Is It Really Necessary for GPS Receivers to Store Both Ephemeredes and Almanacs?

摘要

Global Positioning System (GPS) receivers usually store both satellite ephemeredes and almanacs in order to have a fast time to first fix (TTFF). The paper compares the accuracy of satellite positions and velocities calculated based on expired ephemeredes and the one based on the same aged almanacs. Results demonstrate that expired ephemeredes have the same level of satellite orbital calculation accuracy as valid almanacs. Thus, the paper concludes that it is not necessary for GPS receivers to store both ephemeredes and almanacs. A mechanism of selecting either an ephemeris or an almanac for a satellite is also provided. This idea and method can be used to reduce non-volatile memory space usage so that any crowded GPS receiver memory space crises can be alleviated in certain degrees.