Why GPS is in your future
Mark A. Jones ASI 11900 Crownpoint Drive, Suite 100 San Antonio, TX 78233
Origins, changes and current state of the GPS System
Origins & Changes The origins of the present-day NAVSTAR [Navigation Signal Timing and Ranging] GPS system go back to the launch of the Sputnik satellite in 1957. By studying the orbit of the tiny satellite, scientists found that it could be tracked by its radio signal. Derived from this was the opportunity for a person on earth to determine his or her position on the globe by reading the signal from the satellite if the precise orbit of the satellite was known. (Taylor, 2000) The MINITRACK system, which was developed in the late 1950sfor the U. S. Navy’s Naval Research Laboratory’s [NRL] Vanguard Satellite Program, used the signals emitted by Sputnik and later satellites to determine their positions and orbits. This evolved into tracking non-radiating satellites by signals reflected off of them. (U.S. Navy NRL, 2000) The U. S. Navy’s NRL Naval Center for Space Technology (NCST) created the TIMATION (TIMe/navigATION) program in 1964. This program was designed to provide the basis for a navigation system with three-dimensional coverage (longitude, latitude, and altitude) throughout the world. In 1967 the Navy launched the TIMATION-I satellite to lay the foundation for the navigation system. (U.S. Navy NRL, 2000) The presence of an onboard crystal oscillator clock would allow the user to determine the length of time it took each signal to travel from the satellite to the receiver. The user then knew the distance to the satellite. The TIMATION I was followed in 1969 by the TIMATION-II satellite launch. (Beadles, 1998) In 1973 the TIMATION program was merged with the Air Force's 621B program to form the NAVSTAR GPS program. The NRL’s TIMATION III satellite was re-designated as the Navigation Technology Satellite 1 (NTS-1) and was launched in the summer of 1974 in connection with the NAVSTAR effort. NTS-1 had onboard two atomic clocks, while the earlier TIMATION satellites had carried crystal oscillator clocks to test the NRL theory. (U.S. Navy NRL, 2000) Another factor spurring the evolution of the satellite-based navigation system is found in the communication problems U.S. military personnel experienced during the Vietnam conflict. During that period, a system known as LORAN was utilized for locational information. The LORAN system was prone to problems inherent in radio communication such as poor reception during ominous weather or during the night. The U.S. Department of Defense responded to this problem by investing into a system known as the Navy Navigational Satellite System, TRANSIT, which most recently evolved into GPS. (Oguneye, 2000) Although the TRANSIT system was a significant leap forward in GPS technology, it suffered from poor accuracy and was often inefficient. In the early 70's, the United States military began a program that would later be known as the NAVSTAR GPS program. (Taylor, 2000) Research and development work on the NAVSTAR system began in 1973. In April, the U.S. Navy TIMATION system and Air Force System 621B 3-d navigation system were combined in a joint effort to develop a Defense Navigation Satellite System (DNSS). This combined system would later become NAVSTAR. (Beadles, 1998) Unlike earlier systems that had one satellite, the NAVSTAR system would offer several satellites for navigation. Availability of positioning data would be increased, and moving vehicles would not have to wait for extended periods of time to get the necessary readings. Originally named the Navigation Technology Program [NTP], the first four satellites of the NAVSTAR constellation were launched in 1978. (Taylor, 2000) On July 17, 1995, the U.S. Air Force issued a press release announcing that the Global Positioning System satellite constellation had met all requirements for Full Operational Capability. Current State The NAVSTAR GPS system today consists of 24 satellites in six orbits. The satellites are spaced in orbit so that at any time a minimum of 6 satellites will be in view to users anywhere in the world. (US Coast Guard, FAQ page, 2000) There are currently four generations of GPS satellites: Block I, Block II/IIA, Block IIR and Block IIF. The Block I satellites were used for testing the principles of the system. Block II and IIA satellites make up the current constellation. The third generation Block IIR satellites are currently being deployed as the Block II/IIA satellites reach their end-of- life and are retired. Block IIF satellites will be the fourth generation of satellites and will be used for operations and maintenance replacement. (Andrews Space & Technology, 2000). Uses, Strengths, Weaknesses and benefits of employing GPS Uses Uses for GPS range from purely recreational [hiking, backpacking, biking, “geocaching”] to highly complex and detail-oriented tasks [military, navigation, surveying, construction, mapping, agriculture, vehicle tracking]. Equipped with a GPS receiver, a user can utilize any point as a reference to get to another point. Applications for GPS are limited by the imagination of its users. Strengths GPS signals are free to all. This means that no cost locational/navigational data is available to anyone with a receiver. GPS availability is worldwide, 24-hours per day, 365 days per year. Additionally, low entry cost means that suitable user equipment can be relatively inexpensive, dependent upon accuracy requirements Weaknesses GPS may not work well without a relatively unobstructed view of the sky. For example, tree canopies may interfere with good reception of the GPS signals thereby preventing adequate signal capture at the receiver. GPS may be subjected to intentional accuracy degradation by implementation of measures by the U. S. Government. By reactivating selective availability, which will be discussed in the next section, the accuracy levels achieved through the use of consumer-grade GPS equipment will be on the order of ± 100 meters. Benefits Today’s GPS receivers are lightweight and compact thereby allowing ease of transport. GPS permits rapid location determination and provides the ability to quickly relocate specific geographic points. Repeatable locational data collection with minimal equipment set-up time required provides cost-reductions by allowing the user to concentrate on obtaining usable data and move on to other points requiring observation. Elimination of Selective Availability (SA) To understand the impact of the elimination of Selective Availability upon the users of the NAVSTAR GPS system, a brief introduction into how the system works is needed. Please note that the following does not take into account the use of differential GPS [DGPS] techniques; differential GPS is a process by which errors in accuracy are minimized by the use of reference data to correct for anomalies in the data collected in the field. The NAVSTAR GPS system consists of three segments: space, control, and user. The Space Segment consists of 24 operational satellites in six circular orbits 10,900 nautical miles above the earth. The satellites continuously broadcast position and time data to users all over the world. The Control Segment consists of a master control station in Colorado Springs, with five monitoring stations and three ground antennas located around the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The User Segment consists of the receivers and related equipment that allow users to receive the GPS satellite broadcasts and determine their position, velocity and/or time. There are two types of user groups: military or other governmental users and civilian users. Different signals and different levels of accuracy are available to each group as determined by the receiving equipment utilized. Each GPS satellite transmits a highly accurate time signal. Distances from each of the satellites are calculated by determining the differences between the time a signal is transmitted from a satellite and the time it is received. This difference is a measure of the apparent range to the satellite. By utilizing signals from three or more satellites and triangulation, a GPS receiver can determine its location on the surface of the earth. It is within the relationship between the space and user segments that a number of factors affecting the accuracy of the locational solution arise. Some of the most important are: Selective availability [SA]: Selective availability was implemented to intentionally degrade the quality of the GPS signals available to civilian users. A primary intent behind the implementation of SA was the belief that the signals could be easily used by enemy states against the US. The potential accuracy of the system of approximately 20-30 meters is reduced to 100 meters by the presence of SA. (Dana, 1999) Space vehicle clock errors: Each NAVSTAR GPS satellite contains a number of atomic clocks that control broadcast signal generation. The clocks are highly stable but may deviate from GPS time. This deviation is known as satellite clock error. (Kaplan, 1996) Tropospheric delays: The troposphere is the lower part of the atmosphere (ground level to 8 to 13 km) that experiences the changes in temperature, pressure, and humidity associated with weather changes. These changes may affect the propagation of the signals. (Dana, 1999) Ionospheric delays: The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. Changes in the ionosphere may also affect the propagation of signals. (Dana, 1999) Multipath: Multipath is caused by the reflection of 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. (Dana, 1999) Ephemeris data errors: The ephemeris is a prediction of the locations of the satellites upon which the GPS receiver depends for accurate locational determination on the ground.
Sources: http://www.racal-landstar-usa.com/ppt1/sld002.htm http://www.montana.edu/places/gps/1Basic/slide25.html As may be seen above, the most significant sources of errors in the GPS system is that of Selective Availability. With the May 2000 elimination of SA, the single largest source of errors has been removed, so that average accuracies now attainable may be within a ±10- 20 meter range or better. Given that the user’s accuracy requirements fall within this range, even relatively inexpensive consumer-grade GPS receivers may be found suitable for use. For example, if a user wishes to repeatedly visit a given site in a changing landscape where landmarks may not be consistently relied upon, the use of GPS permits the user to navigate back to the area of interest within a relatively small distance. This can be done today with widely available receivers for less than $400. Elimination of SA is but one of the reasons GPS is in your future. Announced by Vice President Al Gore in a press release dated January 25, 1999, a program is now underway to modernize the current GPS system through the transmission of additional data on existing frequencies as well the addition of new frequencies for data transmission. This GPS modernization program will result in significant improvements in GPS positioning accuracy for civilian users with off-the-shelf receivers. This new capability will have a positive effect upon the development of new applications for GPS and help expand the market for GPS equipment and services worldwide. The first of these new signals will be available for use in non-safety-related applications such as automotive navigation, boating and fishing as well as other recreational applications. This will be available beginning with the initial GPS Block IIR satellites scheduled for launch in 2003. (NTIA, 2000) The other signal will be available on GPS Block IIF satellites scheduled for launch beginning in 2005. This new signal, known as L5, falls in a band that is protected worldwide for aeronautical radionavigation, and therefore will be protected for safety-of-life applications. Without the modification of existing systems, the addition of L5 will make GPS a more valuable service for many aviation applications, as well as all ground-based users. (NTIA, 2000) At the current rate of GPS satellite replacement, the new signals will be available for initial operational capability by 2010, and for full operational capability by approximately 2013. (NTIA, 2000) In relationship to spatial data derived from remote-sensing systems, the use of GPS-based data will be dependent upon the overall levels of accuracy required by your project. Current levels of accuracy achievable with consumer-grade receivers preclude their use in a critical manner with more accurate data derived from remote-sensed sources. With the introduction of the two new frequencies mentioned above, however, these types of receivers may be more useful in that the levels of accuracy they will be capable of producing will be more in line with that from remotely-sensed sources. The Aerospace Corporation, 1999 http://www.aero.org/publications/GPSPRIMER/Satellites.html Andrews Space & Technology, 2000 http://www.spaceandtech.com/spacedata/constellations/navstar-gps_consum.shtml Beadles, John T., 1998, Introduction to GPS Applications jbeadles@pobox.com http://ares.redsword.com/gps/apps/index.htm Dana, Peter H., 1994, Department of Geography, University of Texas at Austin http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html Dana, Peter H., 1999, Global Positioning System Overview, http://www.colorado.Edu/geography/gcraft/notes/gps/gps.html#SA Kaplan, Elliot D., 1996, Understanding GPS - Principles and Applications Montana State University Web Site, 2000 http://www.montana.edu/places/gps/1Basic/slide21.html National Telecommunications and Information Administration, 2000 http://pswac.ntia.doc.gov/gps/gpsmodern.htm Oguneye, Kunbi, 2000 http://www.geocities.com/WallStreet/Floor/8106/History.htm Taylor, Robert, 2000 http://www.mapfacts.com/gps/history.asp U.S. Coast Guard Navigation Center Web Site, 2000 http://www.navcen.uscg.mil/gps/geninfo/global.htm http://www.navcen.uscg.mil/faq/gpsfaq.htm U.S. Navy, Naval Research Laboratory [NRL], 2000 http://code8200.nrl.navy.mil/nts.html U.S. Coast Guard, 2000 http://www.navcen.uscg.mil/faq/gpsfaq.htm | ||||||||||||||||
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