Errors in GPS Errors in GPS GPS is currently the most accurate positioning system available globally. Although we are talking about extreme precision and measuring distances by speed of light, yet there are several sources of errors in GPS. The designers of GPS did a great job of protecting it from potential problems, but even so, some small errors cannot be entirely eliminated by the basic system. Geodetic Coordinate system Question of Error? Accuracy? With Reference to WHAT? Geodetic Coordinate System: A set of rules for specifying how coordinates are to be assigned to positions on the surface of the earth. Defined by X,Y,Z on an Ellipsoid Sphere Geoid Equal gravity Ellipsoid What we like to think the earth is shaped like Equipotential gravimetric surface (sealevel) Assuming sea level everywhere Best fitting mathematical shape http://sensoryoutput.com/2010/01/11/geoid/ R. Khosla Fall Semester 2015 1
Geodetic Height Ellipsoid It is an imaginary sphere similar to the shape of earth. [Earth is not a perfect sphere] Ellipsoid is a simple geometric surface that can be defined by mathematical equation Ellipsoid vs Geoid Ellipsoid Mathematical Definition Simple Geometrical Surface Described By 2 Parameters Cannot be sensed by instruments Geoid Physical Definition Complicated Surface Described by infinite # patterns Can be sensed by instruments Best fitting ellipsoid Ellipsoidal surface Geoidal surface Simple terms: Geodetic height is the difference between the Ellipsoid height and height above mean sea level (elevation). Geodetic coordinate systems DMA: Defense Mapping Agency since 1960 WGS 60, 66, 72, 84 New ones are coming as our understanding of the earth improves R. Khosla Fall Semester 2015 2
What s so special about GPS heights GPSHeights are defined with respect to the Ellipsoid NOT the geoidal height or Mean Sea Level what we are used to in our daily lives The height of the surface of the infinitely lumpy actual surface of the Earth above (or occasionally below) sea level is referred to as Orthometric Height GPS heights are not much accurate No SV s below the earth is visible The earth is opaque One sided geometry results in ½ Optimal configuration Vertical accuracy typically ½ that of horizontal Best fitting ellipsoid Ellipsoidal surface Orthometric surface Geoidal surface Sources of GPS Errors Ionospheric and Atmospheric Errors Satellite Clock Errors Orbital Errors Multipath Errors Receiver Errors Induced Errors or Selective Availability Ionospheric and Atmospheric Errors 20 000 km Satellite orbit 200 km 50 km Inonosphere Ions Troposphere Water vapor R. Khosla Fall Semester 2015 3
Ionospheric and Atmospheric Errors Ionosphere is a blanket of electrically charged particles 50 200 kms above the earth surface. These particles affect the speed of light and effect the speed of GPS radio signals. Refraction Ideal Real situation + + + + + + + + + + + + + + + + + Mitigating Atmospheric Errors L1 Carrier (higher freq.) L2 Carrier (lower freq.) By using both frequencies, it is possible to infer the error generated by the atmosphere. Mitigating Atmospheric Errors Using a 15 mask to minimize refraction error 15 R. Khosla Fall Semester 2015 4
Multipath errors Multipath errors Selective Availability (S/A) The degradation of position accuracy by the DoD Done to deny hostiles high accuracy positioning Done by: Offsetting SV clocks Injection of Ephemeris error (orbital position) With S/A On: ~100 meters(2drms) With S/A Off: ~20 40 meters (2DRMS) DRMS: Distance Root Mean Square 1DRMS = 68% 2DRMS = 95% R. Khosla Fall Semester 2015 5
Sources of error 1 SV clock error (0 to 1.5 m) 2 Orbital error (1 to 5 m) 3 Atmospheric refraction Ionosphere: 0 to 30 m Troposphere: 0 to 30 m 4 Multipath (0 to 1 m) 5 Receiver noise (0 to 10 m) 6 Selective availability (0 to 70 m) GPS Error Budget Satellite Clock error 0 1.5 m SV Ephemeris error 1 5 m Ionospheric Refraction 0 30 m Tropospheric Refraction 0 30 m Multipath 0 1 m Receiver Noise 0 10 m Selective Availability 0 70 m Dilution Of Precision: Above x 1 6 Total Error with Selective Availability on: ~100 m Total Error with Selective Availability off: ~28 m = ~ 90ft Geometric Precision The position can be better or worse depending on which satellites the receiver is using to make a position measurement. Not that one satellite is better than another It depends on the relative angle of satellite in the sky In simple terms, the wider the angle between satellites the better the measurement. R. Khosla Fall Semester 2015 6
Good DOP Dilution of Precision Poor DOP Dilution of Precision Dilution of Precision Area of ambiguity R. Khosla Fall Semester 2015 7
Dilution of Precision Area of ambiguity Precision is diluted in a larger area Dilution of Precision GDOP = Geometric Dilution of Precision PDOP = Position Dilution of Precision (Most commonly used) 3D VDOP = Vertical Dilution of Precision HDOP = Horizontal Dilution of Precision TDOP = Time Dilution of Precision QUALITY Very good DOP 1 3(considered good) Good 4 5 Fair 6 Suspect >6 R. Khosla Fall Semester 2015 8