Consumer GPS Comes of Age
By Martyn Hodgson
One of the most popular new gadgets being used by Biological Recorders in the last couple of years are hand-held GPS receivers. GPS, or Global Positioning System, receivers allow the user to locate their position anywhere on the earth’s surface to remarkably high accuracy. Two recent changes, (one gradual, one sudden) have improved the accuracy of GPS receivers greatly. For less than £100 you should now be able to pin-point your position to within about 20m almost anywhere on earth, using equipment the size of a mobile telephone.
The basics of how GPS works
What follows is very simplistic. A fuller explanation (still simplified, but quite understandable) can be found at Trimble Navigation’s web site.
In the 1970’s the American Department of Defence launched 24 satellites into geostationary orbit. From anywhere on earth, if you had good enough eyesight, you could see at least 5 of these satellites (assuming there aren’t buildings or mountains etc. in the way). From Britain there seem to be about 7 or 8 ‘visible’. These satellites transmit radio signals which the GPS receiver picks up. The GPS receiver uses these radio signals to triangulate its location, based on its distance from the satellites. This is requires incredibly accurate timing (an error of less 40 billionths of a second) as the distances are calculated from the ‘time lag’ between the signals.
To make all this possible, there are a number of factors that must be taken into account. The most important being that each satellite must know its exact location. This is managed by 5 ground stations which monitor the satellites and tell them what adjustments need to be made to their signals to compensate for ‘drift’. At the users end, the GPS receiver has to allow for delays in the radio signals as they pass through the earth’s atmosphere.
In order for all this to work, a GPS receiver needs to be able to receive transmissions from at least 4 satellites. As mentioned earlier, ignoring the effects of buildings, mountains etc., at the location with the poorest coverage on earth you can still see 5 satellites. So, in theory, where ever you are you should be able to get your location, with no loss of accuracy.
Recent changes that have improved GPS
Over the last two or three years, small ‘consumer’ GPS receivers have really taken off. They are being put into cars, telephones, computers, and dedicated hand held GPS units. This has largely been made possible by improvements in the computing power available in lower (electrical) power computer processors.
However, there has been a big drawback. It is called the ‘Selective Availability Program’. Because GPS covers the world, and anyone can use it, the American Government decided that a fuzziness (signal degredation) would be deliberately added to the transmissions. This limited its accuracy for conventional receivers to 100m or so. In the mean time, electronics engineers have been busily working out ways of getting around the Selective Availability Program. The commonest method is called Differential GPS. If you are interested in DGPS then see the details on Trimble Navigation’s web site.
The big news this year though, is that from 1st May 2000, the Selective Availability Program was switched off permanently. This won’t make DGPS redundant as it still offers higher accuracy (an error of about 1.5m), but for Biological Recorders, standard GPS is now even better. For details regarding the decision to switch off Selective Availablity, see the Interagency GPS Executive Board web site.
Accuracy of standard GPS today and its limitations.
A standard GPS receiver will cost you less than £100. It should be about the size of mobile telephone (but it won’t fry your brain - I hope). With the Selective Availability Program switched off, and a clear sky all around you, you should be able to get a horizontal accuracy of at least 13m, and a vertical accuracy of 16m (these numbers are based on figures given at www.trimble.com/gps/howgps/moreinfo/aa_m3s.htm).
However, there are a number of other factors that can seriously mess up your readings. A GPS receiver needs a clear line of sight to the satellites it is using. Heavy cloud and rain certainly slows down the time taken to get a reading, but it will get there after a minute or so. With my receiver I have managed to get a position in 20 seconds at sea on a sunny day, compared with 70 seconds in low cloud on a rain swept hillside. A similar effect is caused by heavy tree cover. More importantly still, steep valley sides, cliffs, buildings, or anything that blocks your view of parts of the sky can stop your GPS receiver from making a reading at all. Remember, you need to be able to see at least 4 satellites, and there are currently only 24 covering the earth. The number of satellites is due to increase in the next year or two, but even so, if you can only see a small amount of the sky you’re probably in trouble.
Other standard features
Even with hand held GPS receivers, there are a number of other software features that generally come as standard. These include a speedometer (quite fun when you are a car passenger), compass and route plotting. This last feature allows you to either preload points and then the GPS receiver guides you to between them, or to simply mark points as you reach them. You can then use these points to retrace your route.
Conclusions
In spite of their limitations in enclosed surroundings, hand held GPS receivers are potentially very useful for biological recorders. There is no excuse now for inaccurate, or missing, map references (all of the consumer versions I’ve seen will give readings in OSGB format). You don’t even need a map! Actually I suspect that we’ll start to notice slight errors and discrepancies in UK maps. You’ll certainly start to see errors in maps abroad. Ordnance Survey admit themselves that if you scale one set of maps to another, they don’t always match!
I wouldn’t dare recommend any model of GPS as they change so regularly, but a browse through the internet, or Maplin catalogue, should give you a good pointer to what is available. However, as a starting point, I think that the two manufacturers whose products are most widely available in the UK are Garmin and Magellan.
Futher information
The following web sites may be of interest:
