Ephemeris data comprises the precise orbital parameters broadcast by navigation satellites that enable GNSS receivers to calculate satellite positions at any given time. This information is fundamental to the positioning process because accurate knowledge of satellite locations is essential for computing the receiver’s position through trilateration, the mathematical process of determining position from distances to known points.

Each GNSS satellite continuously broadcasts its ephemeris within the navigation message, providing receivers with the mathematical elements needed to compute that satellite’s position in three-dimensional space at any moment. These parameters include orbital elements such as semi-major axis, eccentricity, inclination, right ascension of ascending node, argument of perigee, and mean anomaly, along with correction terms that account for perturbations to the ideal Keplerian orbit caused by gravitational irregularities, solar radiation pressure, and atmospheric drag.

Ephemeris data is characterized by high accuracy but limited validity. Broadcast ephemeris is typically valid for four hours and must be refreshed regularly as satellites transmit updated parameters. If a receiver operates with outdated ephemeris, computed satellite positions will be inaccurate, directly degrading positioning performance. This is why time-to-first-fix (TTFF) can be prolonged during cold starts, the receiver must collect current ephemeris from the navigation message, which takes approximately 30 seconds per satellite.

For high-precision applications, broadcast ephemeris accuracy (typically 1-2 meters) may be insufficient. Precise ephemeris products computed by organizations like the International GNSS Service (IGS) using global networks of tracking stations achieve centimeter-level orbital accuracy. These precise products, available in rapid, final, and ultra-rapid varieties with decreasing latency and increasing accuracy, enable precise point positioning (PPP) and are essential for geodetic research, reference station operations, and other demanding applications.