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GPS navigation units are becoming cheaper, smaller, and better.  If you don't already have one, chances are you'll soon be wanting one.

The increasing variety of models and features makes it harder to make the 'right' choice.  This information will help.

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An Introduction to GPS Navigation

You'll never be lost with a GPS Navigation unit

This GPS (Block IIF) satellite, along with 23 others, all located 11,000 miles above the earth enables you to know exactly where you are.

Amazing technology made simple and easy with a modern GPS receiver.

This is the start of a new series on GPS - additional articles to be published in coming weeks.



GPS receivers are one of the modern miracles of our age.  They receive signals from satellites 11,000 and more miles away and from that information instantly calculate where you are to an incredible accuracy of sometimes within ten feet or less.

Small sized, light weight, easy to operate, and increasingly affordable, they are becoming more commonly included in new vehicles, and a wide range of standalone units makes it easy for anyone to add a GPS unit to their vehicle.

No More Getting Lost, Ever

A typical business trip for me often involves flying to an unfamiliar city, arriving after dark, and then struggling to drive an unfamiliar car to an unknown location, with an inadequate rental car company supplied map and ambiguous directions balanced on my knee.

Even if such an arrangement doesn't lead to an accident, it surely can lead to much frustration, missed turns, and getting hopelessly lost (invariably in a bad part of town).

Or, when on vacation overseas, the quaint old towns in Britain and Europe are very difficult to navigate through.  Winding roads, one way streets, no left turns, combine with one's own errors at map reading to make traveling by car frustrating and unpleasant.

In both these cases, and any other time you're needing to know where you are and/or where to go, a modern GPS Navigation Unit can completely solve the problem.  Its 'moving map' display shows you exactly where you are on the map, and it will highlight the turns you need to make to get you where you need to go.  And if you should still miss a turn, that's not a problem either.  The unit will quickly recalculate the best revised way to get you where you are going.

GPS units have transformed the ease and ability with which we all can travel.  If you don't already have one, you should get one.

What a GPS Receiver Can Do For You

A GPS receiver calculates your location (and perhaps also your altitude) based on the time it takes for radio signals to reach the receiver from different transmitters.

It can also calculate your speed and direction based on changes in your location, and many GPS units add 'trip computer' functions such as telling you the average, maximum and instantaneous speed, and the distance you've traveled.  The GPS information is usually more accurate than your car's speedometer and odometer, so this is a convenient way of checking them to see if they're accurate.

When you've set a destination, the GPS can then provide an estimated time of arrival and distance remaining to travel.  Better GPS's make an uncannily accurate prediction of your arrival time because they 'know' how much of the journey is on freeways, how much on regular highways, and how much on congested city streets, and the sort of speeds you can travel at on these different types of road.

As part of their map information, a GPS will typically also be able to tell you where nearby restaurants, hotels, gas stations, and other 'points of interest' are.

Realtime location based information

More sophisticated GPS units link up with real time information about things like freeway traffic conditions and road works to help you avoid congested areas, and can even provide local weather forecasts, and information on nearby cinemas and what they are currently screening.

A wonderful feature on some GPS units is telling you not just where nearby gas stations are but also their current prices for gas.  This can save you both time and money each time you need to fill up, but note that not all gas stations report their pricing to the MSN Direct service so the information is incomplete.

How GPS Works

A standard GPS unit receives signals from a 'constellation' of satellites in low earth orbit.  More advanced GPS units supplement these signals with extra signals from ground based transmitters, giving them more accuracy.

In theory there are a minimum of 21 satellites and three working spares in six different orbit paths, all inclined 55 from the equator, which is enough to ensure there are always at least four visible from anywhere in the world.  In practice there are usually more than 24 satellites aloft and operational (eg 27 at present), and it is common to have many more than four satellites visible at any given time and place.

The satellites weigh just under a ton each, are 10,900 miles above the earth, and circle the earth once every 11 hrs 58 minutes, meaning they're in a slightly different orbit every time.

Satellites have a typical life span of 7.5 years before replacement, so every year, on average, 3.25 satellites need to be replaced.

GPS satellites broadcast a signal on two frequencies, 1575.42 MHz (L1) and 1227.6 MHz (L2).  Your satellite receiver uses the information in these signals to work out the distance to each satellite, and then solves a simple trigonometric equation to calculate its position on the earth based on its distance from the various satellites it receives signals from.

The accuracy of a GPS unit is truly astonishing when you consider how it is calculated.  With accuracy of approximately 40', this means the unit can discriminate delays in receiving a radio signal as small as a twenty five millionth of a second (this is the time it takes a radio wave to travel 40', and based on information from satellites in orbit 10,900 miles above the earth's surface.

How Accurate is GPS

The accuracy of a GPS receiver is based on a number of factors.  Most of these factors can't be controlled - for example, random variations in the ionosphere which slightly impact on the speed of radio signal propagation (which is estimated to be the largest single variable, providing a potential error of about 13.5').  A couple of errors can be optimized but not eliminated (getting as accurate a clock as possible, and optimizing the receiver circuitry) but these factors are already as close to perfect as possible (representing, between them, about an 8.5' error).

The theoretical error radius between where the GPS calculates you as being and where you actually are is generally considered to be in the realm of 40' to 50', assuming no additional aggravating factors.

Prior to May 2000, the accuracy of civilian GPS units was deliberately downgraded (by sending a less exact signal, termed SA - selective availability) so that accuracy was limited to about +/- 100', but since then, GPS accuracy is no longer downgraded and most receivers can theoretically establish accuracy to within about 40' - 50'.

Some GPS units will display a constantly changing theoretical accuracy of their information, so you know how reliable the information is.  This information isn't exact, but if the GPS unit is showing a theoretical 30' accuracy, you know it is giving you a better answer than if it is showing a theoretical 300' accuracy.

The actual GPS accuracy is determined by several factors :

How many satellites the GPS receiver 'sees' in the sky

There are at least 24 GPS satellites circling the earth (the number varies depending on when satellites fail and how many spares are already in orbit waiting to be deployed).

These are not 'geosynchronous' satellites; in other words, rather than staying fixed above the earth in a certain point, like a tv satellite, they are all the time moving relative to our positions on the planet.  Sometimes there will be satellites almost directly overhead, and other satellites may be just above the horizon in any direction, or anywhere else in the sky.

The number of satellites your GPS receiver can 'see' (and receive signals from) is regularly changing, depending on where the current 'cloud' of satellites are located relative to you, and what obstructions there are between the GPS receiver's antenna and the satellites (signals are strictly line of site only).  In theory, and assuming no massive obstructions to the line of site path, you can always receive signals from at least four satellites, and often will have eight satellites, and sometimes even more.

A GPS needs to receive signals from at least three satellites before it can calculate your position in two dimensions, and at least four satellites before it can calculate your position in three dimensions (ie including your height/altitude as well as your latitude and longitude).

Where the satellites are located

Because of the trigonometric calculations your receiver must do to work out its location from the information it receives from the satellites, it helps if the satellites are evenly spread around the sky.

If the satellites were all in a straight line, the calculation would be much less reliable than if they were spread around, with some satellites in front, some to the left, some to the right, and some behind you.

If the GPS has D-GPS or WAAS capability

These two extensions to basic GPS functionality are explained below.  For now, suffice it to say that if your GPS receiver has - and is using - either of these two enhancements, its accuracy will be appreciably better than if it is not using either.

If the GPS also uses dead reckoning

Some GPS units, especially those pre-installed in cars, supplement their GPS calculations with dead reckoning for times when they don't have sufficient reliable satellite information.  If the GPS knows how fast your car is driving, and in what direction (and it can obtain this information from the speedometer and the position of your steering wheel) it can make reasonably accurate estimates for where the car is and where it is going to, especially when it adds the assumption that you're driving on the roads rather than going offroad.

It is also possible for a GPS to calculate some of this data itself by use of internal accelerometers that can tell it whether you're speeding up or slowing down, and if you're turning or going straight ahead.

This dead reckoning quickly becomes less and less accurate, but usually is only needed for short stretches of time - for example, if you're going through a tunnel, or if you're in a city 'canyon' with tall buildings all around you blocking your view to the satellites.

If the GPS has a 'snap to roads' feature

It is possible, due either to errors in the map data about where the road is exactly located, or due to limitations in the GPS accuracy (or due to both factors together) that the GPS may end up thinking that you are driving not on the road but perhaps 100' to one side of the road - through buildings or whatever.

Because of this, many GPSs offer a 'snap to roads' feature to tell the GPS that anytime it calculates that your car is traveling close to and parallel to a road, it should assume you are actually on the road itself.  This can be both a benefit and a hindrance.

It is a benefit because it makes it easier for you to see exactly where you are.  But it can also be a hindrance, because sometimes you end up tricking the GPS into making a wrong assumption.  For example, if you are in a car park close to the road, and then drive through the carpark, parallel to the road, the GPS will likely assume you're on the road itself.  And then when you drive out of the carpark to the street, the GPS might then move you over to the next block, and for the next considerable time might be showing you driving along the wrong block (this happens to me on occasion).

How is Accuracy Measured

There's an important thing to understand when talking about accuracy.  A full statement of the accuracy of a unit comprises two pieces of information - the distance accuracy being claimed, and the percent of time the unit is accurate to within that limit.

But most GPS accuracy claims only tell you the first part of the statement - the distance accuracy being claimed, and don't tell you what percent of time the unit is expected to be accurate within that limit.

Because accuracy errors are semi-random, sometimes the error can be zero, and so a person could say 'this unit is accurate to within six inches' and be correct some of the time - the key factor in that statement is the (usually unstated) issue of how much of the time the accuracy level is achieved.

The higher the percent of time the accuracy standard is being achieved, the broader the accuracy tolerance that needs to be given.  For example, a device may indeed be accurate to within 6", but only 1% of the time.  It might be accurate to within 3' 25% of the time, to within 6' 50% of the time, and to within 10' 75% of the time, to 20' 95% of the time, and nearly always to within less than 100'.

So the accuracy claim can vary widely, depending on what percentage of the time it must be met.  Scientists usually choose to adopt a 95% or higher rating (they often call this a 'confidence level'), but marketeers may feel that 51% is enough for a 'better than half the time' concept to be established.

Basically, we suggest you don't consider accuracy claims as being relevant in your choice of GPS units, because most units have similar accuracy - indeed the vast majority of units use exactly the same chips to decode and calculate the unit's position.

Two Enhancements to Standard GPS Accuracy

There are now two different types of enhancements that can further improve the accuracy of a GPS unit, sometimes giving you an accuracy of as little as 5' between your actual position and your computed position.

And if you think that is amazing (which of course it is), military (and commercial surveying) applications offer even greater accuracy.


DGPS, short for Differential GPS, adds a land based transmitter to the satellite based transmitters.  The land based transmitter is typically very much closer to your receiver, and the signal has a shorter more reliable path to travel to get to your receiver, allowing for more accurate location calculations.

Accuracy is typically improved to about 5' - 15', depending on how close you are to the DGPS beacon.

The two disadvantages of DGPS for most users are the need to have a second separate receiving unit to get the DGPS signal, and the need to pay an ongoing fee to be able to use the DGPS service.


WAAS, short for Wide Area Augmentation System, is a more recent enhancement and in many respects can be considered a replacement for DGPS, and may offer better accuracy too.

A network of fixed ground stations continually monitor the GPS satellites in the sky, and compare the calculated location given by the satellite data with the actual location of the ground station.  It then calculates continually varying correction factors and rebroadcasts those over extra satellites.

If you have a WAAS enhanced GPS receiver, it will receive the normal satellite signals and also the WAAS correction signals, and by applying the WAAS correction factors to its calculation, can come up with a much more accurate result - typically giving you accuracy of about 10' - 20' both in terms of location and altitude.

WAAS is free and requires no additional receivers.  Most of the modern middle and higher end receivers have WAAS capabilities.  A European equivalent of WAAS is EGNOS (European Geostationary Navigation Overlay Service).

Strangely enough, our testing of WAAS equipped units (eg Garmin Nuvi 680 and Garmin Nuvi 660) have shown very little difference in claimed accuracy as to with WAAS enabled or disabled.  We had both these two units side by side and would alternate between having one unit with WAAS on and the other with WAAS off, or both units with WAAS on or off.

There was almost no appreciable difference in claimed accuracy, with the 680 usually showing slightly better accuracy than the 660, no matter which unit had WAAS on or off.

It may be fair to say that default regular GPS is so accurate these days that WAAS no longer offers the significant improvement in accuracy that it formerly did.

Altitude Accuracy

Due to how location is calculated, GPS units are not as accurate when displaying altitude as they are when displaying latitude and longitude, and more satellites are required to get any type of result (a minimum of four to calculate 3D positions compared to three satellites for 2D positions).

Altitude accuracy can generally be expected to be at least 50% worse than for the 2D accuracy.

This accuracy is compounded by the fact that the reference 'zero' level - sea level - isn't constant around the planet.  The earth isn't exactly spherical, and has various bulges and hollows.

What this means is you may sometimes be driving alongside the ocean and see your GPS telling you you're at an improbable 250ft below sea level (or an equally improbable 250 ft above sea level).

Mapping Accuracy

If your GPS is suggesting that your car is driving off the road on its map display, don't immediately blame your driving or the GPS.

It is equally likely that the map data in the GPS isn't completely accurate, and maybe the GPS is truly showing where your car is located in terms of absolute latitude and longitude data, but the map data in the unit thinks the road is somewhere other than where it truly is.

Map data also ages - new roads and freeway exits are added, and information in 'Point of Interest' databases (lists of restaurants, gas stations, tourist attractions, etc) also changes quite rapidly as businesses are opened, sold, or closed.

Almost all GPS receivers use mapping data from one of only two different suppliers.  The better established supplier of mapping data is NavTeq, and the newer supplier is Tele Atlas.

Until recently many people believed the NavTeq data to be more reliable than the Tele Atlas data, but it seems the gap has narrowed and there's little to choose between them.  More important than which supplier provides the mapping data for your GPS are considerations about how recently it was updated, and what the policies (and costs) are for you to acquire ongoing updates into the future.


GPS receivers can be as small as to fit into your shirt pocket, and priced for $200 or less (we'll be reviewing a good $200 unit in a couple of weeks).

Their twin functions - of showing you where you are, and helping you know how to get to where you want to be - make them invaluable and essential any time you're driving somewhere you're not 100% familiar with.

If you don't already have one, use the information in the second part of this series about how to choose a GPS unit (due to be published on 23 March) and get one.

Read more in the GPS articles series

Coming soon - see the links at the top right of the page to visit other articles in our GPS series.

Related Articles, etc

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Originally published 16 March 2007, last update 21 Jul 2020

You may freely reproduce or distribute this article for noncommercial purposes as long as you give credit to me as original writer.


GPS Related Articles

About GPS
Beginner's Guide to GPS part 1
Beginner's Guide to GPS part 2
Beginner's Guide to GPS part 3
How to Choose a GPS Navigation Receiver
GPS Receiver Reviews Menu
Enhanced GPS Data services
GPS Based Realtime Gas Prices
Using a GPS internationally
Dash Express review
Garmin Nuvi 660 review
Garmin Nuvi 680 review
Garmin StreetPilot 7200 review
GlobalSat GV-370 review
Hertz NeverLost Portable GPS review
Landrover LR3 built-in GPS review
Plenio VXA-3000 review
TomTom Go 930 GPS review
Trafficgauge Mobile Traffic Map review
Via Michelin X-930 review





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