Short answer
The elapsed time from receiver power-on (or warm/cold start) to the first valid position fix. Cold start (no almanac, no recent position, no time) is typically 30–60 seconds; warm start (with valid almanac and approximate position) is 5–15 seconds; hot start (with current ephemeris) is <1 second.
Detailed explanation
TTFF measures the latency from receiver power-on to first valid position. It's a critical user-experience metric for navigation devices, especially mobile and IoT applications where the receiver is power-cycled frequently to save battery.
Three start scenarios are defined: cold start (no almanac, no recent position, no recent time — receiver must search for satellites blind), warm start (receiver has a valid almanac and approximate time / position but no current ephemeris), and hot start (receiver has valid ephemeris from <2-4 hours ago and can immediately compute satellite positions).
Typical TTFF: cold start 30–60 seconds (limited by needing to download a complete ephemeris from at least one satellite, which broadcasts at 50 bit/s over 30 seconds per satellite per frame), warm start 5–15 seconds (skips almanac download), hot start <1 second (skips ephemeris download too).
TTFF is sensitive to signal level — a weak-signal cold start can take many minutes or fail outright. A-GNSS (Assisted GNSS, where the receiver gets ephemeris over cellular instead of from the satellites) is a common acceleration: cellular phones do this routinely to deliver sub-second cold-start TTFF.
Related terms
C/N₀
The ratio of received signal power to noise power spectral density at the receiver. Expressed in dB-Hz (because the ratio has units of Hz). A clean rooftop GPS L1 receiver should see 45–50 dB-Hz on strong satellites; below 25 dB-Hz the receiver loses track.
DOP
A geometric quality metric expressing how the spatial distribution of visible satellites amplifies ranging errors into position errors. Lower is better: PDOP < 4 is good, < 2 is excellent. PDOP depends only on satellite geometry, not signal quality.
Ephemeris & Almanac
Two related types of orbital data broadcast by GNSS satellites. Ephemeris gives precise (cm-class) satellite positions, validity 4 hours. Almanac gives coarse (km-class) positions for the entire constellation, validity weeks. Receivers need both for cold start.