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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?
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