This section was edited by Kenneth Foote, Department of Geography, University of Texas Austin.

This unit is part of the NCGIA Core Curriculum in Geographic Information Science. These materials may be used for study, research, and education, but please credit the author, Peter H. Dana, and the project, NCGIA Core Curriculum in GIScience. All commercial rights reserved. Copyright 1998 by Peter H. Dana.

Your comments on these materials are welcome. A link to an evaluation form is provided at the end of this document.

Topics covered in this unit

• This unit provides an overview of the Global Positioning System including
  • a description of the space, control and user segments.
  • a description of the basic services provided by GPS.
  • a discussion of position and time determination from GPS signals.
  • a discussion of GPS error sources and methods for overcoming some GPS errors.

• The overview discusses GPS project planning and costs.

• The overview does not discuss details of GPS signals and data formats, but does provides references to relevant sources.

Learning Outcomes

• After learning the material covered in this unit, students should be able to:
  • List the major GPS segments as defined by the Department of Defense.
  • Explain how a GPS receiver computes position and time from GPS signals.
  • Describe the major error sources for GPS positioning projects.
  • Explain the various forms of Differential GPS.
  • Propose suitable equipment and processes for various levels of positioning accuracy.

Instructors' Notes

Full Table of Contents

Metadata and Revision History

1. The Global Positioning System: a Satellite Navigation System

• The Global Positioning System is an earth-orbiting-satellite based system that provides signals available anywhere on or above the earth, twenty-four hours a day, that can be used to determine precise time and the position of a GPS receiver in three dimensions.

• GPS is funded by and controlled by the U. S. Department of Defense (DOD) but can used by civilians for georeferencing, positioning, navigation, and for time and frequency control.

• GPS is increasingly used as an input for Geographic Information Systems particularly for precise positioning of geospatial data and the collection of data in the field.

• Effective use of the GPS system does require training, appropriate equipment, and knowledge of the limitations of the system.

• Some technical topics concerning GPS signals and data formats go beyond the scope of the present overview, but are addressed in  sources listed in the references including the http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

1.1. Segments of the Global Positioning System

1.1.1. Space Segment

• The Space Segment of the system consists of the 24 GPS satellites.
  • These space vehicles (SVs) send radio signals from space.
  • Their configuration provides user with between five and eight SVs visible from any point on the earth.
  • Figure 1. GPS Satellite
  • Figure 2. GPS Constellation

1.1.2. Control Segment

• The Control Segment consists of a system of tracking stations located around the world.
  • These stations measure signals from the SVs, compute orbital data, upload data to the SVs, then the SVs send data to GPS receivers over radio signals.
  • Figure 3. GPS Master Control and Monitor Network
  • Figure 4. GPS Control Monitor

1.1.3. User Segment

• The User Segment consists of the GPS receivers and the user community.
  • GPS receivers convert SV signals into position, velocity, and time estimates.
  • Four satellites are required to compute the four dimensions of X, Y, Z (position) and T (time).
  • Figure 5. Four GPS Satellite Solution
  • GPS receivers are used for navigation, surveying, time dissemination, and other research.
  • Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals.
  • Figure 6. GPS Navigation

1.2. GPS Positioning Services

1.2.1. Precise Positioning Service (PPS)

• Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System.
  • The PPS provides (95% of the time) a 22 meter horizontal accuracy, a 27.7 meter vertical accuracy, and a 100 nanosecond time accuracy.
  • Authorized users include U. S. and Allied military, certain U. S. Government agencies, and  selected civil users specifically approved by the U. S. Government.

• Civil users worldwide use the SPS without charge or restrictions.
• Most receivers are capable of receiving and using the SPS signal.
• Prior to May 2, 2000, The SPS accuracy was intentionally degraded by the DOD by the use of Selective Availability (SA).
  • With SA the SPS provided (95% of the time) a 100 meter horizontal accuracy, a 156 meter vertical accuracy, and a 340 nanoseconds time accuracy.
  • Without SA the SPS provides a much improved performance, perhaps as good as 20 meters horizontal and 30 meters vertical. No new specification for the SPS without SA has been issued as of 7/01/2000.

1.3. GPS Satellite Signals and Data

• The SVs transmit two microwave carrier signals.
  • The L1 frequency (1575.42 MHz) carries the navigation message, the SPS code signals known as the C/A (coarse acquisition) Code, and the P (precise) Code used for the PPS.
  • The L2 frequency (1227.60 MHz) carries the P Code used for the PPS. The phase difference between the P-Code on L1 and L2 is used to measure the ionospheric delay by PPS equipped receivers tracking both frequencies.
  • A C/A Code modulates the L1 carrier phase.
    • The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code.
    • This noise-like code consisting of a repeating sequence of 1023 bits modulates the L1 carrier signal.
    • There is a different C/A code PRN for each SV.
    • GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code
  • Figure 7. GPS Signals

• The GPS Navigation Message consists of time-tagged data bits marking the time of its transmission by the SV and includes:
  • Clock data parameters describe the SV atomic clock and its relationship to GPS time.
  • Ephemeris data parameters describe SV orbits for short sections of the satellite orbits.
  • An ionospheric model that is used in the receiver to approximates the phase delay through the ionosphere at any location and time.
  • The amount to which GPS Time is offset from Universal Coordinated Time. This correction can be used by the receiver to set UTC to within 100 nanoseconds.
  • Figure 8.  Navigation Data Bits

2. Using GPS

2.1. One Receiver Using Civilian Code-Phase Tracking

• The receiver tracks the satellites by aligning a set of receiver-generated C/A Codes with the received C/A Code sequences from the satellites.

• These measurements of code alignment times are called pseudo-ranges because they not actual range measurements, but are relative times of arrival all offset by the receiver clock bias common to each C/A code generated in the receiver.

• The GPS receiver gathers and interprets the Navigation Message transmitted by the SVs it is tracking, computing a position for each satellite at the moment of C/A code transmission.

• The measured pseudo-ranges are corrected for SV clock bias, ionospheric delay and other offsets.

• The coordinates of the receiver are computed by finding a position where the set of pseudo-ranges intersect when a common receiver clock offset is accounted for.
  • Figure 9. Intersection of Pseudo-Ranges

• GPS time in the receiver is computed from the receiver clock offset that allows the corrected pseudo-ranges to converge at the receiver position.

• Four satellites (normal navigation) can be used to determine three position dimensions and time.

2.1.1. Position

• Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates.

• Figure 10. ECEF X, Y, and Z

• Position in XYZ is converted within the receiver to geodetic latitude, longitude and height above the ellipsoid.

• Figure 11. Geodetic Coordinates

• Latitude and longitude are usually provided in the geodetic datum on which GPS is based (WGS-84).
  • Receivers can often be set to convert to other user-required datums.
  • Position offsets of hundreds of meters can result from using the wrong datum.
  • Receiver position is computed from the SV positions, the measured pseudo-ranges, and a receiver position estimate.
  • Four satellites allow computation of three position dimensions and time.
  • Three satellites could be used determine three position dimensions with a perfect receiver clock.
    • In practice this is rarely possible and three SVs are used to compute a two-dimensional, horizontal fix (in latitude and longitude) given an assumed height.
    • This is often possible at sea or in altimeter equipped aircraft.
  • Five or more satellites can provide position, time and redundancy.
  • Twelve channel receivers allow continuous tracking of all available satellites, including tracking of satellites with weak or occasionally obstructed  signals.

2.1.2. Time

• Time is computed in the same solution as position and is used to correct the offset in the receiver clock, allowing the use of inexpensive oscillators in low-cost receivers.

• Time is computed in SV Time, GPS Time, and UTC.
  • SV Time is the time maintained by each satellite's atomic clocks.
  • SV clocks are monitored by ground control stations and occasionally reset to maintain time to within one millisecond of GPS time.

• SV Time is set in the receiver from the GPS signals.
  • SV Time is converted to GPS Time in the receiver using the SV clock correction parameters.

• GPS Time is a "paper clock" ensemble of the Master Control Clock and the SV clocks.
  • It is measured in weeks and seconds from 24:00:00, January 5, 1980 and is steered to within one microsecond of UTC.
  • GPS Time has no leap seconds and is ahead of UTC by several seconds.

• Universal Coordinated Time (UTC) is computed from GPS Time using the UTC correction parameters sent as part of the navigation data bits.

2.1.3. Velocity

• Velocity is computed from change in position over time, the SV Doppler frequencies (the change in carrier frequency due to the combined movement of the satellites and the receiver), or both.

2.2. GPS Errors

• GPS errors are a combination of noise, bias, blunders.
  • Figure 12. Noise, Bias, and Blunders

2.2.1. Noise Errors

• Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter).

• Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution.

2.2.2. Bias Errors

• Bias errors result from Selective Availability and other factors.
  • Selective Availability (SA) is the intentional degradation of the SPS signals by a time varying bias.
  • SA is controlled by the DOD to limit accuracy for non-U. S. military and government users.
  • The potential accuracy of the C/A code of around 30 meters is reduced to 100 meters (95% of the time).

• Other Bias Error sources:
  • SV clock errors uncorrected by Control Segment can result in one meter errors in position.
  • Tropospheric delays: 1 meter position error.
    • The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes.
  • Unmodeled ionosphere delays: 10 meters of position error.
    • The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air.
  • Multipath: 0.5 meters of position error.
    • Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite.
    • Multipath is difficult to detect and sometimes hard to avoid. Care in antenna placement at fixed sites, special antenna configurations, and special tracking techniques can help sometimes.

2.2.3. Blunders

• Blunders can result in errors of hundred of kilometers.
  • Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers.
  • User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters.
  • Receiver errors from software or hardware failures can cause blunder errors of any size.

• GPS ranging errors are magnified by the range vector differences between the receiver and the SVs.
  • Poor GDOP, a large value representing a small unit vector-volume, results when angles from receiver to the set of SVs used are similar.
    • Figure 13. Poor GDOP
  • Good GDOP, a small value representing a large unit vector-volume, results when angles from receiver to SVs are different.
    • Figure 14. Good GDOP

• GDOP is computed from the geometric relationships between the receiver position and the positions of the satellites the receiver is using for navigation.

• GDOP Components:
  • PDOP - Position Dilution of Precision (3-D)
  • HDOP - Horizontal Dilution of Precision (Latitude, Longitude)
  • VDOP - Vertical Dilution of Precision (Height)
  • TDOP - Time Dilution of Precision (Time)

• While each of these GDOP terms can be individually computed, they are formed from covariances and so are not independent of each other.
  • A high TDOP, for example, will cause receiver clock errors which will eventually result in increased position errors.

• 2.4. Satellite Visibility
  • GPS satellite signals are blocked by most materials. GPS signals will not mass through buildings, metal, mountains, or trees. Leaves and jungle canopy can attenuate GPS signals so that they become unusable.
  • In locations where at least four satellite signals with good geometry cannot be tracked with sufficient accuracy, GPS is unusable.
  • Planning software may indicate that a location will have good GDOP over a particular period, but terrain, building, or other obstructions may prevent tracking of the required SVs.
    • Figure 15. Good predicted GDOP,  Poor Visibility

  2.5. Differential GPS (DGPS) Techniques

  • The idea behind all differential positioning is to correct bias errors  at one location with measured bias errors at a known position.
  • A reference receiver, or base station, computes corrections for each satellite signal for all satellites in view.
  • DGPS receivers require software that can apply individual pseudo-range corrections for each SV prior to computing a position solution.
  2.5.1. Differential Code-Phase GPS (Navigation)
  • Differential corrections may be used in real-time or later, with post-processing techniques.
    • Real-time corrections can be transmitted by radio link.
    • The U. S. Coast Guard transmits DGPS corrections over radiobeacons covering much of the U. S. coastline.
    • Private companies broadcast corrections by ground-based FM-radio signals or satellite radio links.
    • Corrections can be recorded for post processing.
    • Many public and private agencies record DGPS corrections for distribution by electronic means.
  • To remove Selective Availability (and other bias errors), differential corrections should be computed at the reference station and applied at the remote receiver at an update rate of five to ten seconds, fast enough to keep up with the rapid changes in the SA bias.
    • Figure 16. Differential Code-Phase Navigation
    • Figure 17. Errors Reduced by Differential Corrections
  • DGPS is not able to eliminate all sources of error discussed in the next section.
  • Bias errors are less common at great distance from the reference receiver.
  • 300 to 500 km are considered reasonable reference-remote separations for Code-Phase DGPS.
  2.5.2. Differential Carrier-Phase GPS (Surveying)
  • Positions can also be calculated by tracking the carrier-phase signal transmitted by the SVs
    • Figure 18. Carrier Phase Tracking
  • All carrier-phase tracking is differential, requiring both a reference and remote receiver tracking carrier phases at the same time.
  • In order to correctly estimate the number of carrier wavelengths at the reference and remote receivers, they must be close enough to insure that the ionospheric delay difference is less than a carrier wavelength.
  • This usually means that carrier-phase GPS measurements must be taken with a remote and reference station within about 30 kilometers of each other.
  • Using L1-L2 ionospheric measurements and long measurement averaging periods, relative positions of fixed sites can be determined over baselines of hundreds of kilometers.
  • Special software is required to process carrier-phase differential measurements.
  • Carrier-phase tracking of GPS signals has resulted in a revolution in land surveying.
  • A line of sight along the ground is no longer necessary for precise positioning.
  • Positions can be measured up to 30 km from reference point without intermediate points.
  • This use of GPS requires specially equipped carrier tracking receivers.
    • Figure 19. Differential Carrier-Phase Positioning
  • Post processed static carrier-phase surveying can provide 1-5 cm relative positioning within 30 km of the reference receiver with measurement time of 15 minutes for short baselines (10 km) and one hour for long baselines (30 km).
  • Rapid static or fast static surveying can provide 4-10 cm accuracies with 1 kilometer baselines and 15 minutes of recording time.
  • Real-Time-Kinematic (RTK) surveying techniques can provide centimeter measurements in real time over 10 km baselines tracking five or more satellites and real-time radio links between the reference and remote receivers.

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  3. GPS Project Costs

  • Receiver costs vary depending on capabilities.
    • Small civil SPS receivers can be purchased for under $200.
      • Most output NMEA sentences with position information for use with computer serial ports.
      • Many can accept DGPS corrections from real-time sources.
    • Receivers that can store files for post-processing with base station files cost more ($2000 to $5000).
    • Receivers that can act as DGPS reference receivers and carrier phase tracking receivers (and two are often required) can cost many thousands of dollars ($5,000 to $40,000).
    • RTK systems require two receivers and radio links and may cost $60,000.
    • Military PPS receivers may cost more or be difficult to obtain.
  • Other costs include the cost of multiple receivers when needed, post-processing software, and the cost of specially trained personnel.
  • Project tasks can often be categorized by required accuracies which will determine equipment cost.
    • Low-cost, single receiver SPS projects (100 meter accuracy)
    • Medium-cost, differential SPS code Positioning (1-10 meter accuracy)
    • High-cost, single receiver PPS projects (20 meter accuracy)
    • High-cost, differential carrier phase surveys (1 mm to 1 cm accuracy)
    • High-cost, Real-Time-Kinematic (1 cm) with real time accuracy indications
    • Figure 20. GPS Applications, Costs, and Signals

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  4. Review and Study Questions

  4.1. Essay and Short Answer Questions

  • Essay Questions
    • Will GPS technology really make much difference to most GIS applications?
    • What GIS applications can make the best use of GPS technology? Which application will be affected the least?
    • To what extent is the problem of georeferencing a major obstacle to the creation of global GIS?
  • Short Answer
    • No matter how inexpensive and wide-spread GPS technology becomes, why will it not entirely solve the problem of creating precise and accurate GIS datasets?

  4.2. Multiple-choice questions

  Choose the best or most appropriate answer(s) to the question.

  • What is selective availability?
    • 1. The limited window of time during which GPS signals are within line-of-sight of a receiving antenna.
    • 2. The intentional degradation of GPS signals is to deny full access to unauthorized users.
    • 3. The Department of Defense classification of GPS users with access to the encrypted satellite signal.
  • What is differential GPS?
    • 1. A method for correcting GPS measurements by comparing bias errors between a known location and the position of a "roving" GPS receiver.
    • 2. The variance between Code and Carrier Phase GPS positioning.
    • 3. Multipath or imaging problems that cause position errors in a GPS code-tracking receiver.
    • 4. The design variations between the US (GPS) and the Russian (GLONASS) satellite positioning systems.
  • High accuracy, survey quality GPS is usually associated with:
    • 1. Differential code phase tracking.
    • 2. Low-cost GPS.
    • 3. Differential carrier phase tracking.
    • 4. No post-processing software.
  • The latitude, longitude, and altitude displayed by a GPS receiver represent:
    • 1. An estimate of the receiver's antenna position.
    • 2. The height above mean sea level.
    • 3. The three-dimensional position fix with millimeter accuracy.
    • 4. The height above the reference ellipsoid.

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  5. Reference Materials

  5.1. Print References

  • Global Positioning System Standard Positioning Service Specification, 2nd Edition, June 2, 1995, available on line from United States Coast Guard Navigation Center. This is a primary source for details about GPS C/A code implementation.
  • GPS Joint Program Office. 1997. ICD-GPS-200C: GPS Interface Control Document. ARINC Research. This document contains additional information about GPS P-code implementation.
  • Hoffmann-Wellenhof, B. H. Lichtenegger, and J. Collins. 1994. GPS: Theory and Practice. 3rd ed. New York: Springer-Verlag. A review of GPS for the surveyor. Chapters on planning GPS surveys.
  • Institute of Navigation. 1980, 1984, 1986, 1993, 1998. Global Positioning System monographs. Washington, DC: The Institute of Navigation. A complete set of papers in five volumes that cover all aspects of GPS receiver design, GPS applications, and DGPS specifications.
  • Kaplan, Elliott D. ed. 1996. Understanding GPS: Principles and Applications. Boston: Artech House Publishers. A complete description of GPS and GPS applications. A good choice for a single GPS reference work.
  • Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New York: John Wiley & Sons. This book covers GPS surveying in detail including network error analysis and carrier phase details.
  • Parkinson, Bradford W. and James J. Spilker. eds. 1996. Global Positioning System: Theory and Practice. Volumes I and II. Washington, DC: American Institute of Aeronautics and Astronautics, Inc. A very complete set of papers spanning all of the GPS literature. Full of engineering details as well as application specific chapters.
  • Wells, David, ed. 1989. Guide to GPS positioning. Fredericton, NB, Canada: Canadian GPS Associates. One of the first overviews of GPS positioning techniques. Still useful as a guide to surveying theory.

  5.2. Web References

  5.2.1. US Federal Agencies

  • United States Coast Guard Navigation Center
  • NOAA National Geodetic Survey Division Home Page

  5.2.2. University-Related Pages

  • Sam Wormley's GPS Page
  • Alfred Leick's GPS Page
  • University NAVSTAR Consortium (UNAVCO)

5.2.3. Other Relevant Web Pages

• The Institute of Navigation
• The following are related sections by the same author in The Geographer's Craft Project at the University of Texas Austin:
  • Dana, Peter H. 1995. Global Positioning System Overview, http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
  • Dana, Peter H. 1995. Geodetic Datum Overview, http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html
  • Dana, Peter H. 1995. Coordinate Systems Overview, http://www.colorado.edu/geography/gcraft/notes/coordsys/coordsys_f.html
  • Dana, Peter H. 1995. Map Projections Overview, http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html

Evaluation

Citation

Dana, Peter H. (1997) Global Positioning System Overview, NCGIA Core Curriculum in GIScience, http://www.ncgia.ucsb.edu/giscc/units/u017/u017.html, posted August 28, 1997.