How The GPS System Works

How The GPS System Works

Some background about how the GPS system works, and make it work for you

How The GPS System Works 

How the GPS System Works

There is lots of technical information available elsewhere on the web if you’re that way inclined, but there are some key pieces of information that you need to know so that your receiver works when you need it to.

To get a two dimensional fix, aka a grid reference or position on a map, you need to have at least three satellites available. This is important information to know, as you need to have a clear view of the sky with as broad an horizon as possible. So, you may be able to look up and see the sky, but the satellites may be located closer to the horizon than you can see. Position yourself appropriately.

The positional fix from satellites should be accurate to within a few metres 95% of the time. Always bear in mind that the position shown on your GPS receiver could be within that 5% that is less accurate. If your life depends on the GPS position, wait a few minutes, maybe even switch it off and on, and see if the position is still the same. To get a positional and height fix, you need to have at least four satellites in view, all with good signal strength. Height measures are suppose to be approx within 20m 95% of the time

GPS units also have limitations, which you will have noticed if you powered one up and it took an age to acquire a signal and even longer to provide a position. There are reasons for this.

Ephemeris and Almanac Data

GPS satellites transmit two types of data, being ephemeris and almanac data. Almanac data describes to the GPS receiver the approximate position of the orbiting satellites, and such information is updated regularly by the USAF. Receivers need that kick-start to know where in the sky to look for satellites, which is in part based on your geographical position at the time. For this reason, if you’ve not used your receiver for a few days, or have travelled approx 600 km, the initial satellite acquisition will be what’s known as a ‘cold start’. Almanac data is valid for a maximum of 60 days, and most be downloaded if required. On older units, that download could take up to an hour.

Ephemeris data consists of two types of data; time and position. Basically, the satellites send a message every few seconds, which consists of the time that the message was sent, and position of the satellite at the time sent. From this information, the GPS receiver can calculate its distance from the satellite by subtracting the time sent from time received, as the speed travelled by the signals is a constant speed. Combine that calculation messages from at least two other satellites, and the receiver calculates its position by triangulation.

Ephemeris data is updated about every 2 hours and remains valid about 4 hours, so if you have your GPS receiver switched off for more than that time, it will typically take a little longer to get a satellite fix. This is referred to as a warm start, and may take a couple of minutes to get a positional fix.

If you travelled 600 miles or more, or your GPS unit no longer has an accurate time, a new almanac will be downloaded so that your unit knows where to look in the sky for satellites, hence taking longer to obtain a positional fix. This is a cold start. You can sometimes kick-start a cold start by manually entering a grid reference to the unit.

By contrast, if your GPS unit is switched off for only a few minutes, or not at all, then the positional fix will be quick, taking seconds. This is a hot start.

Map Datums

GPS receivers calculate co-ordinates in latitude &longitude, but the software of the unit can convert such co-ordinates into something more useful for land based navigation. In the UK we use the British National Grid, but other conversion systems exist, call datums. For UK use you must set your receiver to the OSGB or OSGB36 datum, which will give you a UK grid reference. If you selected, for example, UTMS/WGS84 datum, a grid reference would be calculated, but it would be of no use to anyone.


Some modern GPS units now come equipped with in-built mapping, meaning that an accurate location can be shown on a map on screen too, and not just represented by a grid reference. It’s even possible to get Ordnance Survey maps on some units, giving the use a rich graphical experience and accurate position. You can see why it is tempting to rely on a GPS unit for the whole day in the hills, but remember that your GPS Unit should be there as a secondary navigational aid, not the primary one.