GPS: The Basics Darrell R. Dean, Jr. Civil and Environmental Engineering West Virginia University Expected Learning Outcomes for GPS Explain the acronym GPS Name 3 important tdt dates in history of GPS Name the 3 components of GPS system Explain geometric basis for position calc. Explain how signal travel time is meas. Name 2 codes used to measure range List the general requirements for GPS work Feb. 2010 Darrell.Dean@mail.wvu.edu 2 Dr. Darrell R. Dean, Jr., P.S. 1
Expected Outcomes (Cont.) State the required number of satellites to map a point. Name two types of GPS receivers List 3 attributes of GPS receivers Explain positional accuracy for different receivers and different techniques Feb. 2010 Darrell.Dean@mail.wvu.edu 3 What Is GPS? Global Positioning System NAVSTAR NAVigation System using Timing and Ranging Radio based Navigation System Using Satellites Developed by the U.S. Department of Defense Feb. 2010 Darrell.Dean@mail.wvu.edu 4 Dr. Darrell R. Dean, Jr., P.S. 2
GPS Attributes Output can include position (latitude, longitude, height), velocity, and time (PVT) All weather, any place (?), and any time One way ranging system Requires precisely synchronized clocks on each end Atomic clocks in the satellite and a quartz clock in the receiver Feb. 2010 Darrell.Dean@mail.wvu.edu 5 GPS History Developmental satellites began launch in 1978 Operational satellite launches start in 1989 Full Operational Capability (24 satellites) in 1995 Selective availability turned off May 1, 2000 Feb. 2010 Darrell.Dean@mail.wvu.edu 6 Dr. Darrell R. Dean, Jr., P.S. 3
GPS System Components Space User Equipment Satellites -- Block II/IIA/IIR/IIF Ground Antenna User Master Control Station Ground Monitor Station User Equipment From: Moore & Parisi, Master GPS WRC-2000 Modular Briefing, MITRE, Dec. 1999 Master Control Station Feb. 2010 Darrell.Dean@mail.wvu.edu 7 Space Segment 24-30 Satellites Altitude: 20,200 km Orbital Period: 12 hrs Orbital Plane: 55 deg Number of Planes: 6 Vehicles per plane: 4 From: Moore & Parisi, Master GPS WRC-2000 Modular Briefing, MITRE, Dec. 1999 Feb. 2010 Darrell.Dean@mail.wvu.edu 8 Dr. Darrell R. Dean, Jr., P.S. 4
Ground Control Segment Five monitoring stations Precise position known in WGS 84 (datum) Passively tracks satellites in view Accumulates range or distance data to the satellites Data transmitted to the master control station Feb. 2010 Darrell.Dean@mail.wvu.edu 9 Ground Control Segment (Cont.) Three ground antennas at 3 of the monitoring stations tti Transmits updated information to each satellite One master control station Processes range data to precisely determine satellite orbits and positions Updates each satellite s navigation message Feb. 2010 Darrell.Dean@mail.wvu.edu 10 Dr. Darrell R. Dean, Jr., P.S. 5
Ground Control Segment Feb. 2010 Darrell.Dean@mail.wvu.edu 11 User Segment From: Moore & Parisi, Master GPS WRC-2000 Modular Briefing, MITRE, Dec. 1999 Feb. 2010 Darrell.Dean@mail.wvu.edu 12 Dr. Darrell R. Dean, Jr., P.S. 6
How Does GPS Work? 3 To measure travel time, GPS needs very accurate clocks. In addition to knowing the distance to a satellite, a user 4 needs to know the satellite s s location. 5 As the GPS signal travels through the ionosphere and earth s atmosphere, it gets delayed. 2 To trilaterate, GPS measures distance using the travel time Trilateration from of a radio signal. 1 satellites is the basis of the system. X From: Moore & Parisi, Master GPS WRC-2000 Modular Briefing, MITRE, Dec. 1999 Feb. 2010 Darrell.Dean@mail.wvu.edu 13 How Does GPS Work? (Cont.) Uses basic equation: distance = rate x time Approximate rate for radio waves = 186,000 miles per second Satellite transmits a signal with a time code Receiver locks onto the signal From Mar. 96 P.O.B Feb. 2010 Darrell.Dean@mail.wvu.edu 14 Dr. Darrell R. Dean, Jr., P.S. 7
How Does GPS Work?(Cont.) Signal used to calculate pseudo range or distance. With distances to 3 satellites known, trilateration can be used to calculate a position (latitude, longitude, and height) From Mar. 96 P.O.B. Actually, four satellites are required because a calculation for time is also needed Feb. 2010 Darrell.Dean@mail.wvu.edu 15 System Signal Characteristics Pseudo random noise code (PRN) modulated on two carrier frequency waves. Navigation messagesatellite ephemeris and time corrections also modulated don carrier frequency From Feb. 96 P.O.B. Feb. 2010 Darrell.Dean@mail.wvu.edu 16 Dr. Darrell R. Dean, Jr., P.S. 8
System Codes (PRN Codes) C/A code: course acquisition code Associated with standard d positioning system(sps) SPS available to all users P(y) code: precision code Associated with precise positioning system (PPS) PPS for military users L2C code: for civilian use recent update Provides improved accuracy Feb. 2010 Darrell.Dean@mail.wvu.edu 17 System Protection Procedures Selective availability (S/A) reduces accuracyofpositions from system Introduce error in satellite clock (dither) Introduce error in orbit data (epsilon) Turned off since May 1, 2000 Anti spoofing (A S) guards against fake transmissions of satellite data Encryption of the p code to form the y code or P(Y) Feb. 2010 Darrell.Dean@mail.wvu.edu 18 Dr. Darrell R. Dean, Jr., P.S. 9
GPS Receivers Signal tracked Code mapping grade & consumer grade Carrier frequency survey grade Code + Carrier frequency Number of frequencies Single or dual No. of channels (no. of satellites tracked at one time) 6 to many. Feb. 2010 Darrell.Dean@mail.wvu.edu 19 GPS Receivers Cont. External or internal antenna Accuracy vs. ease of signal acquisition Data logging integratedfield computer Enter and store attribute data (data dictionary) Point averaging Point nesting e.g. change from line feature to point feature w/o interrupting line Offset capability compass/range finder for remote point location Storage ability for large projects Feb. 2010 Darrell.Dean@mail.wvu.edu 20 Dr. Darrell R. Dean, Jr., P.S. 10
GPS Positioning Methods Autonomous vs Relative positioning Static positioning Real time correction capability Wide area augmentation system (WAAS) Satellite Based Augmentation System(SBAS) Radio (RTCM) broadcasts from beacons or communication satellites, some free some require a subscription fee (DGPS) RTK Single base RTK Network connection Wireless (cell phone) data connection Commercial networks Public entity (DOT, e.g.) networks Post Processing Feb. 2010 Darrell.Dean@mail.wvu.edu 21 Positional Accuracy (in general) Accuracy Receiver Method/Processing 3 5 mm Survey Relative positioning /static, RTK 1 cm Grade Submeter 1 3m 2 5 m 2 5 m > 5 m Mapping Grade Mapping Grade Consumer Grade Consumer Grade, others Autonomous/DGPS, real time or post processing Autonomous / WAAS/DGPS Autonomous /WAAS Autonomous No DGPS No WAAS Feb. 2010 Darrell.Dean@mail.wvu.edu 22 Dr. Darrell R. Dean, Jr., P.S. 11
Factors Affecting Accuracy Type of receiver Code based/carrier phase Dual frequency (L1 & L2) Methodology Autonomous positioning Relative positioning (static vs. RTK) Processing: Real time or post processing Signal interruption (trees, bldg., topo.) Feb. 2010 Darrell.Dean@mail.wvu.edu 23 Accuracy Factors (Cont.) Atmospheric factors Ionosphere significant error factor dual freq. receivers min. effect due to ionosphere Multipath reflections of the signal Geometric configuration of satellites Position dilution of precision (PDOP) Feb. 2010 Darrell.Dean@mail.wvu.edu 24 Dr. Darrell R. Dean, Jr., P.S. 12
From: Map. Sys., Gen. Ref. by Trimble Nav. Limited, Jan. 2000 Feb. 2010 Darrell.Dean@mail.wvu.edu 25 From: Map. Sys., Gen. Ref. by Trimble Nav. Limited, Jan. 2000 Feb. 2010 Darrell.Dean@mail.wvu.edu 26 Dr. Darrell R. Dean, Jr., P.S. 13
From: Map. Sys., Gen. Ref. by Trimble Nav. Limited, Jan. 2000 Feb. 2010 Darrell.Dean@mail.wvu.edu 27 What s Required to Use GPS? Receiver Mission planning Free download available See reference sheet Check number of satellites and location Check satellite geometry position dilution ofprecision (PDOP) Feb. 2010 Darrell.Dean@mail.wvu.edu 28 Dr. Darrell R. Dean, Jr., P.S. 14
Enter Survey Location Feb. 2010 Darrell.Dean@mail.wvu.edu 29 Number of Satellites Feb. 2010 Darrell.Dean@mail.wvu.edu 30 Dr. Darrell R. Dean, Jr., P.S. 15
Number of Satellites (Cont.) Feb. 2010 Darrell.Dean@mail.wvu.edu 31 Elev. and Azimuth of Satellite Feb. 2010 Darrell.Dean@mail.wvu.edu 32 Dr. Darrell R. Dean, Jr., P.S. 16
Elev. and Azimuth (Cont.) Feb. 2010 Darrell.Dean@mail.wvu.edu 33 PDOPs for Survey Date Feb. 2010 Darrell.Dean@mail.wvu.edu 34 Dr. Darrell R. Dean, Jr., P.S. 17
PDOPs (Cont.) Feb. 2010 Darrell.Dean@mail.wvu.edu 35 PDOPs (Cont.) Feb. 2010 Darrell.Dean@mail.wvu.edu 36 Dr. Darrell R. Dean, Jr., P.S. 18
Number of Satellites by Interval Feb. 2010 Darrell.Dean@mail.wvu.edu 37 Number by Interval(Cont.) Feb. 2010 Darrell.Dean@mail.wvu.edu 38 Dr. Darrell R. Dean, Jr., P.S. 19
Number by Interval(Cont.) Feb. 2010 Darrell.Dean@mail.wvu.edu 39 What s Required (Cont.)? Clear view of sky Log positions in field, enterattribute data Download spatial data and attribute data Post process spatial data as necessary Post processing software Base station files Create required products and services Feb. 2010 Darrell.Dean@mail.wvu.edu 40 Dr. Darrell R. Dean, Jr., P.S. 20
Conclusions Proper equipment and methods are necessary for precise surveying with GPS Open conditions and a clear view of the sky are required for GPS surveying Relatively inexpensive GPS receivers may be used for collecting survey data and feature attributes for management purposes Feb. 2010 Darrell.Dean@mail.wvu.edu 41 Dr. Darrell R. Dean, Jr., P.S. 21