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Almanac data stored in the receiver of the satellite navigation system NAVSTAR/GPS is used for the:
  • A
    recognition whether Selective Availability (SA) is operative
  • B
    assignment of received PRN-codes (Pseudo Random Noise) to the appropriate satellite
  • C
    fast identification of received signals coming from visible satellites
  • D
    correction of receiver clock error

GNSS almanacs and ephemerides both form the navigation message transmitted by each satellite.
The navigation message consists of 5 subframes, where each frame is made up of 10 words and takes up 6 seconds to download.
The navigation message frame structure is as follows:

Ephemerides:

  • contains information on week number, satellite accuracy and health, age of data, satellite clock correction coefficients, orbital parameters

  • valid two hours before and two hours after time of ephemeris (toe). The toe can be thought of as when the data was computed from the GNSS control segment

  • Used for real time satellite coordinate computation which is required in position computation


Almanac:

  • contains less accurate orbital information than ephemerides

  • valid for a period of up to 90 days

  • Used to speed up time to first fix by 15 seconds (compared to not having almanac stored)


Thus, the receiver is capable of computing a position without having an almanac present.
The almanac helps out with fixing the satellites for the first time but that is about it.
The ephemerides however are vital for positioning computation.

The ephemerides are also used for post-processing as like was stated earlier, the ephemerides allow for the satellite position computation which is required in order to triangulate the receiver's position.
Thus, it is very common for clients to log the ephemerides of each constellation they are tracking with the receiver.

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