THE GLOBAL POSITIONING SYSTEM

THE GLOBAL POSITIONING SYSTEM

AND

ITS APPLICATIONS

Prof. Madhav N. Kulkarni, Lt. Col.(R)

kulkarni@civil.iitb.ernet.in

Department of Civil Engineering

Indian Institute of Technology, Bombay.

Powai, MUMBAI ¨C 400076.

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THE GLOBAL POSITIONING SYSTEM AND ITS APPLICATIONS

CONTENTS

1.

INTRODUCTION

2.

SYSTEM DESCRIPTION

2.1

2.2

2.3

2.4

2.5

2.6

2.7

3.

General

Historical Background

GPS Segments

Features of GPS Satellites

Principle of Operation

Present status

Accuracies with GPS and Comparison with other Techniques

SURVEYING WITH GPS

3.1

Methods of Observations

3.1.1 Absolute Positioning

3.1.2 Relative Positioning

3.1.3 Differential GPS

3.1.4 Kinematic GPS

3.2

GPS Receivers

3.2.1 Navigation Receivers

3.2.2 Surveying & Mapping Receivers

3.2.2 Geodetic Receivers

3.3

Computation of Coordinates

3.3.1 Transformation From Global to Local Datum

3.3.2 Geodetic Coordinates to Map Coordinates

3.3.3 GPS Heights and Mean Sea Level Heights

4.

APPLICATIONS OF GPS

5.

GPS IN INDIA

6.

CURRENT AREAS OF RESEARCH & FUTURE DEVELOPMENTS

7.

LIST OF REFERENCES

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Abbreviations

C/A Code

DMA

DoD

DGPS

EDM

GIS

GDOP

HDOP

IGS

ISS

MCS

MSs

NASA

NAVSTAR GPS

NCC

NGS

NNSS

NSWC

P Code

PDOP

PPS

PRN

SLR

SPS

SV

ULS

UTC

VDOP

VLBI

WGS

Coarse Acquisition Code

Defence Mapping Agency, U.S.A.

Department of Defense, U.S.A.

Differential Global Positioning System

Electronic Distance Measuring instrument

Geographical Information System

Geometric Dilution of Precision

Horizontal Dilution of Precision

International GPS Service for Geodynamics

Inertial Surveying System

Master Control Station

Monitor Stations

National Aeronautical and Space Administration, U.S.A.

Navigation Satellite Timing & Ranging Global Positioning System

NAVSTAR Control Centre

National Geodetic Survey, U.S.A.

Navy Navigation Satellite System

Naval Surface Weapons Centre

Precision Code

Position Dilution of Precision

Precise Positioning System

Pseudo Random Noise

Satellite Laser Ranging

Standard Positioning System

Space Vehicle

Up Load Station

Universal Coordinated Time

Vertical Dilution of Precision

Very Long Baseline Interferometry

World Geodetic System

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THE GLOBAL POSITIONING SYSTEM AND ITS APPLICATIONS

1. INTRODUCTION

The Global Positioning System (GPS) is a satellite-based navigation and surveying system

for determination of precise position and time, using radio signals from the satellites, in realtime or in post-processing mode. GPS is being used all over the world for numerous navigational

and positioning applications, including navigation on land, in air and on sea, determining the

precise coordinates of important geographical features as an essential input to mapping and

Geographical Information System (GIS), along with its use for precise cadastral surveys, vehicle

guidance in cities and on highways using GPS-GIS integrated systems, earthquake and landslide

monitoring, etc. In India also, GPS is being used for numerous applications in diverse fields like

aircraft and ship navigation, surveying, geodetic control networks, crustal deformation studies,

cadastral surveys, creation of GIS databases, time service, etc., by various organisations.

The Navigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS)

developed by the U.S. Department of Defense (DoD) to replace the TRANSIT Navy Navigation

Satellite System (NNSS) by mid-90¡¯s, is an all-weather high accuracy radio navigation and

positioning system which has revolutionised the fields of modern surveying, navigation and

mapping. For every day surveying, GPS has become a highly competitive technique to the

terrestrial surveying methods using theodolites and EDMs; whereas in geodetic fields, GPS is

likely to replace most techniques currently in use for determining precise horizontal positions of

points more than few tens of km apart. The GPS, which consists of 24 satellites in near circular

orbits at about 20,200 Km altitude, now provides full coverage with signals from minimum 4

satellites available to the user, at any place on the Earth. By receiving signals transmitted by

minimum 4 satellites simultaneously, the observer can determine his geometric position

(latitude, longitude and height), Coordinated Universal Time (UTC) and velocity vectors with

higher accuracy, economy and in less time compared to any other technique available today.

GPS is primarily a navigation system for real-time positioning. However, with the

transformation from the ground-to-ground survey measurements to ground-to-space

measurements made possibly by GPS, this technique overcomes the numerous limitations of

terrestrial surveying methods, like the requirement of intervisibility of survey stations,

dependability on weather, difficulties in night observations, etc.. These advantages over the

conventional techniques, and the economy of operations make GPS the most promising

surveying technique of the future. With the well-established high accuracy achievable with GPS

in positioning of points separated by few hundreds of meters to hundreds of km, this unique

surveying technique has found important applications in diverse fields.

2. SYSTEM DESCRIPTION

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2.1 General

The NAVSTAR Global Positioning System is a satellite based navigation system being

developed and maintained by the DoD since 1972, for providing extremely accurate 3-D position

fixes and UTC information to properly equipped users anywhere on or near the Earth, at any

time, regardless of weather conditions. Uncertainties in positions of GPS satellite and timing

signals, imposed due to security reasons by DoD, and other error sources, are expected to limit

accuracy of determination of absolute position of observation station in real time mode to few

meters, with few minutes of observations; however, various modes of observations and data

analysis available and being developed, would yield accuracies better than few mm. in relative

positions for base lines up to 2000 km, with few hours of observations, at minimum cost. The

system consists of three segments: Space Segment, Control Segment and User Segment. The

satellites continuously transmit dual frequency navigation signals consisting of information of

satellites position with time tag, along with other data, which is periodically uploaded in satellite

memory from the Control Segment. The User Segment receives navigation signals from at least

4 satellites, available any time globally, allowing the user to simultaneously solve 4 independent

range-difference equations to yield his position - latitude, longitude and height and also the time.

The versatility, accuracy, cost-efficiency and economy offered by the system make GPS the most

suitable system for many different applications in various fields.

2.2 Historical Background

The TRANSIT NNSS - the satellite navigation system operational prior to GPS, was

launched in 1958 by the U.S. Navy. It became operational in 1964 and was made available to

civilian users in 1967. The system, comprising 5 satellites at 1075 km altitude, was phased out in

the early 90s. This system has now been replaced by the NAVSTAR GPS in an extensive multibillion dollars project launched in 1972 as a Joint Services Program of U.S. Air Force, Navy,

Army, Marines and Defence Mapping Agency; in three phases. The GPS system became fully

operational and available to the commercial users by early 90s.

2.3 GPS Segments

The Global Positioning System basically consists of three segments: the Space Segment, The

Control Segment and the User Segment.

2.3.1 Space Segment

The Space Segment contains 24 satellites, in 12-hour near-circular orbits at altitude of about

20000 km, with inclination of orbit 55¡ã. The constellation ensures at least 4 satellites in view

from any point on the earth at any time for 3-D positioning and navigation on world-wide basis.

The three axis controlled, earth-pointing satellites continuously transmit navigation and system

data comprising predicted satellite ephemeris, clock error etc., on dual frequency L1 and L2

bands (see Figs. 1 & 2).

2.3.2 Control Segment

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