Use of Global Positioning System navigational aids

  • How GPS works
  • Accuracy
  • Some questions to ask when buying a GPS
  • GPS uses and limitations

The Global Positioning System or GPS is the modern day equivalent of celestial navigation. The sextant has become the GPS receiver and 24 orbiting satellites have replaced the moon and the sun. The United States Department of Defence initially established GPS. When made available to the civilian population, it was immediately recognised by seafarers as the opportunity to abandon their sextants. With the recent reduction in size and price of GPS, more people are venturing into the world of navigation with this handy tool. Before you do however, it is important to understand a few basic fundamentals of their operation and their constraints.

How GPS works

The GPS fixes its position on the ground by referring to satellites which orbit the earth in six precise orbits. By sending a signal to, and receiving a return message from those satellites, the GPS is able to fix its position by way of time and distance calculations.

A visual example of the GPS constellation in motion with the Earth rotating. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time.

Accuracy

At present, accuracy of the GPS is generally accepted as being ± 20 metres in general outdoors use with hand-held GPS units.

The GPS unit must be in contact with a minimum of three satellites to make the calculations required. With three satellites, the position is referred to as a two dimensional (2D) position, and is not sufficiently accurate for most purposes, as a 2D position requires the GPS to make assumptions about some of the variables, adding error. To effectively fix a three dimensional (3D) position, the GPS must be able to see and communicate with at least four satellites. That is, it must have a clear unobstructed view of the satellites.

For greater accuracy, the satellites need to be spaced equally around the sky and not all overhead or to one side of the sky, which can occur when under trees, near mountains or in canyons or valleys. This can affect the GPS’s ability to accurately calculate position. Many of the good GPS units give an indication of the number and position of satellites being used at any given time, yielding a good indication of likely accuracy.

Another variable which affects the accuracy of the GPS is the information it is given by the user about where it is and the map being used. When setting up your GPS it is important to enter the correct time zone and map datum for the map you are using. This information should be checked prior to use and may need to be updated whenever the unit suffers a total power loss, or if changing to maps with a different datum.

Some questions to ask when buying a GPS

  • Does the brand have a good reputation?
  • Does it have a backlit screen which is easy to read and contains all the information required without having to scroll through several screens?
  • Can it switch between portrait and landscape (vertical and horizontal) display on the screen?
  • Is it simple to operate (even with gloves on)?
  • Is the unit water and dust proof?
  • What temperature range can it operate within?
  • Does it have a battery indicator which accurately indicates the charge remaining in the batteries?
  • How long do batteries last under normal usage?
  • Will the unit accept disposable batteries as well as rechargeable ones?
  • Is there a simple ‘on’ button or a ‘switch on’ sequence which avoids accidental start ups while in the pack?
  • Does it switch off or shutdown automatically?
  • Will the weight of the GPS be the ‘straw that breaks the camel’s back’?
  • Is the model designed to work well under tree cover?
  • Will the screen indicate if I have 2D or 3D resolution?
  • Will the unit allow manual addition of variables such as altitude?
  • Is there a visual representation of the number of satellites in use?
  • Does it have a lithium backup battery so any stored data is not lost when changing batteries?
  • Does it have a warranty, and servicing centres in Australia?

The Bush Search and Rescue Victoria website offers some information about two types of GPS unit.

GPS uses and limitations

By pre-setting waypoints to correspond with locations identified on the route card, a leader can use the GPS as a tool to navigate from point-to-point throughout the day. Alternatively the GPS could be carried as a confirmation tool when rest points are reached or ‘geographic embarrassment’ sets in. Likewise the GPS can be used to locate a point of interest in difficult terrain.

It is vital for bushwalkers and ski tourers to recognise GPS’s limitations. As mentioned, the GPS is dependent on being able to ‘talk’ to satellites. Unfortunately, the locations in which bushwalkers and ski tourers demand that this conversation take place are at times beyond the capabilities of hand-held units.

Most current GPS models have difficulty penetrating the canopy of overhead vegetation. Some users state that this problem is greater when the canopy is wet. If the GPS is unable to make contact with its required number of satellites, no position will be given. Similarly if the unit is being used in a gorge or in steep country, the walls of the valley in conjunction with the canopy may reduce the ‘window’ of sky available to the unit. Once again the result may be an inability for the GPS to ‘talk’ to enough satellites.

The units are dependent on their own power source to function effectively. As with all battery powered equipment, they will not work with flat batteries. Most batteries have poorer performance in cold conditions.

Conclusion

Navigators need to understand the use of the equipment and its limitations. The accuracy of navigation using GPS is only as good as the equipment, which needs to be suited to the task. Before leaving for trips, leaders should acquaint themselves with and become proficient in the use of the GPS if they are intending to place any degree of reliance on this equipment. More importantly, they must have a sound knowledge of map reading and navigating without the use of GPS, for reliance on any one piece of equipment could be disastrous.

Further reading

Letham, L. 1998. GPS Made Easy 2nd edn. Rocky Mountain Books: Calgary, Canada.

McElroy, S., Robins, I., Jones, G., Kinlyside, D. 1998. Exploring GPS: A Users Guide. GPS Consortium.