In modernised GPS, the same type of contents as the legacy navigation message (NAV) is transmitted but at higher rate and with improved robustness. The "legacy" message and the first three of the modernised GPS are civil messages, while the MNAV is a military message. The ephemeris and clocks parameters are usually updated every two hours, while the almanac is updated at least every six days.īesides the "legacy" L1 C/A navigation message, four additional new messages have been introduced by the so called GPS modernisation: L2-CNAV, CNAV-2, L5-CNAV and MNAV.
It includes the Ephemeris parameters, needed to compute the satellite coordinates with enough accuracy, the Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions, the Service Parameters with satellite health information (used to identify the navigation data set), Ionospheric parameters model needed for single frequency receivers, and the Almanacs, allowing the computation of the position of ”all satellites in the constellation”, with a reduced accuracy (1 - 2 km of 1-sigma error), which is needed for the acquisition of the signal by the receiver. The Navigation Message provides all the necessary information to allow the user to perform the positioning service. Hernandez-Pajares, University of Catalunia, Spain.Įvery satellite receives from the ground antennas the navigation data which is sent back to the users through the navigation message.
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