Time to First Fix (commonly abbreviated to TTFF) is the process of time in which it takes a GPS enabled device to obtain signal from enough satellites to determine accurately its location.
A GPS receiver must have three sets of data before it can accurately locate itself.
- GPS satellite signals from multiple satellites
- Almanac data
- Ephemeris data
As the location of satellites are constantly changing due to their varied orbital patterns, a gps enabled device often has to spend time searching for them.
GPS optimally requires clear line of sight in between the devices antenna and the satellites, free of obstructions.
Various conditions can affect the Time to First Fix, including the environment, weather, humidity and whether the GPS device is operating indoors or outdoors.
GPS Signal Lock. Warm, Cold & Hot
If a GPS device is new, has been switched off for extended periods of time, or has been moved for a long distance whilst switched off, it will take longer to acquire these required data sets and get a Time to First Fix.
This occurs because the GPS data is outdated and needs to refreshed, with up-to-date information downloaded to the device.
Cold Start
In a cold or factory start, the gps receiver has inaccurate estimates of, or is missing altogether the following:
- Current position
- Visibility of any of the GPS satellites
- Velocity
- The time
As a result such, the receiver must search for any & all satellites.
When the device acquires signal from a satellite, it can then begin to obtain the approximate location of all other satellites.
Information regarding satellite position is referred to as the almanac.
The almanac data can be received from any of the GPS satellites, and is valid for up to 180 days.
Warm Start
A warm or normal start entails the GPS device has the following
- Accuracy of within 20 seconds for timing
- Positioning of within 100km
- Velocity within 25 m/s
- Almanac data is valid
In a warm start the device must acquire a signal from each of the required satellites to obtain the orbital information.
Each satellite broadcasts its ephemeris data on an interval of 30 seconds, and will remain valid for no longer than 4 hours.
Hot Start
A GPS device that can hot start is one that has valid positioning, timing, almanac data & ephemeris data.
When a device can hot start, it is commonly in a type of standby state, ready to rapidly acquire satellite signals.
Whilst in the device in a stand-by host start state, the time required of a receiver to calculate a position fix is often referred to as Time to Subsequent fix (TTSF)
A hot start will ensure that initial location data is the most accurate.
Conclusion
Manufacturs of GPS devices use multiple types of techniques in attempt to speed up the TTFF, including the use of technology known as assisted GPS (aGPS or A-GPS)
Using aGPS, a device can download and store satellite almanac and ephemeris data via mobile network connection from the mobile operator.
This is as opposed to using satellites for the almanac which are not always readily available.
In a situation where a gps tracking device that is being used to monitor a vehicles location, a slow TTFF is not ideal.
This is because a vehicle can drive multiple kilometres before gps signal is acquired.
As a result, a slow TTFF is not recommended for monitoring distance traveled or in anti-theft situations gps tracking situations.
A short Time To First Fix, whether it be cold, warm or hot is highly desirable for GPS accuracy.