Short answer
A 3-D Cartesian coordinate system with origin at Earth's centre of mass, X axis through the prime meridian / equator, Z axis along Earth's rotation axis. The native frame in which GNSS receivers compute satellite and user positions before projecting to lat/lon.
Detailed explanation
ECEF (sometimes called "geocentric Cartesian") is the rectangular coordinate system fixed to the rotating Earth. The origin is at Earth's centre of mass; the X axis passes through the intersection of the equator and the IERS Reference Meridian (close to but not exactly the historical Greenwich meridian); the Z axis aligns with the Earth's instantaneous rotation axis; and the Y axis completes a right-handed frame.
Every GNSS receiver works in ECEF internally. Satellite positions broadcast in navigation messages are ECEF; the receiver's user position solution starts as ECEF; the conversion to lat/lon/altitude happens only at the end for human display. This is why ECEF shows up in raw GNSS APIs, RINEX files, and any precision-grade processing pipeline.
Position differences in ECEF are direct metres on each axis. For static-baseline geodesy and short-baseline interferometric work (multi-antenna direction-finding, attitude determination), ECEF differences are the natural unit — no projection distortion, no datum nonsense.
Conversion between WGS84 lat/lon/alt and ECEF is closed-form forward (N × cos × cos for X, etc.) and Bowring iterative inverse — see the Coordinate Converter tool for an interactive conversion or the source for the math.
Try it interactively
Coordinate Converter
Open the toolRelated terms
WGS84
The reference ellipsoid and datum used by GPS and the de-facto GNSS standard. Semi-major axis a = 6378137 m, flattening f = 1/298.257223563. Position differences from local datums (NAD83, ETRS89, GDA2020, CGCS2000) range from tens of cm to several metres depending on region.
UTM
A worldwide projection system that divides Earth into 60 6°-wide longitude zones, each projected as a transverse Mercator centred on its meridian. Distances and areas inside one zone are easy linear-metric computations; the projection is valid 80°S to 84°N latitude.
MGRS
A compact alphanumeric grid coordinate system built on top of UTM, used by NATO, US DoD, aviation, and search-and-rescue. A typical MGRS reference like 33TUM1234567890 encodes a position to 1 m precision in 15 characters — survives voice transmission and SMS reliably.