GLOBAL POSITIONING SYSTEM GPS TSM February 2014 Inés 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 satellites Distance among them A B User: A or B? One does not make sense But, we do NOT need other satellite

How it works (II) Knows where each satellite is Deal dataExchange 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) Synchro nization? +1 satellite more

errors Retarded signals because of ionosphere/atmosphere Clock problems Multi-way effect Diluted precision Selective 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: obal obal 

Questions?