WEIGHTED DIFFERENTIAL GPS METHOD FOR REDUCING EPHEMERIS ERROR

A method to cancel the effect of GPS ephemeris error on user position is proposed. In this method, the baseline vectors from the reference s tations to the user are estimated without adjusting the GPS ephemeris. The user position is computed by adjustment using differenced data fr om the user and each station separately and averaging the results with weights inversely proportional to the lengths of the baselines. Alter natively, the differenced data can be averaged in a similar manner bef ore the user position is estimated. The averaging procedure cancels mo st of the ephemeris error because the error is proportional to the len gth of the baseline. A numerical simulation is performed to demonstrat e and evaluate the method. Two reference stations with perfectly known locations are assumed to be separated by several hundred kilometers. A user receiver with poorly known location is placed between the stati ons. The GPS ephemeris is assumed to have errors typical of the broadc ast values. Simulated pseudoranges between the receivers and the GPS s atellites are generated as data. The user positions are first estimate d separately using data from the user and each station and then averag ed. The averaging reduces the error by about one order of magnitude.