Why won't my GPS get a satellite fix?

There are many reasons why a GPS might stop working, Some explanations and general pointers are listed here

In this article the following assumptions have been made :-

1a) The correct GPS power supply / batteries are being used
1b) The GPS powers up and shows signs of attempting to get a satellite fix
1c) The GPS is outside and has a clear 'view' of the sky (no trees, buildings etc)

When the GPS is turned on it can be in one of several states of 'readiness', e.g:-

2a) No Time/Date, Almanac or Ephemeris data (Factory Start)
2b) No Ephemeris data (Cold Start)
2c) Time/Date, Almanac, Ephemeris data all present (Warm Start)

The amount of time it will take to get a fix from each of the above will vary as the early units can only look for one satellite at a time, newer units can look for 12 or more in parallel, on some units an Almanac can be downloaded to the unit to speed things up, When working normally even the oldest units quote 8Mins - 2Mins - 20 Seconds for 2a - 2b - 2c above, but due to internal clock drift and other problems it can take a lot longer

Almanac . . A rough 'Picture' of which satellites the GPS should expect to see in the sky, the whole 'Picture' is transmitted by all satellites and is valid for several months as long as the GPS hasn't moved more than a few hundred miles and has the correct time / date

Ephemeris . . Very detailed data of a single satellite containing position, path, clock correction data etc. Each satellite supplies only its own data and it is only valid for 30 Minutes

In order to calculate a position the GPS needs a VERY accurate clock, This type of atomic clock is very expensive, bulky and power hungry, so this is not built into the GPS, instead it has a crystal clock similar to one in a watch, this is normally enough to receive data from a satellite and then synchronise it's internal clock with the satellite's clock, after synchronisation the GPSs clock, is accurate enough to calculate the distance to a satellite, with 3 more satellite distances, the GPS can calculate a position

Having got a fix, the GPS can store clock offset data to fix any drift in it's internal clock, however in some cases the GPS's internal clock can drift so far out that it can't 'see' a satellite to synchronise with in the first place, especially if it hasn't been used for a long time

It is important that the data the GPS receives for time/date, Almanac, Clock Offset, Ephemeris etc. is not lost, if it is then the unit has to start from scratch, this data is retained using a small back-up battery or a non volatile memory system. In older units the combination of loss of data and clock drift is enough to push the GPS into a situation where it will never get a satellite 'fix' without being re-calibrated

A faulty back-up battery can stop the unit working even when the main batteries/Power supply are in full working order and replacing this battery can fix some problems

In older units it is even more important to ensure data is not lost because if current time/Date info. is lost the unit may encounter the 'Week Rollover' problem, this is where the unit displays the wrong date and in many cases there is no solution for this, the loss of the correct date sometimes prevents the unit getting a 'fix', in others the unit works just as before, but any time stamps provided by the unit will be incorrect

So, how do you fix the problem?

In some cases a new back-up battery will fix the problem, other things to try when getting a 'fix' is different temperatures, the GPS's internal clock is temperature sensitive and may have less 'drift' when colder or warmer than normal

In all cases you will need fully charged batteries (where applicable) and a clear outdoor position with no trees buildings or anything else blocking a view of the sky, the next thing you need is time, a modern unit shouldn't need more than hour where-as an old GPS38 could take 12 hours, so leave it as long as possible

If you manage to fix your GPS you will have to ensure the unit does not lose it's data again or you will be back to square one, the backup battery may be past it's best but you can help matters by keeping the main batteties in good condition and getting a new 'fix' as regularly as possible