,
Indonesia (2025). Legend for "flags" field: {{ulist|1=E - ephemeris received Part of an orbiting satellite's broadcast includes its precise orbital data. Originally, the
US Naval Observatory (USNO) continuously observed the precise orbits of the GPS. As a satellite's orbit deviated, the USNO sent the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain its most recent
ephemeris. Modern systems are more direct. The satellite broadcasts a signal that contains orbital data (from which the position of the satellite can be calculated) and the precise time the signal was transmitted. Orbital data include a rough
almanac for all satellites to aid in finding them, and a precise ephemeris for this satellite (determined with the help of ground stations). The orbital
ephemeris is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an
atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission of three (at sea level) or four (which allows an altitude calculation also) different satellites, measuring the time-of-flight to each satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time using an adapted version of
trilateration: see
GNSS positioning calculation for details. Each distance measurement, regardless of the system being used, places the receiver on a spherical shell centred on the broadcaster, at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where the shells meet, a fix is generated. However, in the case of fast-moving receivers, the position of the receiver moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that angle corresponds to the distance which the signal travels through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centred on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as
Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
Einstein's theory of
general relativity is applied to GNSS time correction. The net result is that time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds per day. Multiple SatNav systems can be combined in forming a position solution. Because the different systems have different time references, the receiver originally needed to estimate one additional inter-system time difference parameter for each additional system added, increasing the number of satellites required for a hybrid position fix. Since the early 2010s, a new generation of navigational messages broadcast by many satellites now include "GNSS Time Offset" parameters for converting between time references, allowing receivers to forgo this estimation when needed. Broadcast parameters are based on the estimates of the satellite's owners and their accuracy vary across systems. ==Applications==