George Ritchie

Global Positioning System -GPS

Global. All-weather, 24hour, satellite based navigation system operated by the US Department of Defence for Military and Civilian use.

24 GPS satellites in 12 hour orbits at a orbital height of 20,200 Km, held in 6 orbital planes.

GPS – The Basics

Triangulation is the basis To triangulate, the GPS receiver measures distance

using radio signal travel times Accurate Time is achieved by solving unknowns Need to know where each satellite is in sky Finally correct for signal delays through space.

Positioning Calculation

Passive Ranging to the Satellites enables computation of User’s Position.

Distance from User’s GPS Antenna to each Satellite calculated. 4 satellites required as a minimum to calculate User’s Latitude,

Longitude and height.

Positioning Accuracy

Positioning accuracy stand alone better than 25 metres, typically 5 to 15 metres.

Accuracy depends on: GPS System Biases Number of Satellites in view Geometry of Satellites in view Receiver Quality

Different Applications require different Accuracies.

Differential Accuracy

Accuracy can be improved down to 5mm by using a variety of Differential techniques.

Corrections from both Coastal Beacons and Satellite can achieve sub-metre accuracy.

Correction from 2nd High tech Receiver can achieve 5mm accuracy. Catch is the cost! Stand alone with No Differential - 5 to 15 metres – cost from £100 Differential Sub-Metre – costs from £2000 Differential 5mm – cost from £25000 Need to Identify Accuracy Requirement.