- Mon Sep 16, 2013 7:06 am
GPS receivers need orbital data about each GPS satelite vehicle (SV). That data changes on a regular basis so it is transmitted by the SV. To get all the needed data takes a while. How long depends on many factors, but 2 minutes is in the right ball park. Now if the GPS receiver already has that data, then it does not need to wait to download it before it can start providing solutions. This is the difference between a warm and cold start. During a cold start, the GPS receiver has no cached data. A warm start, the receiver has cached data and and start after just a few seconds of synchronization. There is also a hot start where the receiver starts to produce results right away, but needs even more cached information.
The cached information is only valid for a few hours. Even if you maintain backup battery power to the receiver, by morning that data will be invalid and your GPS will start with a cold start.
There are two ways to get around your long duration (2 minutes) for that first fix. The first is to buy a GPS unit that optimizes the time to first fix. Top of the line units have TTFF of around 20-30 seconds with clear view of the sky. The second is to download from the web the information needed by your GPS (the almanac, ephemeris, and time) and upload that to your GPS prior to wanting your first fix. You will now have a warm start scenario that should provide a solution in just a few seconds (assuming satelite visibility!).