The Time To First Fix (TTFF) is a measure of performance of a GNSS receiver that accounts for the time elapsed from the GNSS receiver switch-on until the output of a navigation solution.

In order to compute a navigation solution, the GNSS receiver needs to track valid signals of at least four satellites to get ranging information and to decode the navigation message transmitted in the signal. The TTFF depends on the starting conditions of the receiver as given below:

• Cold start: the receiver has no information and therefore it needs to do a full search of the sky for all satellites and then compute the navigation solution.
• Warm start: the receiver has a valid almanac (either stored from a navigation message recently decoded or obtained via other means, e.g. via RTCM messages).
• Hot start (Re-acquisition): the receiver has both accurate ephemeris and information on frequency offsets as well as an accurate initial solution.

The TTFF values for different initial conditions of Septentrio GNSS receivers can typically be found in the datasheet as shown below.

Cold Start, Warm start and Hot start (Re-acquisition) values for Septentrio receivers

Please note that the considered environment will also have a critical impact. In harsh environments, such as urban canyons, the signals may suffer interruptions or even obstruction which can increase the TTFF values.

摘錄自：

https://customersupport.septentrio.com/s/article/What-is-TTFF-and-what-are-the-TTFF-values-for-Septentrio-GNSS-receivers