What is Time to First Fix (TTFF)?

What is TTFF? What is Cold Start, Warm Start and Hot Start?

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- everything RF

Aug 25, 2020

Time to First Fix (TTFF) is the time required for a GNSS device to acquire satellite signals and navigational data to calculate its position or fix. A GNSS device must have three sets of data before it can provide accurate position information - GNSS satellite signals, almanac data, and ephemeris data.

Almanac data contains satellite vehicle orbital information and allows GNSS receiver to predict which satellites are overhead when they’re looking for GNSS signals. Using almanac data saves time as the receiver can concentrate on those satellites that it can see and ignore others that are out of its view.

Ephemeris data is the orbital position of each satellite. This data contains information on week number, satellite accuracy, and health, age of data, satellite clock correction coefficients, orbital parameters. It guarantees real-time satellite coordinate computation which is required in position computation.

Time to first fix (TTFF) has three starting conditions that are usually specified for GNSS receivers:

Cold Start: In this case, no information is available at the receiver, and therefore the receiver entails a full search of the sky for all satellites. This usually happens when we use a new GNSS receiver or one that has recently been factory reset. The process usually takes 2 to 4 minutes.

Warm Start: In this case, the receiver has valid almanac data (stored from a navigation message recently decoded or through assisted GNSS) and knows the time. However, it has no ephemeris data stored or the stored data has become outdated. TTFS for a warm start is usually around 45 seconds. A warm start can happen when the device has been off for a day or so.

Hot Start: In this case, the receiver has both accurate ephemeris data and almanac data. In this mode, the receiver is able to quickly track the overhead satellites and needs to download a minimum amount of data to establish a position. The "Hot Start" TTFF is also called "Time to Subsequent Fix" (TTSF). This is the shortest TTFF and just takes a few seconds.

GNSS manufacturers use various techniques to speed up TTFF, including downloading and storing almanac and ephemeris data via a wireless network connection from the mobile operator instead of through satellites. This is called assisted GNSS.