How Does It Work?
The GPS system has three main segments:
- The User Segment – This is the GPS receiver you hold which reads the satellite signals and calculates its position. Note the receiver only receives – it does not transmit anything.
- The Space Segment – This is the constellation of between 24 and 32 satellites orbiting the earth. These are US military satellites in mid-altitude orbits with a highly accurate clock on board. They constantly transmit the time and the satellite’s position in space (along with other information).
- The Control Segment – This is a control centre in the USA, run by the United States Air Force 50th Space Wing. This organisation monitors, maintains and oversees the GPS system.
For detailed information about how GPS calculate position see the many excellent references on the web such as Wikipedia or How Stuff Works. However key points to note about how GPS work as they have direct implications on their use are:
- You need to have a lock on 4 at least satellites to determine a position. If you have less than 4 satellites then your GPS will still show a position but it can be very inaccurate. If you have more than 4 satellites the GPS can average between the satellites to give a slightly more accurate position.
- A GPS is the most accurate clock you are ever likely to have. It syncronises its clock to the atomic clocks in the satellites, meaning your GPS will display the time extremely accurately (as long as you have set the time zone correctly!)
- GPS works by receiving signals from satellites in the sky above. Therefore a GPS works best when it can see large areas of sky. Things which can cause problems for reception are:
- Valleys, cliffs, gorges etc.
- Tree cover, particularly wet tree cover.
- During rain or snowfall
- Cold conditions (due to normal alkaline batteries not working effectively. Lithium batteries are good for cold conditions).
- A GPS does not transmit anything to determine its position. it calculates its position purely on the signal it receives from the satellites.
- A number of systems are available to allow GPS to achieve higher accuracies. The most common ones being “Wide Area Augmentation System” (WAAS) and “Differential GPS” (DGPS) systems. Both of these systems have the potential of improving the GPS accuracy to 1m or even better, but the systems are not widely available in bushwalking areas. And of course – why bother? The 15m accuracy from the normal system is all a bushwalker needs.
What Makes a Good Bushwalking GPS?
There are many applications of GPS – from in-car navigation systems to fish-finders – and they all have their own features but there are some key features which makes some GPS stand out for bushwalking use. These key features are:
- A high sensitivity receiving system – GPS being used by bushwalkers is used in gorges, under heavy tree cover and in the rain. This means a bushwalking GPS needs a more sensitive signal receiving system than is found in most GPS. As of July 2009 the best GPS receive systems are the SiRF STAR III series. A comparison of a Garmin GPSMap 60CSx and a Garmin Etrex made in 2002 (before the new high sensitivity receiver mode was available) showed the 60CSx had far better accuracy and reliability in valleys and under tree cover.
- A rugged and water resistant case – they get a beating in the bush so make sure they can take it. Best leave the GPS enabled iPhone back in the car I think.
How Do You Use a GPS?
A GPS does not replace map and compass for navigation in the bush, but it is an excellent additional tool. It can speed up decision making and reduce mistakes. Route-finding, map reading, map to ground and compass skills are still very important skills.
Before you use a GPS you MUST check the following:
- Check the map datum in the GPS matches the map you are using. For topographic maps in NSW commonly used bushwalking the datums to use are “Australian Geodetic Datum 1966” or AGD66 for first and second edition 1:25000 topographic maps (with the map on one side of the paper only); or “Geocentric Datum of Australia 1994” or GDA94 for third edition 1:25000 topographic maps (with the aerial photo on the back).
- Check the position coordinates. Most bushwalkers use “Universal Transverse Mercator” or UTM coordinates, as this is just an extension of the normal grid reference. Note the last digit in the UTM easting and northing is metres.
- Make sure the GPS has a lock on at least 4 satellites, preferably more.
Only after these three things have been done correctly will a GPS give you an accurate position.
For further details about how to use GPS see The Department of Lands “Exploring GPS”, Garmin’s GPS Guide or GORP. An excellent way to really hone your GPS skills is by geocaching. This is the sport of finding “caches” which people have hidden and published the GPS coordinates of the location. Many of these caches require very careful and accurate use of a GPS to find so are an excellent way to sharpen your skills.