Page 2 - GNSS Simulation - A Guide to Testing
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Time To First Fix (TTFF)
What is Time to First Fix?
When a GNSS receiver is turned on, it searches for satellite signals and this process is called
acquisition. Once it has acquired signals it tracks them to calculate its position, or fix. The time
taken from switch on to a valid location fix is known as Time to First Fix or TTFF.
In order to calculate a valid location fix, the GNSS receiver requires almanac data and ephemeris
data in addition to the GNSS satellite signals.
Almanac Data
Almanac data contains coarse orbital and status information for all satellites within a
constellation. Every satellite broadcasts the almanac data for the whole constellation so that a
receiver can approximate the locations of all satellites and determine which it should track. By
only focussing on those above the elevation mask, the receiver narrows its search and saves time
on acquiring a position fix.
Ephemeris Data
Ephemeris data contains very precise data on the orbital path and position of each satellite,
including clock correction data. This allows a receiver to accurately locate an individual satellite
at any time. Each satellite only broadcasts its own ephemeris data.
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