In GNSS and RTK systems, multiple timing metrics are used to describe receiver behavior during startup, reset, or recovery from signal interruptions. Among these, Time To First Fix (TTFF) and Time To First Ambiguity Fix (TTFAF) are the most common. Although they sound similar, they describe fundamentally different phases of the positioning process and should not be used interchangeably.
This article explains what each metric represents, how TTFAF is measured, and why clear terminology is essential when discussing high‑precision GNSS performance.
Time To First Fix (TTFF)
Time To First Fix (TTFF) describes how long a GNSS receiver takes to produce its first valid position solution after being powered on or reset. This includes satellite signal acquisition, decoding of navigation data such as time and ephemeris, and computation of a position, velocity, and time (PVT) solution. TTFF is therefore a general startup metric that reflects how quickly a receiver can provide an initial position from a user’s perspective. Importantly, TTFF does not imply any particular level of accuracy. A receiver may report a fix very quickly, but that fix may only be accurate to the meter level.
Time To First Ambiguity Fix (TTFAF)
Time To First Ambiguity Fix (TTFAF) measures something fundamentally different. TTFAF describes the time required for a receiver’s carrier‑phase ambiguity resolution algorithm to converge and resolve integer ambiguities, resulting in an RTK fixed solution. Unlike TTFF, TTFAF is not a startup metric. It applies once the receiver is already initialized, tracking satellites, and receiving correction data, and it ends when the first RTK fixed solution is achieved. Because ambiguity resolution is a discrete algorithmic event, TTFAF is a precise and repeatable measure of high‑precision GNSS performance.
How TTFAF Is Measured
To ensure that TTFAF reflects only ambiguity resolution behavior, it is typically measured using repeated hot starts. In a hot start, position, time, and ephemeris information are preserved, which excludes satellite acquisition and navigation message decoding from the measurement. This isolates the ambiguity resolution process as the only variable. By performing many hot starts, a statistically meaningful dataset can be collected, allowing TTFAF to be evaluated not only in terms of average performance but also in terms of reliability and repeatability. For this reason, TTFAF is commonly reported using percentile metrics such as p50, p90, or p95 rather than a single value.
Summary
TTFF and TTFAF serve different purposes and must be interpreted accordingly. TTFF describes how quickly a receiver delivers its first navigation solution, while TTFAF describes how quickly carrier‑phase ambiguities are resolved and a fixed RTK solution is achieved.