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Your GPS unit - the science behind it and how spatial data can be shared

June 08, 2011
Your GPS unit - the science behind it and how spatial data can be shared

By Patrick Burke

For most of us, if our car or motor-home behaves in an unusual manner, we can usually work out why, due to having a basic understanding of how it works. Similarly, when our Global Positioning System (GPS) does the same, some people may either shake it, tap it a few times or simply turn it off.

This article should help build your understanding of how GPS works, as well as providing background on how the science behind GPS operates. It will also provide information on what you can do with the data you collect during your travels.


The system was originally built for use by the military and is now shared by millions throughout the world. There are 24 satellites (with three more as spares), which slowly orbit the earth twice per day at about 11,000 km per hour and at 19,000km above the earth.


The satellites are positioned so that at any time, a number of these are visible to a GPS unit. The position of each satellite is continually checked against known ground transmission points and other satellites, all of which use synchronised time clocks. Our GPS units contain an almanac of each satellites position at any given point in time and are capable of calculating how long it takes to receive a signal.

Put simply, it measures the time a signal was sent and the time the signal was received.

We know the speed at which the signal passes through the atmosphere, so we can calculate the distance we are from the satellite. When this calculation is completed for a number of satellites, we can pinpoint our location to within a few metres.

This may be fine in theory, but we do not always have clear skies or uninterrupted signals to a satellite system. In cities the signals may be reflected off a number of buildings before it reaches us; in stormy weather the signal will travel through more dense atmosphere; in dense forest the readings would be impacted by trees. In all of these scenarios the signal will take longer than expected to get to us and if we use this incorrect time in our calculation, we will get large errors in our location.


A number of GPS owners have commented that the coordinates on their GPS change even if they are standing still. This can be due to a variation in the number of satellites they are receiving data from, or through atmospheric interference.

If you require accurate coordinates and you are experiencing a number of variations, I suggest you place the GPS unit on a stable surface and over a space of 10 minutes or so, regularly record the coordinates and then average these out. The usual rule of thumb is that in good conditions you should get accuracy within 10 metres in the horizontal plane and within 20 metres with elevation.

Although you may be able to find your house using your GPS unit, you should not expect to be able to accurately locate the property boundary. A surveyor uses more advanced GPS devices through which they access ground base stations, which have known locations, as well as the satellite constellation. These systems can provide the surveyor accuracy within a few centimeters.

GPS units are used in many other situations such as emergency beacons, taxi companies, tracking aircraft flights, finding the quickest albeit not always the shortest route for ambulance and other emergency vehicles. Geographic Information Science (GIS), which is one of the Spatial Sciences, is used in conjunction with GPS units and other databases containing information which is spatially referenced.

Asus 636 charges from the lighter socket
Asus 636 charges from the lighter socket


Governments use GIS to plan where new hospitals, schools and other facilities will be needed. The data from the census indicates the geographic location of the population where the services are required or will be required in the future. Geological and environmental data are used to determine the best locations for railways, highways, bridges, dams, airfields and infrastructure supporting features such as mines.

In todays competitive consumer environment, large retailers use GIS to map consumer purchases. You may have been asked at the checkout to give your post code.

This information may used by the company to determine which port to have the overseas products delivered to. If 70% of an imported product has been sold in Brisbane, there is little point having the product delivered to Melbourne. This post code information is also used regarding advertising material, allowing similar items to be advertised in a catalogue which is customised for areas where large numbers of a particular type of product have already been sold.

As can be seen from the examples above, data is valuable, however, its value is increased greatly when it can be associated with a geographic location. This is what GIS is all about - using data which has a geographic location attribute with other data in the same area of interest.


The everyday GPS unit is used in many instances to collect data for businesses and Government. The value is increased again when a number of users or data collectors share their data with one another, or add it into one central dataset.

There are currently thousands of GPS units fitted to vehicles of travelers and holiday makers, who all have a wide variety of experiences and interests on a daily basis. Most modern GPS units allow the operator to connect to their own computer and extract information such as way points or Points of Interest entered by the user, or which have been recorded during their travels.

This file is usually a simple text file which can be edited, saved and even uploaded to the internet to share with other people who have the same interest.

When you have found that magic FREE camping spot, next to the river where you caught that Barramundi while your partner was browsing the nearby craft shops, you can share it with your friends!

There are diverse groups of people with similar interests who would love to know where things are, whether it be a winery, coffee shop or even a bridge to avoid. There are lots of interest groups that have websites where you can upload your data for others to use, or download information for your next trip.

There are so many people collecting information as they travel down the highway - sharing it is great fun and helps others to enjoy the experiences and places you have found!

Hope you find this of assistance.

Editors Note: Patrick Burke is aSpatial Analyst working for the Snowy Mountains Engineering Corp in Brisbane.

The Asus in left-handed landscape mode
The Asus in left-handed landscape mode