Alastair's GPS Page

This page contains links to information about the Global Positioning System (GPS.) GPS uses a number of satellites (kindly supplied by the US military :-) to calculate the position of the receiver to an accuracy of around 50 metres (for civilian units.) GPS locators are compact enough to be carried in the hand when walking around.

What does GPS do for you?

When I first looked for a GPS unit I wasn't completely sure what handheld GPS can and cannot do. Now I have owned one for a few weeks, here is my top 10+ list of stuff I would have liked to know more about before buying (from my GPS III - these may not be true for other models):
  1. GPS reports your altitude as well as your position. However altitude is much less accurate than latitude/longitude because the locator selects the set of satellites which gives the best horizontal fix at the cost of the vertical accuracy.
  2. GPS reports your current speed. This is aparrently accurate to 0.1 Knot and modern receivers are capable of measuring speeds upto 999 Knots. Speed is updated on mine every second or so.
  3. Some GPS will plot your position on a chart showing points you have marked as waypoints. You can use this to build maps on GPS units which don't have built in maps of the area you are in by putting waypoints at landmarks you pass, and using the landmarks and the waypoints to navigate from A to B. Look for how many waypoints your GPS can store in the literature - 200 or more is best.
  4. Most GPS can be connected to a PC via a serial cable (look for NMEA output in the manual.) Good PC map software will take this NMEA output and use it to plot your position on the map. The format definition for NMEA is also available on the web so you can write your own programs to do cool stuff with GPS data.
  5. GPS units with 12 parallel channels perform much better in difficult reception situations than single channel multiplex units. If you see a cheap handheld GPS it is worth checking which type it is - it is worth spending a little more for one with 12 channels.
  6. The radio signals used by GPS receivers are relatively low power, and will not travel through solid objects like buildings or metal parts of a car. They will travel through glass and (not too thick) tree cover though. In situations where the coverage is patchy like this a 12 channel receiver works much better.
  7. The GPS stores data from the satellites it was last connected to while it is switched off. When it is switched on again it tries to use this data to make a quick connection. Generally, the less time a GPS has been off the better it will do trying to get a quick satellite lock. You will see times for 'warm' and 'cold' starts in GPS literature - a 'warm' start is one where the GPS has been switched off for only a short time.
  8. My GPS works fine on the dashboard of a car. I have used this several times and I have had very little trouble with loss of reception, even in built up areas.
  9. It is a bit more difficult to get good reception through the window of a train because less of the sky is visible than through a slanted car windscreen. I have managed to do this a couple of times though.
  10. GPS III only: the GPS III comes with inbuilt maps which cover the whole world (see next item.) In the UK the map covers motorways, major A roads, major rail lines, large rivers and lakes and medium to large towns and cities. The maps seem to vary in accuracy and detail with area, but I find them good enough to be useful - especially when I have added my own waypoints to the map.
  11. GPS III only: There are two flavours of GPS III available from Garmin - the US and Rest of World models. The difference is in the inbuilt maps - the US model has high detail of the US and low/no detail of the rest of the world. The rest of world model has things the other way around.

    Coming soon!

    This site is still 'under construction' (just like the rest of the web!) but I have a few ideas of things which I will add very soon:

    Please feel free to mail me with ideas and material for this page.