Basic Principles of GPS

Mathias LemmensEU GIS/Mapping Advisor

Abuja 4th August 2005

Contents

• Introduction

• GPS as a surveying tool

• Methods of Observation

• Accuracy Aspects

• Sources of GPS Error

Global Positioning System (GPS)

• Satellite-based navigation and positioning system, original developed for military purposes by the US Department of Defense in 1972

• Applied in many fields as diverse as geodesy, GIS, precise cadastral mapping, cartography, vehicle guidance and atmospheric research

• The phenomenal progress in receiver hardware is continuously widening the scope of applications of this unique technique.

Advantages of GPS as Surveying Tool

• Transmission of signals that can be "seen" over a far larger area than ground-based systems (no line of sight necessary)

• Weather (transmission signals through cloud and rain) and day-time independent

• They recognise no national boundaries, and hence can be used globally wherever they are visible, on the ground, in the air and at sea.

:

SPACE SEGMENTcomprising the satellites themselves, transmitting the signals necessary for the system to operate.

CONTROL SEGMENTground facilities carrying out the task of satellite tracking, orbit computations, telemetry and supervision necessary for the daily management of the Space Segment.

USER SEGMENTentire spectrum of applications equipment and computational techniques that provide the users with the position results.

Three segments

Methods of Observation

• Infrastructural Support– Stand Alone– Local Differential GPS– Long Distance Differential GPS– Wide Area Differential GPS

Stand Alone GPSPositioning by using one Receiver

• From the distances measured to at least 4 satellites X, Y, Z and the time bias (clock error) are computed

• No additional infrastructure required

• Accuracy is limited

Stand Alone GPS

Local Differential GPS

• Two GPS receivers, one at a known point

• Transmission of correction messages to the other receiver(s)

• Influence of many errors can be reduced

• Much higher accuracy possible than with stand alone

Local Differential GPS

Long Distance Differential GPS

• Relative positioning with the fixed receiver transmitting correction messages to the other receiver(s)

• The further the user is away from the base station the less appropriate the corrections

Wide Area Differential GPS

• An integrated network of base stations over a (part) of a continent is uses, e.g.

• WAAS in the US (Wide Area Augmented System)

• EGNOS in Europe (European Geostationary Navigation Overlay Service

Accuracy Aspects

• Noise

• Bias (systematic error)

• Blunder

.

The width of the function represents the uncertainty. When a coordinate is normally distributed, the 1-s interval to both sides about the mean (m) contains about 68% of the samples, and 95% of the samples (in yellow) will lie within the interval [-1.96s, 1.96s] about the mean

Normal probability density function (pdf).

Accuracy and precision are often used interchangeably; precision refers to only the spread, no matter the bias, whereas accuracy both spread and bias includes

Right, the precision of the observable in dark-blue and the one in light-blue is the same, but the latter has been biased due to for instance a remaining systematic effect.

In the absence of biases (Left) precision and accuracy can be used synonymously.

Position Dilution of Precision (PDoP)

• Dimensionless Number between 1 (best) and infinite (worst)

• Indicates how good is the geometry of the satellite configuration

Quality of GPS measurements controllable by user

• Number of satellites (at least 4)

• Configuration of satellites (PDOP)

• Short travel time through atmosphere (avoid use of low satellites)

• Number of independent observations per station

• Time of Measurement

Thank you very much for your attention

Questions?