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Understanding Almanac and Ephemeris Data

written by: Kristina Dems•edited by: Rhonda Callow•updated: 6/10/2010

A GPS receiver requires almanac and ephemeris data to provide the necessary location information requested by a user. This article describes how these types of data are used and what parameters they contain.

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    What are Almanac and Ephemeris Data?

    Global navigation satellites broadcast two kinds of data called almanac and ephemeris data. Almanac data contains course orbital parameters from all global navigation satellites and it can be used by and transmitted to other navigation satellites. Almanac data is not very precise and it can be several months old. On the contrary, ephemeris data is very precise because of the fact that it uses clock correction technology. For precise navigation information, ephemeris data is used by global positioning systems. It only stays valid for approximately 30 minutes and it is broadcasted by satellites every 30 seconds. Every navigation satellite broadcasts have its own ephemeris data only.

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    What is Almanac Data for?

    Almanac data is used when a GPS receiver is turned on after being inactive or off for 30 minutes or more. It is utilized to establish the position of a GPS receiver at a given time depending on its data in the almanac. After the specified TTFF (Time to First Fix) navigational information, the GPS receiver can select the appropriate navigation satellites for the initial position search.

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    What is Ephemeris Data for?

    From the selected navigation satellites, ephemeris data is collected to provide the requesting GPS receiver a precise location. When the GPS receiver is turned off and turned back on again, locking on a navigation satellite and data transmission will be very quick but only if the ephemeris data is still valid. After the 30-minute lifespan of ephemeris data, all requests will require another recollection of ephemeris data by the appropriate navigation satellites.

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    How Do Almanac and Ephemeris Data Work Together?

    A GPS receiver requires the acquisition of both almanac and ephemeris data to determine a target location. Requesting and transmission of almanac and ephemeris data in RINEX or Receiver Independent Exchange Format, is the bulk of the communications between GPS receivers and navigation satellites. Most GPS receivers, particularly the older models, need to acquire almanac data first before it can request ephemeris data. Almanac data basically tells the GPS receiver where it can get its required ephemeris data. If both almanac and ephemeris data are up to date, the GPS receiver can immediately do a location search even if it has just been turned on.

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    What do Almanac and Ephemeris Data Contain?

    Almanac data contains ten parameters that describe satellite orbital characteristics over long periods of time that can extend up to several months. This is enough to provide the GPS receiver with the approximate orbital information, position and Doppler frequency which describes the shift in movement of navigation satellites. Ephemeris data provides a description of the relationship between the GPS time and the satellite clock. These parameters along with location information, specified by GIS or Geographic Information System, in the ephemeris data provide the GPS receiver precise location and time data.

    You may also be interested in learning about SBAS (Satellite Based Augmentation System) and GPRS (General Packet Radio Service).