GPS - HOW IT WORKS

(GLOBAL POSITIONING SYSTEM)

The following notes provide a very brief summary of the primary features of GPS (Global Positioning System).

We hope our clients - existing and future - will find this information helpful in understanding GPS. After reading this page - if you would like to know more about GPS you can contact us, or seek further information on the Web starting from the links on our Links page.

For information on the new Ordnance Survey's UK GPS 'system' and reference datum, including our own practical tips and 'warnings', please click here, but to fully understand that page we suggest you read this page first!

The notes start with the technical - 'how it works' and continue with information on field work, recent developments, plus a summary of the pros and cons of GPS.

Feel free to print this page and keep as a reference document. Note also that we would be pleased to arrange lunchtime seminars at your premises, or more formal training sessions for your staff.

STOP PRESS - A quick note on selective availability. The US Department of Defence has now removed the deliberate degradation of GPS signals. The accuracy of using a single GPS unit (in navigation mode) will now be in the order of +/- 10m for horizontal position - but worse for heighting. Millemetric accuracy is possible using 2 or more high quality instruments. See below for further information.

System comprises 24 satellites orbiting at 20,200 km above the Earth. Fewer satellites in earlier years - now full constellation. The satellites are tracked from Earth stations. Their predicted course - the ephemeris - is transmitted to the satellites which is then relayed back to Earth for use in all GPS receivers.

2 radio frequencies broadcast from the GPS satellites. A known code is modulated onto the 2 main carrier waves. Comparing the broadcast code, with code generated in the receiver enables a distance to be measured to the satellites. A minimum of 4 satellites are required. 3 distances provides a position (resection), but 4 satellites enables correction of clock errors.

Single receiver gives Navigation accuracy of +/- 10m. (Previously +/- 100m before 'selective availability' was switched off). Height value may be many times worse. Two or more receivers will give ‘differential’ position (DGPS). Highest quality receivers accurate to 5mm +/- 1ppm for position, under ideal conditions. (ie 6mm in 1km, 15mm in 10km).

Note that hand held receivers in differential mode may achieve +/- 1m for position - worse for height. Beware of ‘generalised’ transformations for OSGB36 - 24m variation across UK. However, will the accurate GPS result fit the OS map - which is often less accurate ?

UK GPS services (Ordnance Survey) - Click here for further information.

Position and differential results provided in WGS84 co-ordinate system, which is the best approximation to the Earth’s shape. WGS84 must be transformed to ‘local’ map projection eg OSGB36 or UTM.

Sub divided into 2 methods:-

STATIC True static (lines over 20km) half an hour or more to observe.

Rapid static (lines < 20km) 5 minutes for 5km with accuracy +/- 10mm.

KINEMATIC True kinematic (continuous measurement of 3D position).

Stop & go (point position within 2 seconds - latest equipment 0.2 seconds).

Other considerations:- GDOP (Geometric dilution of precision). Greatest accuracy is achieved when the satellites form a strong geometric pattern.

HEIGHTS Beware - high accuracy results are possible but only with great care. Spirit levelling measures heights relative to the Geoid (mean sea level). GPS measures heights relative to the ellipsoid (mathematical model approximating to the Earth’s shape). The geoid/ellipsoid separation must be determined and modelled to enable conversion of GPS (ellipsoid) heighting to geoid heights. (eg above OS datum).

ADDITIONAL INFORMATION

REAL TIME GPS To compute differential GPS results, the data from 2 different receivers (often called the base station and the rover) must be combined. This is called post processing. The data gathered at the base station can be broadcast by terrestrial radio to the rover and processed almost instantaneously. Thus positions and heights can be obtained in ‘real time’ - often known as Real Time Kinematic (RTK). Distance is restricted by radio power - limited in UK to 0.5 watts, giving 1 to 10km depending on the terrain.

AREA WIDE DGPS Permanent base stations have been established by various service providers and the data or ‘differential corrections’ are broadcast over a localised area for other users to work with. eg Focus FM (with 10m and 1m subscription service) Racal and Omnistar broadcast the corrections via geo-stationary communication satellites. UK coastal regions covered by Trinity House (free) service. Soon to be covered by Ordnance Survey permanent base stations.

GLONASS/GPS COMBINED The Russians established a similar system to the US. Known as GLONASS (Global Orbiting Navigation Satellite System). 24 satellites orbiting at 19,100km, they are dual frequency like GPS. However only 12 to 14 currently operational. 3 new satellites launched in December 1998 has boosted international confidence in the system. GLONASS uses a different reference datum - SGS90. The transformation of SGS90 to WGS84 is now well established and thus combining the 2 systems gives more satellites and enables operation in more difficult locations - eg urban canyons, overhanging trees.

ORDNANCE SURVEY - NATIONAL REFERENCE FRAME The Ordnance Survey have not maintained bench marks for over 18 years, and now the ‘trig’ pillars are not maintained. A system of GPS co-ordinated points have been established throughout UK. Computer programs enabling transformation of WGS84 to OSGB36 and conversion of ellipsoid to OS level datum are available from the OS. This program will not be just a useful tool. This software will become the National Reference Datum for co-ordinates and heights. For further information and advice - click here.

Advantages

High accuracy.

Works without intervisibility (In the dark, rain, fog etc).

Long range. (But accuracy is proportional to the distance from your base station).

Rapid results.

3 dimensions with one measurement.

Disadvantages

Need to see the sky (can’t use it underground ! )

Beware height conversions to geoid. (Plenty of scope for major mistakes here!)

Ease of use - training is essential (tank squadron in the Gulf war was 600m out of position - using Australian grid setting on their hand held GPS).

Accuracy can better than the local control. Results are often downgraded to ‘fit’ the control.