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GPS Tutorial # 1Overview
Darren Sessions
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GPS OverviewGPS System
Calculating a Position
Receiver Outputs
Accuracy and Availability
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GPS SystemNAVSTAR GPS
Navigation Satellite Timing and Ranging Global Positioning SystemFunded and controlled by U. S. Department of Defense.
Comprised of three segmentsSpace SegmentControl SegmentUser Segment
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Space SegmentSpace segment consists of the GPS space vehicles (SVs).Nominally 24 SVs plus spares.
Each vehicle has a12 hour orbit.Repeats same ground track daily.6 orbital planes with 4 vehicles each.Planes are equally spaced 60 degrees apart.Inclined 55 degrees from equatorial plane.20,200 km above the earth.5 to 8 SVs visible from anywhere on earth.
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Space Segment
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Control SegmentSVs are controlled by five system tracking stations
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Control SegmentStations monitor and measure signals from the SVs which are incorporated into orbital models for each of the satellites.The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite.The Master Control station uploads updated ephemeris and clock data to the SVs.
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User SegmentCivilian
SPS - Standard Positioning Service Uses single frequency L1Uses C/A code only
MilitaryPPS - Precise Positioning Service
Uses two frequencies L1/L2 Uses C/A code and P-code
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GPS System ReviewSpace Segment
Control Segment User Segment
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GPS OverviewGPS System
Calculating a Position
Receiver Outputs
Accuracy and Availability
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Calculating a PositionMeasure distance to satellites.
Obtain satellite positions.
Perform triangulation calculations. (Trilateration)
Adjust local clock bias.
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Measuring DistanceDistance = Velocity * Time
Velocity is that of a radio wave.Time is the travel time of the signal.
Measure the travel time.Receiver generates the same codes as the satellite (PRN codes).Measure delay between incoming codes and self generated codes.D = Speed of light * measured delay.
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Measuring DistanceDistance = Time Delay * Speed of Light
Time Delay
Signal Generated at Satellite
Received Signal
Signal Generated by Receiver
Receiver Generated Signal with phase delay
(Code Phase)
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Calculating a PositionMeasure distance to satellites.
Obtain satellite positions.
Perform triangulation calculations.
Adjust local clock bias.
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Satellite PositionsOrbital data (Ephemeris) is embedded in the satellite data message.
Ephemeris data contains parameters that describe the elliptical path of the satellite.
Receiver uses this data to calculate the position of the satellite. (X,Y,Z)
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Elliptical path of Satellite
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Calculating a PositionMeasure distance to satellites.
Obtain satellite positions.
Perform triangulation calculations.
Adjust local clock bias.
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Triangulation in 2DIf location of point A is known, and the distance to point A is known, desired position lies somewhere on a circle.
Could be anywhere along circle
A
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Distance to two points are known.Desired position is in one of two locations.
Triangulation in 2D
Or Here
Could Be Here
AB
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Triangulation in 2DDistance to three points are known.Position is known!
AB
C
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Distance to two points is known.
Triangulation in 3D
Could be anywhere along ellipse
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Triangulation in 3DDistance to 3 points are known.Intersects at 2 points.
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Calculating a PositionMeasure distance to satellites.
Obtain satellite positions.
Perform triangulation calculations.
Adjust local clock bias.
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Clock Bias
Time Delay
Signal Generated at Satellite
Received Signal
Signal Generated by Receiver
Receiver Generated Signal with phase delay
(Code Phase)
Clock bias results in incorrect time delay measurement
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Time ErrorLocal clock and Satellite clock are not synchronized.
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Clock BiasFourth satellite will not intersect with the first three.
Local clock is not synchronized to SV clocks.
Clock error is from a single source.Common to all satellites.
Apply a clock biasAdd or subtract a constant to all SV pseudo ranges.
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Clock Bias
SV 3
SV 9
SV 31
SV 7
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Calculating a Position ReviewMeasure distance to satellites.
Use pseudo rangesObtain satellite positions.
Decoded ephemeris from satellite message.Perform triangulation calculations.
Need at least 3 satellites for triangulation.Adjust local clock bias to find position.
Need 4th satellite to adjust bias.Position is now known!
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GPS OverviewGPS System
Calculating a Position
Receiver Outputs
Accuracy and Availability
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Receiver OutputsTypically receivers provide two different formats.
NMEA (Nation Marine Electronics Association)ASCII FormatDefines a set of standard messages.
Proprietary FormatTypically BinaryNo limit on information transmitted.
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Receiver OutputsPosition, Velocity, Time (PVT)
PositionLatitude ddmm.mmmmLongitude dddmm.mmmmAltitude m
VelocitySpeed knots Heading degrees
Time (UTC)Date dd/mm/yyTime hh/mm/ss.sss
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Receiver OutputsSatellite information
Satellite ID or PRNAzimuthElevationSignal Strength
Dilution of precision (DOP)PDOPHDOPVDOP
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GPS OverviewGPS System
Calculating a Position
Receiver Outputs
Accuracy and Availability
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Differential GPS Maritime Differential GPS (DGPS)
Managed by the U.S. Coast Guard (USCG)Employs ground stations along the coasts with known fixed locations.Corrections are transmitted from ground stations at low frequencies (200-500kHz).Requires an additional Differential Beacon Receiver (DBR) and an additional antenna.Accuracy is a function of the distance from the ground station.
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Differential GPS Wide Area Augmentation System (WAAS)
Managed by the FAACommunicates with several ground stations.Provides atmospheric corrections. Early warning of GPS failures.Same frequency as GPSHigher data rate 250 HzSatellites are in geostationary orbits.
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Ionoshperic Delay
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Differential GPSEuropean Gestationary Navigation Overlay System (EGNOS)
Managed by the European tripartite groupCorrections for GPS and GLONASS
Multi-Function Transport Satellite Space Based Augmentation System (MSAS)
Managed by the Japanese Civil Aviation Bureau (JCAB)
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Geostationary Satellites
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Accuracy
25 ns40 ns340 nsTime Accuracy
to UTC(2σ)
7.6 m30 m140 mVertical Position
Accuracy(2σ)
7.6 m20 m100 mHorizontal Position
Accuracy(2σ)
WAASGPS w/o SAGPS with SAParameter
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Signal Availability
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Signal Availability
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Further ReadingElementary
http://www.trimble.com/gps/index.html
Novicehttp://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
Experthttp://www.gmat.unsw.edu.au/snap/gps/gps_survey/principles_gps.htm