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Basic Principles of GPS. Mathias Lemmens EU GIS/Mapping Advisor Abuja 4 th August 2005. Contents. Introduction GPS as a surveying tool Methods of Observation Accuracy Aspects Sources of GPS Error. Global Positioning System (GPS).

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Basic Principles of GPS

Mathias Lemmens

EU GIS/Mapping Advisor

Abuja 4th August 2005


  • Introduction

  • GPS as a surveying tool

  • Methods of Observation

  • Accuracy Aspects

  • Sources of GPS Error

Global positioning system gps
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
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.

Three segments

  • :


    • comprising the satellites themselves, transmitting the signals necessary for the system to operate.


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


    • entire spectrum of applications equipment and computational techniques that provide the users with the position results.

Methods of observation
Methods of Observation

  • Infrastructural Support

    • Stand Alone

    • Local Differential GPS

    • Long Distance Differential GPS

    • Wide Area Differential GPS

Stand alone gps
Stand Alone GPS

Positioning 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

Local differential 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

Long distance 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
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
Accuracy Aspects

  • Noise

  • Bias (systematic error)

  • Blunder

Normal probability density function (pdf).

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


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
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
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