Unit 2. GNSS computational methods I - Coarse positioning »
2b. Satellite orbital information: ephemerides
Each GNSS constellation has its own particular way of providing positional information. In the case of the U.S. GPS constellation, every GPS satellite broadcasts a “navigation message,” which contains the information needed to identify ALL the satellites and compute each one’s approximate position in orbit.The network of ground-based observing stations constantly tracks the satellites and computes predictive models of each satellite’s orbit. The predicted orbital parameters for a given satellite are recorded in a docuмent called an “ephemeris” (plural, “ephemerides”). Since satellite orbits vary over time, each ephemeris needs to be updated regularly. This is carried out by the ground-based Master Control Station, and transmitted to the GPS satellite constellation.
An ephemeris provides the parameters that can be used to calculate the position of a satellite along its orbit at any moment in time.
An almanac provides the ephemerides for an entire constellation of satellites
The navigation message broadcast by each satellite includes its own ephemeris (called the “broadcast ephemeris”), updated at frequent intervals. It also contains the “almanac,” a less-frequently updated collection of the ephemerides for ALL GPS satellites in the constellation. Because the almanac is updated at much longer intervals, the orbital parameters are less accurate than the broadcast ephemeris from each individual satellite..
Topic: Global Navigation Satellite Systems -- tutorial (Read 27732 times)