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How navigation works

GPS is the world’s first satellite navigation system. It was developed by the U.S. Government’s Department of Defense, who gave GPS its official name: the NAVSTAR system (Navigation Satellite Timing and Ranging).

GPS consists of 3 key elements:

  • Satellites in space

  • Monitoring Stations on Earth

  • And last but not least, you and your GPS receiver.


The satellites
GPS has 24 satellites that circle Earth in six orbital paths, sending out radio signals from their position in high orbit, 12,600 miles/ 20,300 kilometres above our heads. Being so high, each satellite’s signal covers a large area of the earth’s surface and their orbits have been ‘choreographed’ so that your GPS receiver back on Earth is always getting signals from at least 4 satellites, the number you need to pinpoint your location.

The monitoring stations
There are 5 monitoring stations: the master station in Colorado Springs, USA and four unmanned stations. One on Hawaii, the other three in remote locations as close to the equator as possible: Ascension Island in the Mid Atlantic; Kwajalein in the Pacific and Diego Garcia Atoll in the Indian Ocean. The 4 unmanned stations receive constant data from the satellites and forward it to the master control station, which ‘corrects’ the data and then sends corrected data back up to the GPS satellites.

GPS signals
Your GPS receiver picks up signals from GPS satellites to work out your location. The last, important step in the process is of course you making use of that information. Each satellite transmits low power radio signals on different frequencies for different users. The signals travel by so-called ‘line of sight’. This means they pass through clouds, glass and plastic, but not usually through solid objects, such as buildings.

How navigation works

Put simply, your TomTom device consists of two distinct elements: the software and the hardware.

A small computer inside your device ensures the software runs smoothly. Depending on the device you’ve got, the software is to be found on either the SD card or the hard disk. The hard disk itself can store some 20GB of data.

A ‘boot loader’ in the computer searches the hard disk or SD card for the software and your map data. It then transfers the software to the 64MB internal RAM memory in your TomTom device and starts the software. Only that part of your map that’s actually needed at that moment is then loaded. When you consider that in 1991 an IBM personal computer ran on only 16MB, you realize what a powerful little device your TomTom actually is!

A Linux system in the device makes sure the hardware functions properly. The hardware itself starts the GPS and the navigation application. The navigation application then reads whatever settings you have installed, such as your preferred voice and your last chosen route.

The GPS module in your TomTom device makes sure that the satellite signal is translated into co-ordinates pinpointing your exact location on the map.

Once everything has been ‘started up’, the GPS module in your device calculates where you are from the satellite signals it receives. The satellites constantly send out signals and the module picks up those that are nearest to it.

As you know, the GPS module works out its position by calculating its distance from at least four different satellites. But while your TomTom device may know its distance from these satellites, it still doesn’t know exactly where you are until it also knows where the satellites are positioned. Even then it’s not straightforward, as each satellite is constantly moving in orbit around the earth.

This last problem is solved by the fact that the GPS signal the satellites send out contains so-called ‘almanac’ information. Information about such things as the altitude of the satellite, which satellite it is, its position in relation to the other satellites, and so forth. Using this information, your GPS module can translate these signals into co-ordinates, which it then sends on to the navigation application.

Which is where your GPS module really comes into its own. Inside the module is a small, highly sensitive GPS chip that can receive and register signals even when it’s in very inaccessible locations, such as down narrow alleys, amongst high buildings or in dense woods. Which obviously greatly improves the accuracy and consistency of your TomTom device.

How navigation works

The world is made up of countless millions of roads, from huge motorways to quaint country lanes. The mapping of all these routes, as well as all the various street directions and other details that affect them, is carried out by governments and other statutory organizations across the globe. Their maps form the basis of all maps, including the digital maps for your TomTom.

The advantage of digital mapping

Digital maps can be updated
Apart from the fact that traditional maps are less convenient and less interactive and therefore less efficient than digital maps, one of the main reasons traditional paper maps are being superseded by digital maps, is that a paper map cannot be updated. On average, 5% of roads are altered in some way every year. So with a paper map that’s only 2 years old, you have close to a 1 in 10 chance of being wrongly directed with each reading you take! In fact, given the time-lag between getting the data for the maps and then drawing them up, getting them typeset, printed, distributed and so on; a new paper map is already out of date before the ink on it is dry. The on-going challenge to digital mapmakers is to reduce the delay between a change occurring in the road system and its appearing in the map in your navigation system. The leading digital map suppliers (there are several, of which TeleAtlas and Navteq are the largest) employ literally hundreds of people to meet this challenge and make your digital map as up-to-date and accurate as possible.

More than just directions..
Even the earliest maps enhanced their basic cartography with additional information they hoped would be of interest to their readers. So with medieval maps you often see written across areas on the edges that were as yet unexplored warnings like ‘Hic sunt dracones’ meaning: ‘Here be dragons”. The huge range of information digital maps can provide (with traffic signs, prohibited manoeuvres, vehicle restrictions, post/zip codes, house number ranges, points of interest, tourist information, speed camera data, and much more) is just another example of how much more user-friendly they are than traditional maps. So whereas even the best traditional maps simply show you where you are, and perhaps indicate road signage etc (as it was at the time of printing), digital maps take the information you get to another level altogether:

  • With route calculation – the best route according to the driver’s criteria (speed, scenery, fuel-consumption, etc), distances, points of interest, and so on.

  • With route guidance – real-time information, for example on traffic flow (and non-flow!) and safety cameras; and support, for example with negotiating complex intersections, preparing you in advance for turn warnings; and much more.

Keeping digital maps up to date

There are 3 main ways to collect the data to develop and update digital maps:

  1. Fieldwork. Data collectors driving the road networks of the world, recording changes and discrepancies.

  2. Analysis of aerial and satellite imagery.

  3. Customer feedback. At TomTom, for example, any map feedback from customers via the website are reported to TeleAtlas, who look into every single case and , where appropriate, actually send inspectors out to physically check the locations.