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GPS tutorial basics
GPS data resources and links
GPS Status and Outage Information
GPS status Information concerning maintenance outages, satellite launches and removals, and notices of changes to GPS operations are posted on this website run by the US Coast Guard. This information is generated by the Air Force 2nd Space Operations (2SOPs) as operators of the GPS system.
SBAS (WAAS and EGNOS)
Information and current status on WAAS (Wide Area Augmentation System) and EGNOS (European Geostationary Overlay Service).
The National Marine Electronics Association (NMEA) is a U.S. standards committee that defines data message structure, contents, and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment. NMEA 0183 is the standard data communication protocol used by GPS receivers.
RTCM SC-104 (Radio Technical Commission for Maritime Services Special Committee 104) recommendations define the correction message format and various different correction message types.
GPS Signal Specifications