between carrier phase and pseudorange. Instead, these differences are defined by the data analyst through selection of the appropriate measurement biases to be estimated. For example, when processing carrier phase ranges the analyst should elect to estimate the carrier phase range ambiguities while the estimation of clock biases would be more appropriate to the processing of pseudorange data. The elimination of the clock dithering component of Selective Availability may be accomplished by the utilization of any of the difference data types which involve two receivers observing the same GPS spacecraft. Two different levels of carrier phase automatic cycle slip removal are available. Pseudorange aided cycle slip removal may be exercised in the earliest phases of data processing when reformatting and formulation of difference measurements are performed. During the parameter determination process, a fine level of cycle slip removal utilizing orbit relaxation may be elected. GPS observations may be input into MicroCosm® in receiver independent RINEX-1 & RINEX-2 formats. GPS data in these formats may be clock corrected using the MicroCosm® FixClockTM program. Other formats will accommodate not only GPS data but also a wide variety of other tracking data types. In addition to the determination of the GPS orbits, the MicroCosm® Software also provides the capability to determine geodetic parameters. In particular, it may be used to provide an improved determination of the coordinates of any given subset of the receivers providing data to the solution. It can even be used to provide improved estimates of the gravitational model of the Earth as required by TOPEX/POSEIDON. Because MicroCosm® is not solely a GPS processing program it also provides an ideal platform for the comparison of geodetic solutions using competing measurement techniques such as GPS, satellite laser ranging and DORIS Doppler. MicroCosm® may also be used to perform combined solutions using any of the above competing techniques. Back to MicroCosm® Home Page |