Time to first fix

Concerning a Global Positioning System (GPS) receiver, Time To First Fix (TTFF) is a specification detailing the time required for a GPS receiver to acquire satellite signals and navigation data, and calculate a position solution (called a fix).

The TTFF is commonly broken down into three, more specific or "real world" scenarios. As defined in the GPS equipment guide:

* Cold or Factory: The receiver is missing, or has inaccurate estimates of, its position, velocity, the time, or the visibility of any of the GPS satellites. As such, the receiver must systematically search for all possible satellites. After acquiring a satellite signal, the receiver can begin to obtain approximate information on all the other satellites, called the almanac. This almanac is transmitted repeatedly over 12.5 minutes. Almanac data can be received from any of the GPS satellites and is considered valid for up to 180 days. Manufacturers typically claim the factory TTFF to be 15 minutes.

* Warm or Normal: The receiver has estimates of the current time within 20 seconds, the current position within 100 kilometers, its velocity within 25 m/s, and it has valid almanac data. It must acquire each satellite signal and obtain that satellite's detailed orbital information called ephemeris data. Each satellite broadcasts its ephemeris data every 30 seconds, and is valid for up to four hours.

* Hot or Standby: The receiver has valid time, position, almanac, and ephemeris data, enabling a rapid acquisition of satellite signals. The time required of a receiver in this state to calculate a position fix may also be termed "Time to Subsequent Fix" (TTSF)

ee also

* Global Positioning System (GPS)
* GPS signals
* High Sensitivity GPS

References

* US Coast Guard, Navigation Center's [http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf NAVSTAR GPS User Equipment Introduction] .