What is TTFF? Understanding Time to First Fix

Written by: Kristina Dems • Edited by: Rhonda Callow
Published Jun 7, 2010 • Related Guides: Gps Receiver

TTFF (Time to First Fix) is a peg in determining which GPS receivers give us location information faster. The faster the TTFF is, the better and more satisfying service we get.

What is TTFF?

TTFF, or Time To First Fix, is a specification that details the required time for a GPS, or Global Positioning System, receiver to acquire signals and navigation information from satellites and the time to calculate a solution for a certain position, which is also called a fix. Three specific scenarios make up TTFF. These scenarios are called cold or factory, warm or normal, and hot or standby.

TTFF Scenarios

In a cold or factory scenario, the GPS receiver is offering inaccurate estimates of position, velocity, time or the satellite reception. This scenario may also mean that the GPS receiver is missing, malfunctioning or still fresh from the manufacturer. In this scenario, the GPS receiver must acquire a stable signal from the satellite to get reliable information required for global navigation. The usual TTFF for this scenario is approximately 15 minutes.

In a warm or normal scenario, the GPS receiver is ready to get a signal from the source satellite. It can request navigation data at any time through a stable connection. The transmission of data, including the time it takes to find and connect to the satellite, requires an approximate 30 seconds.

In a hot or standby scenario, the GPS receiver already has a fix on the source satellite so it can immediately request navigation data without spending time in finding and connecting the satellite. Depending on the number of channels that the GPS receiver uses simultaneously, it can acquire data in no time at all.

Almanac and Ephemeris Data

GPS satellites offer two kinds of data to GPS receivers that they can use to get a location fix. Almanac data is usually not very accurate and may be as old as several months. This kind of data is easier to obtain because it usually is just sitting there on a satellite, ready to be accessed and downloaded. Ephemeris data is more precise, even featuring clock correction to make the data as current as possible. It has a shorter lifespan, given that it is updated more often than almanac data. The TTFF of GPS receivers to access and acquire both these kinds of data varies. This is a good peg for people who want to purchase a GPS receiver. The shorter TTFF of a GPS receiver, the better.

Speeding Up TTFF

GPS receiver manufacturers usually speed up their TTFF by using wireless network connections to download and store both ephemeris and almanac data. Wireless network connections are usually faster than data acquisition using satellites. GPS receiver users can minimize slowing down of TTFF by keeping the receiver on and using it to get location fixes. Turning the receiver off for long periods of time or not using it after a great distance is already traveled since it was turned on will effectively slow down TTFF.

TTFF in the Bigger Picture

Time To First Fix represents our modern fixation on knowing where we are or where everything is located. We want fast and reliable location fixes so we rely on GPS receivers to assist us in this pursuit. TTFF is what dictates which GPS receiver should we purchase and use. TTFF helps us in establishing our location identity and in satisfying our craving for instant location fixes.

Berend Harmsen Jan 25, 2011 9:45 AM

GPS TTFF

I stumbled on the article about GPS TTFF definitions and I think it gives a wrong impression on some of the fundamentals of how GPS works: the way it's written the suggestion is given that GPS receivers somehow 'request' data from a satellite.

This never happens. GPS receivers are passive devices (in relation to the GPS satellites). The satellites continually cycle through the data they transmit regardless of whether a receiver is listening or not.

The difference between factory/cold/warm/hot only refers to the quality of the data that the receiver already posesses:
full Almanac: takes 12.5 minutes to download
Ephemeris: cycles every 30 seconds
and so on.
The more accurate the downloaded data is, the more accurate the assumptions the receiver can make about the expected data coming from space, and hence the faster the TTFF.

When GPS receivers get a fix in a little over 30 seconds, it means that it had accurate Almanac, but no valid ephemeris. With valid ephem and accurate clock, a fix should be obtained in less than 10 seconds.

In each case position determination is a process of iteration, whereby the receiver essentially 'tries to fit assumed positions' with the incoming satellite data.

It is important to realize that GPS satellite signals are incredibly weak, and the bitrates are very low (50 bps), so the receiver actually has very little data to work with, hence the strong reliance on clever assumptions.

Look at it like this: considering the weakness (and noisyness) of the satellite signals, in order for a position to be calculable by the receiver, it basically needs to know where it is in order to know what the signals from space would look like (it needs to factor things like doppler shifts: satellites moving towards you are blueshifting, the ones moving away are redshifting).

So it needs to know where it is to be able to calculate where it is...
It's actually a miracle that it works at all.

...last words to use up max. text available to me...

....