NAVDATA, or Navigation Data, refers to the digital message content broadcast by GNSS satellites that provides receivers with the information needed to compute positions. This data stream includes essential elements such as satellite ephemeris (orbital parameters for calculating satellite positions), satellite clock corrections, ionospheric delay models, constellation health status, and timing information, all transmitted continuously as part of the navigation signal structure.

The navigation message is modulated onto the carrier signal at a relatively low data rate (typically 50 bits per second for legacy GPS signals), meaning that collecting complete navigation data takes significant time. A complete GPS almanac takes 12.5 minutes to receive, while current ephemeris for one satellite requires approximately 30 seconds. This collection time explains why GPS receivers experience extended time-to-first-fix (TTFF) during cold starts when they possess no prior knowledge of satellite positions or current time.

Different GNSS constellations structure their navigation messages differently, though all contain similar fundamental information. GPS navigation data is organized into frames and subframes containing ephemeris, clock parameters, and almanac data for the entire constellation. Galileo uses I/NAV and F/NAV message types with different content and update rates. BeiDou employs D1 and D2 navigation message types. Despite structural differences, all navigation messages serve the common purpose of enabling receivers to determine satellite positions and correct for satellite clock errors.

Modern GNSS applications increasingly supplement broadcast navigation data with precise products from ground-based processing. Broadcast ephemeris accuracy of 1-2 meters and clock accuracy of several nanoseconds is adequate for standard positioning but insufficient for centimeter-level applications. Precise ephemeris and clock products from organizations like the IGS achieve centimeter-level orbital accuracy and sub-nanosecond clock accuracy, enabling Precise Point Positioning (PPP) applications. Real-time correction services similarly provide improved orbit and clock information that supersedes broadcast navigation data for high-precision positioning.