Discussion of basic measurement and filtering approaches for stationary grond-based GNSS Integrety and health monitoring

Discussion of basic measurement and filtering approaches for stationary grond-based GNSS Integrety and health monitoring

Zwartbol, T.

National Aerospace Laboratory NLR

1995-12-30

This report discusses approaches for integrity monitoring of GNSS radio navigation signals. GNSS integrity monitoring is defined as, Ref.9,: "A GNSS subsystem which enables the timely detection and indication of malfunctions in GNSS operations to ensure that the user is aware whether or not the system is operating within its specified performance limits". An integrity failure of GPS or GLONASS radio navigation signals leads to erroneous navigation, in spite of a correctly functioning receiver, while the user is not aware of this situation. It is obvious that erroneous navigation causes safety risks. For this reason, radio-navigation systems for civil use are designed such that, in case of integrity failures, the faulty signals are shut down immediately, or that users are timely warned not to use the faulty navigation signals. The (military) GPS and GLONASS radio navigation systems virtually lack such a capability. For a large user community of GPS, such as civil aviation users and maritime users safety is of paramount importance. For this reason the future Global Navigation Satellite Subsystem (GNSS) which will based on use of GPS and possibly GLONASS radio navigation signals, is to be equipped with wide area and local area integrity monitoring capability. Integrity monitoring as envisioned for GNSS functions as follows. An integrity monitoring station continuously (e.g. with one Hz sample rate) performs code phase pseudo range and carrier phase delta range measurements of the SVs in view. The monitoring station also continuously calculates the ranges and delta ranges to the SVs in view, using the accurately known location coordinates of the monitoring station and the GPS time epochs of the measurements. Furthermore, the pseudo range measurements are corrected for the known error sources, which are: ionospheric and tropospheric effects, SV clock bias and monitor station clock bias. A "measurement" of the residual range errors is formed by subtracting the calculated range and the corrections for the known errors from the measured pseudo range. The residual range error is designated "bias". The bias is time varying. Likewise, a "measurement" of the residual range error rate is formed by subtraction of the calculated delta range and the corrections for the known error sources from the carrier phase delta range measurement. The residual range error rate of change is designated "bias rate of change", which is also time varying. The residual range errors (biases) and range errors rate of change (bias rates of change) of the GNSS radio navigation signals are filtered via an set of recursive (Kalman) filters, one for each spacevehicle (SV). The outputs of the filters can be checked against the specified performance limits in order to determine whether or not all signals can be safely used for navigation during a particular phase of flight or that a failure has occurred and one or more signals should not be used for navigation. A dynamic model for the bias and bias rate of change, to be incorporated in the filter state equation, is proposed. The system noise of the proposed dynamic model and the measurement noise are important design parameters for the filter algorithms. Wide area and local area integrity monitoring and accuracy enhancement, are important features of a GNSS, which will open a much broader professional user market for satellite navigation applications. The present report presents some basic approaches in this area. Integrity monitoring and accyracy enhancement are basic parts of GNSS technology. In Europe, the development of user requirements, development of technology, design and implementation of the European part -designated EGNOS, European Geostationary Navigation Overlay Service- of the world-wide GNSS takes place under the aegis of the Tripartite-Group EU, ESA and EUROCONTROL. EGNOS is part of the European approach to an integrated Communication Navigation and Surveillance and Air Traffic Management (CNS/ATM) system within Europe. Participation in the technology development and design and implementation of H such systems will provide opportunities for the Dutch industry.

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Nationaal Lucht- en Ruimtevaartlaboratorium

Campus only

NLR CR 95676 L

Aerospace Engineering Reports

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(c) 1995 National Aerospace Laboratory NLR