GPS

GLOBAL POSITIONING SYSTEM

GPS

TSM 352 - February 2014Inés Resano Goizueta

Introduction

GPS: NAVSTAR GPS (NAVigation System with Time and

Ranging Global Positional System) After Second World War developed by USA army to determine exactly any position

of their soldiers.

GLONASS (Soviet Union Russian Federation)

GALILEO (Europe)

How it works

1. Intersection of 3 spheres 2 points

We need to know:Position of satellitesDistance among them

A B

User: A or B? One does not make sense But, we do NOT need other satellite

How it works (II)

3 components (segments) to resolve Spatial: 24 satellites in 6 orbits (20200 km, p 12h,

atomic clock) At least: 4 satellites over each point (>15▫) all time

Control: Fixed stations define and track. Master station: update and predict each path (next24h)

User:

Knows where each satellite

is

Deal data Exchange info

Calculus

TRIANGULATION method

Distance receptor / satellite

Time Satellite signal

Position Satellite

Calculus (II)

Triangulation methods Topographic method to determinate the position of a

land point. Measures the distance of this point to some satellites.

Analytical we have to use 4 satellites (exactly) Practically we need only 3. One solution is absurd.

- Receptor in B - Know P1, P2 and P3 Satellite position.- Know r1, r2 and r3 distance

satellite/receptor.- Fourth satellite precision

Calculus (III)

Measurement of distances Time that takes the satellite´s signal to arrive at the

receptor [0,06 s ]

Clocks (satellite and receptor) Satellite´s clocks: Rubidium (), Cesium (), H() Receptor´s clock: Quartz ()

Synchronization?

+1 satellite more

errors

Retarded signals because of ionosphere/atmosphere

Clock problemsMulti-way effectDiluted precisionSelective availability (S/A)Anti Spoofing (A-S)

errors (II)

Ionosphere and atmosphere issues The speed of the signal through the ionosphere could

be dismissing and vary depending on its density and the height.

errors (III)

Clock problems Satellite (atomic): small & master correction Receptor: be aware & vary FOURTH SATELLITE

Multi-way effect Reflections: signals do not go straight to the receptor.

errors (IV)

Diluted position: Depending on the shape of the figure that they form.

GOOD!

BAD!

errors (V)

Selective availability (SA) by USA: Dithering: in order to avoid the access to good

precision. About 100 m of dispersion May 2000 was deactivated.

Anti-spoofing (A-S) It is not allow the access to P code. Signal is called Y

code Only USA Army and their allies can read the Y code.

Thank you

References María González de Audicana Amenábar, UPNA notes –

topografía (2011) Wikipedia: http://

es.wikipedia.org/wiki/Sistema_de_posicionamiento_global

http://en.wikipedia.org/wiki/Global_Positioning_System

Questions?