Navigation accuracy is a measure of how well a Global Positioning System (GPS) receiver can determine its location by receiving signals from GPS satellites. Accuracy depends on which satellites the receiver is able to track and their spatial configuration as well as the accuracy of the GPS signals themselves. There are also other factors that go in to the total navigation error, comprising what is known as the GPS error budget. The Navigation Accuracy Library and the Navigation Accuracy Advanced Library model most aspects of the GPS error budget to high fidelity.
The documentation for this topic is split between the Navigation Accuracy Library and the Navigation Accuracy Advanced Library.
The Navigation Accuracy Library considers many sources of error in a GPS error budget. GPS almanacs, which define GPS satellite orbital positions, and Performance Assessment Files (PAFs) and Prediction Support Files (PSFs), which specify GPS ephemeris and clock errors can be used to determine receiver positioning performance. Receiver performance can be characterized by dilution of precision, assessed or predicted accuracy and number of satellites tracked. In addition, GPS satellite outages can be taken into account using the U.S. Air Force's Satellite Outage File (SOF). Different navigation related data files can be loaded and used, including SP3 and RINEX files. Also, the library contains models for tropospheric loss and ionospheric loss for single frequency receivers.
The Navigation Accuracy Advanced Library builds on the Navigation Accuracy Library but also considers how communication links contribute to GPS errors. GPS transmitter, signals and a GPS receiver communications front end are modeled, allowing for GPS link budgets to be used in analysis and to determine tracking using Carrier to Noise thresholds. Using these features, GPS receiver performance can be determined in the presence of complex communications environments as well. Transmissions from GPS satellites and jammers can be modeled, and GPS Figures of Merit such as dilution of precision and predicted navigation accuracy can be determined using these additional communications constraints. A communications specific receiver noise model is also included in the library.