Getting to Grips with a Global Positioning System (GPS)

Tom Burke

Content

• Introduction

• Background

• GPS’s & the GPS Systems explained

• Limitations of a GPS handheld device

• Making your choice – a buying guide

• Basic user guide

• Useful links - resources

IntroductionAims• By the end you should feel you have enough knowledge (or know

where to look for further research) to be in a position to choose & use the basics a handheld GPS system should you wish.

Objective• To give an overview of the Global Positioning Satellite System &

how a Handheld Global Positioning System works.• To provide you with an informative guide to the functions, features &

user interfaces (UI’s) of the various hardware & software options.• To provide you with a basic user guide for handheld GPS’s.

BackgroundGlobal Positioning Satellite System (GPSS):

• The global positioning satellite system was developed by the US Military during the cold war in order to increase the accuracy of intercontinental ballistic missiles.

• In order to limit civilian use the they degraded the signal to reduce accuracy.

• In 1995 the US GPS system became fully operational.

• In May 2000 selective availability was removed to give an average accuracy of 15m.

• In 2007 the Russian GLONASS system (another GPSS) was scheduled to become fully operational.

• As of March 2008, the system is not fully available, however it is maintained and remains partially operational with 16 operational satellites.

Background cont.

• However it would appear that both systems are still in their infancy with a little way to go before completion.

• Aside from the US & Russian systems, two other systems are in the ‘pipe line’.

• The European one which is called GALILEO and was planned for completion in 2010.

• The other system is being built by China and is called Beidou. It’s completion was planned for 2008 when it would be providing an “open” level of service with 10m accuracy.

• With Galileo, the second Satellite, of the planned thirty was only launched in April, they are now predicting an completion date of 2013 with the third satellite scheduled for launch in 2010.

• It would appear that the Chinese system currently only has 4 Satellites (as at Feb 2007) and will continue to be used for testing.

Satellite Based Augmentation Systems (SBAS)

Background cont.

• They work using geostationary* satellites and ground based relay stations to improve the signal quality from the GPSS.

• These systems are designed to increase the accuracy of GPS’s; typically by a factor of around 5 which equates to a position fix to 3m.

• Because the system is based on fixed positions, coverage availability is limited.

*geostationary – in a fixed position above a point on the earth

• You also require a compatible device which will show in the specification as WAAS (US) or EGNOS (European) compatible.

How the GPS System Works• GPSS uses a system of

satellites (the US system uses 24) 11,000 miles above us in space.

• These are circling along a number of axis’s around the globe at 1,000 miles per hour sending radio signals back to earth.

• Usually at least 6 satellites are visible at any one time.

• A GPS Receiver can log on to 12-14 satellites simultaneously.

How the GPS System Works cont.

• A GPS needs to log on to at least 4 satellites and, by sharing a combination of distance, time and position information between the satellites, gives the user a fix of latitude, longitude and elevation.

• This is known as a 3D fix.

Points to Note

• A handheld GPS is an excellent ADDITION to your traditional navigation methods.

• It will never replace the map & compass

• It will give you a position fix even in hill fog, blizzard or ‘white out’.

• The service a GPS uses is completely free bar the cost of replacing batteries.

Limitations of a GPS

• Additional FeaturesMemory expansion slots for saving data

• Make & ModelThe make and model will govern what your GPS will be capable of. Obviously you are restricted by what is available but ensure you choose a make and model that suits your current and future needs.

The features & design will affect its performance & flexibility.

• Standard Features & Design

Included or additionally bought mapping softwarePC link-up for upload & download of data and loading maps or new firmware

Things like battery type (power) and the antenna type used will affect signal strength.

Memory size, colour screen, electronic compass, barometer & altimeter can enhance the users experience.However the compass, barometer and altimeter are only useful if regularly calibrated & calibrated correctly.

• Terrain in your surroundings• Location / Position (in the world)• Battery / Power• Antenna type

Reception is how good the connection to the Satellites are; the more satellites you can gain reception from them, again the more accurate your readings.

The available reception will affect the GPS’s signal strength; the following can affect or block the GPS’s reception & signal strength:NB. Reception is unaffected by bad weather

• Reception Signal & strength The whole system relies on a good line of sight to the satellites: the stronger the signal, the more accurate the position fix will be.

Limitations cont.• User Ability

You will only get out of your GPS what you put in. To be able to use your GPS to its fullest potential you will need to spend time with the manual / or a guide book to learn how it works. Alternatively you can book yourself on a dedicated training course.

Terrain in your surroundings

• Reception cont.Limitations cont.

Location / Position (in the world)

Battery / Power

In some locations the satellite coverage (path routing) is better that others. If your in a poor area you will not get as good / will struggle to get a sufficient signal. This could result in a 2D fix (which does not include elevation) or a False signal.

Reception can be affected by Tree cover, Gullies, buildings, your body and other objects.

• False Signals• Atmospheric conditions• Multipath errors

In order to obtain a strong signal handshake your GPS sends a signal to the Satellite. The strength of the signals will determine which satellites you log onto and this in turn affects the GPS’s accuracy. When there is sufficient power the signal will run at the designed maximum but as the power is lost the GPS is unable to maintain max power which then affects how many satellites it can reach and logs onto.

• Reception cont.

Limitations cont.

• False Signals

• Atmospheric conditionsThis can affect the speed of the GPS signals to the satellite and cause your handset to do strange things. This could be as a result of Solar activity in the Ionosphere.

If a false signal is being received, the speed or location could be inaccurate. This can be caused by poor satellite geometry. If all the satellites within your GPS’s line of site are clustered closely together or are situated in a line away from the antenna, it is more difficult to calculate positions.

• Antenna typeThere are two types of GPS aerial – quadrifilar helix (right) and patch (left). A quadrifilar helix aerial is more sensitive and will generally get a signal where a patch will struggle.

• Multipath errorsA multipath error is where the GPS receives two signals from the same satellite. This can be caused by a satellite signal being bounced of rocks or similar in your location.

Buying Guide

1. Zoom In/Zoom OutWhilst viewing the map screen, press to zoom the map view in and out. Look at scale on screen.On any other page use these keys to scroll up and down a list or to move a slider bar.

2. Find/OptionsPress and hold to show the waypoint find menu.Press and release to show the options menu for a page.

3. Thumb Stick(a four way scrolling device and Enter button)The GPS is designed to be used in the left hand with the

thumb stick being operated with the thumb.Press down and release to select highlighted options or data

or to confirm on screen messages.Push up/down/left/right to move through lists, highlight areas,

buttons or icons or to move the map panning arrow on the map screen.

4. Find/OptionsPress and release to cycle through the screen pagesThis button can also be used to ‘quit’ a process when working within a page.Press and hold in to toggle the electronic compass on and off (certain models only).

5. Power/BacklightPress and hold in to switch on and off. When GPS is on, press and release to switch on and adjust backlighting, view the date and time, satellite and battery status. Move slider bar on screen to adjust brightness.

Features Essential1 Desirable2 Not needed3

WAAS enabled      

Datebase for at least 500 waypoints      

Capacity to store at least 20 routes and 10 tracks      

Exchange data between GPS and PC      

Good quality colour screen      

Base map with towns and cities included      

Memory capacity to store extra points of interest and street mapping      

Electronic compass      

Electronic altimeter      

Elevation profile on screen      

Audible proximity alarm      

Antenna type      

Battery type & number required      

Average battery life      

User friendly button and menu (Try before you buy)      

Fits comfortably in the palm of my hand      

Waterproof to IPX7 standard      

Cable, case & lanyard included      

Downloadable software upgrades      

Thinking of the future, will you need the desirable features later?      

Other features you might want to consider:      

Large screen & large images on it (important id you wear glasses)      

Size of GPS      

Weight of GPS      

Geocaching features      

External antennae interface (useful if using in a car in cities)      

Other features?      

1. I will not buy a GPS without these features

2. These features are nice to have if the price is right but not essential

3. I will not pay extra for these features as I will never use them

My Ideal GPS profile

Budget:Minimum: £________

Maximum: £_______