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The Global Positioning System (GPS) is a worldwide, all weather, 24 hr navigation system developed and operated by the US Department of Defense. The system was designed to replace the TRANSIT system, which had several shortcomings, including large time gaps in coverage and low accuracy.
The GPS, officially designated NAVigation System with Timing And Ranging (NAVSTAR), is, as its name states, designed to provide navigation (location, velocity) as well as time. It consists of (nominally) 24 satellites in six orbits at an altitude of about 20,000 km. Since orbital period is directly proportional to the semimajor (i.e. larger) radius of the orbital ellipse, this gives the satellites a period of 11h 58m. Since the earth rotates half way around in 12 hours, the satellite constellation appears to repeat itself every 23h 56m, which is equivalent to 1 sidereal day. This also means that the satellite constellation advances by 2h per month, 24h per year. With the full constellation in place, this is not nearly as important as when a lesser number of satellites were present.
The basic concept of the system is the measurement of ranges from the orbiting satellites. Each satellite broadcasts a signal which contains ephemeris (i.e. where it is) information and a timing signal, among other things. By using four satellites to solve four unknowns (X, Y, and Z, as well as T, the receiver time offset), a position can be computed. The position is in the same coordinate system and datum as the satellites, namely Earth Centered Earth Fixed (ECEF) WGS84. This can be transformed, if needed, to other systems, such as latitude and longitude on NAD83 or NAD27, which is still found on many quadrangles in the US. The National Imagery and Mapping Agency (NIMA) of the US Government has a program available called MADTRAN, which computes datum transformations. The software can be downloaded from NIMA Geodesy and Geophysics page. This software is useful for areas outside of the United States. For use in the US, a more accurate method is to use NADCON, available from the National Geodetic Survey. Whereas MADTRAN uses Molodenski's formulas, NADCON contains a database of datum shifts between NAD 1983 and NAD1927 (also between NAD 1983 and NAD 1983 19XX for states with HARN networks). The autonomous accuracy of a single receiver (called point positioning) is about 100 meters horizontal and 150 meters vertically. This degraded accuracy is a result of Selective Availability (SA), which is a purposeful clock dithering and ephemeris manipulation, supposedly to prevent enemy forces from using the full accuracy of the system (15 m). As will be discussed below, Differential GPS (DGPS) renders this degradation totally ineffective (if you are a US citizen, write your elected representatives to get rid of SA completely!).
Some of the larger error sources present in single point positioning are SA, ephemeris errors, satellite clock errors, and tropospheric and ionospheric delays. Note that these errors are common (atmospheric less so as the distance increases) to receivers on the ground in a small area (i.e. several 100 km's). By placing a receiver at a point of known position, the observed ranges can be subtracted from the computed ranges to provide a correction, which is then broadcast over a radio link to be used by remote receivers. This is known as DGPS. In the United States, the US Coast Guard has established a series of DGPS stations covering the coastline, and other agencies are working on covering the inland areas as well. There are also several commercial providers of DGPS corrections, such as Accqpoint, which transmits corrections on FM subcarrier that can be picked up on small FM receivers and pagers, and OmniSTAR,which uses Inmarsat geosynchronous satellites to provide worldwide coverage.
The accuracy of the navigation solution after using the USCG corrections vary depending on the type of receiver used. For example, an inexpensive navigation only receiver might have a DGPS accuracy of 10 meters, while a survey grade receiver might have a DGPS accuracy of better than 1 meter. Many Geographic Information System (GIS) users utilize GPS to collect entities and attribute data using DGPS. For example, a power company may have a base map prepared by a photogrammetric company. The utility company then goes around to each pole in its system, capturing location info, number of circuits, serial numbers, etc. This data is then loaded into the GIS based on the location as determined by the DGPS.
One of the first commercial uses of GPS occurred back in the mid 1980's when carrier phase GPS (interferometry) was proposed as a survey tool by Charles Counselman of MIT. The idea was to be able to measure very accurately a vector between two antennas. The original receivers for this method treated the GPS signal as noise, and merely measured the phase of the signal. The receivers (Macrometer V 1000) cost around $250,000 each, and occupied the back of a jeep. The internal oscillator on the receivers had to be synchronized using an atomic clock or a GOES satellite receiver before and after each day' work. Because of limited satellites available at that time, only one or two sessions per day could be measured. So, the operation of these receivers was a costly endeavor. Even so, it was a breakthrough in the field of surveying, because no longer was it necessary to have intervisibility between stations. All modern survey receivers now use the broadcast ephemeris, therefore eliminating the need for an external time reference. A single frequency survey grade receiver can now be obtained for $5,000-$15,000, depending on features available. Dual frequency receivers are available which use the GPS L2 frequency to measure longer lines, or, more common, to measure short lines in a shorter time period.
GPS surveying has become a common tool for today's surveyors. The key to using GPS for surveying is to determine the integer number of wavelengths from the satellite to the antenna. The receiver can accurately measure the fractional part of the 19 cm L1 wavelength (to about 1%), but the number of wavelengths present when the receiver started tracking is unknown. By having a change in geometry (i.e. over time) of the satellites, or by using dual frequency receivers, these integer biases can be determined. There are several variations, including:
- static - this uses two or more receivers, with antennas setup on tripods and collecting data simultaneously for a period of 30 minutes to several hours. Long lines (500-1000 km) can be measured accurately with dual frequency receivers, precise ephemeris, and longer data sets, 6-24 hours)
- fast static - by taking advantage of the L2 frequency, the processing software can resolve the integer biases in as little as a minute, although most users wait for a minimum of 8 or more minutes. This method is applicable for baseline separations up to about 15 km.
- kinematic - once the integer biases are known, the antenna can be moved as long as at least four satellites are continuously tracked. Stop and Go kinematic is when discrete points are occupied where coordinates are desired. Continuous kinematic means that coordinates are being provided for each epoch (typically 1,5 or 15 seconds), regardless of whether the receiver is moving or stationary. This latter method is used in airborne GPS controlled photogrammetry. An antenna is installed above a camera mounted in an aircraft. While the photo mission is being flown, the GPS is providing a position of the antenna every second. This is interpolated for the instant of exposure, enabling a large reduction in the amount of ground control required. Real Time Kinematic (RTK) uses a telemetry link (i.e. VHF or UHF radio) to broadcast data collected at the base receiver. This is then processed at the roving receiver to provide accurate (centimeter level) coordinates in near real time (latency of 0.1 to 2 seconds). This can be used, for example, for stakeout.
- The above discussion, while very brief, is meant to explain the basic types of GPS. The links below are to the home pages of some of the major US manufacturers of survey grade GPS receivers:
- Allen Osborne Associates, Inc.
- Ashtech, Inc
- Leica Geosystems AB GPS
- Spectra Precision AB
- Trimble Navigation Limited
