GNSS (GPS) for Bushwalkers
The Global Navigation Satellite System (GNSS) receiver is a device which receives signals from a number of satellites above and calculates its position – which is then displayed for the user to transfer to a map, or using a built-in map display plotted on an electronic map. The USA Global Positioning System (GPS) is the best known GNSS but other systems are now being used: The Russian GLONASS, European Galileo and Chinese BeiDou.
GNSS can give positions to high accuracy – the typical consumer grade GNSS is specified to have 15m latitude/longitude position error 95% of the time. This is more than adequate for bushwalking use. In fact the GNSS is usually more accurate than the map you are using to plot the coordinates!
The first GNSS- GPS, was fully established by the US military in 1995 to provide accurate navigation for their forces. It has evolved a major civilian role and now the GPS system is critical for everything from the navigation of ships and aircraft, monitoring the location of police cars and railway locomotives – even providing accurate network timing in CDMA mobile phone systems! Receivers are now being made that can use multiple GNSS so offering potentially increased accuracy and availability.
How Does It Work?
The GNSS system has three main segments:
- The User Segment – This is the GNSS 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 satellites in mid-altitude and geostationary 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 – A manned control centre on Earth that monitors, maintains and oversees the GNSS system.
For detailed information about how GNSS calculates position see the many excellent references on the web such as Wikipedia or How Stuff Works. However key points to note about how GNSS 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 GNSS will still show a position but it can be very inaccurate. If you have more than 4 satellites the GNSS can average between the satellites to give a slightly more accurate position.
- A GNSS is the most accurate clock you are ever likely to have. It synchronises its clock to the atomic clocks in the satellites, meaning your GNSS will display the time extremely accurately (as long as you have set the time zone correctly!)
- GNSS works by receiving signals from satellites in the sky above. Therefore a GNSS 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: but be warned some receivers do not like Lithium batteries).
- A GNSS 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 GNSS 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. But be warned, using these augmentation systems outside of rated coverage areas may decrease accuracy. e.g. using WAAS or EGNOS in Australia will likely decrease accuracy. Anyhow, there is no need to fuss with augmentations systems as 15m accuracy from the normal system is all a bushwalker needs.
What Makes a Good Bushwalking GNSS?
There are many applications of GNSS – from in-car navigation systems to fish-finders – and they all have their own features but there are some key features which makes some GNSS stand out for bushwalking use. These key features are:
- A high sensitivity receiving system – GNSS being used by bushwalkers is used in gorges, under heavy tree cover and in the rain. This means a bushwalking GNSS needs a more sensitive signal receiving system than is found in most GNSS. As of July 2009 the best GNSS 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 GNSS enabled iPhone back in the car I think.
How Do You Use a GNSS?
A GNSS 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 GNSS you MUST check the following:
- Check the map datum in the GNSS 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 GNSS has a lock on at least 4 satellites, preferably more.
Only after these three things have been done correctly will a GNSS give you an accurate position.
For further details about how to use GNSS see Geospatialworld-GNSS, Spatial Services NSW “Exploring GPS”, Garmin’s GPS Guide or GORP. An excellent way to really hone your GNSS skills is by geocaching. This is the sport of finding “caches” which people have hidden and published the GNSS coordinates of the location. Many of these caches require very careful and accurate use of a GNSS to find so are an excellent way to sharpen your skills.