Global Positioning Systems (GPS)
GPS Surveying :
The Global Positioning System (GPS) is a satellite-based navigation and surveying system for the determination of precise position and time, using radio signals from the satellites, in realtime or in post-processing mode. GPS is being used all over the world for numerous navigational and positioning applications, including navigation on land, in air and on the sea, determining the precise coordinates of important geographical features as an essential input to mapping and Geographical Information System (GIS), along with its use for precise cadastral surveys, vehicle guidance in cities and on highways using GPS-GIS integrated systems, earthquake and landslide monitoring, etc. In India also, GPS is being used for numerous applications in diverse fields like aircraft and ship navigation, surveying, geodetic control networks, crustal deformation studies, cadastral surveys, creation of GIS databases, time service, etc., by various organizations.
GPS is primarily a navigation system for real-time positioning. However, with the transformation from the ground-to-ground survey measurements to ground-to-space measurements made possibly by GPS, this technique overcomes the numerous limitations of terrestrial surveying methods, like the requirement of intervisibility of survey stations, dependability on weather, difficulties in night observations, etc.. These advantages over the conventional techniques and the economy of operations make GPS the most promising surveying technique of the future. With the well-established high accuracy achievable with GPS in the positioning of points separated by a few hundreds of meters to hundreds of km, this unique surveying technique has found important applications in diverse fields.
Errors and uncertainty:
Positions are the products of measurements. All measurements contain some degree of error. Errors are introduced in the original act of measuring locations on the Earth's surface. Errors are also introduced when second- and third-generation data is produced, say, by scanning or digitizing a paper map. In general, there are three sources of error in measurement: human beings, the environment in which they work, and the measurement instruments they use.
- Human errors include mistakes, such as reading an instrument incorrectly, and judgments. Judgment becomes a factor when the phenomenon that is being measured is not directly observable (like an aquifer), or has ambiguous boundaries (like a soil unit).
- Environmental characteristics, such as variations in temperature, gravity, and magnetic declination, also result in measurement errors.
- Instrument errors follow from the fact that space is continuous. There is no limit to how precisely a position can be specified. Measurements, however, can be only so precise. No matter what instrument, there is always a limit to how small a difference is detectable. That limit is called resolution.
SURVEYING WITH GPS
Within the span of a few years of its operation, GPS has truly revolutionized the field of surveying, with its potential to replace many conventional surveying techniques in use today. The different methods of surveying with GPS will be briefly described here, along with a review of GPS instrumentation and method of computation of geodetic and map coordinates from the GPS observations. 3.1 Methods of Observations The different methods of observations with GPS include absolute positioning, relative positioning in translocation mode, relative positioning using differential GPS technique, and kinematic GPS surveying technique. 3.1.1 Absolute Positioning In the absolute positioning model, the absolute coordinates of the antenna position (centered over the survey station) are determined using a single GPS receiver, by a method similar to the resection method used in the plane tabling. The pseudo ranges (the satellite-antenna range, contaminated by the receiver clock bias) from a minimum of four satellites are observed at the given epoch, from which the four unknown parameters - the 3-D position of the antenna (x, y, z) and the receiver clock error can be determined. The accuracy of the position obtained from this method depends upon the accuracy of the time and position messages received from the satellites. With the selective availability operational, the accuracy of absolute positioning in real-time was limited to about 100 meters, which has now improved to about 10 to 20 meters, since the SA is switched-off. This can be further improved to a few centimeters level by using post-processed satellite orbit information in the post-processing mode. The accuracy of absolute positioning with GPS is limited mainly due to the high orbit of the satellites. However, very few applications require absolute position in real-time
Relative Positioning: In the translocation mode, with two or more GPS receivers observing the same satellites simultaneously, many common errors, including the major effect of SA, get canceled out, yielding the relative positions of the two or more observing stations to a very high level of accuracy. The length of the baseline between two stations, and also the absolute position of one of the stations, if the accurate position of the other station is known, can be obtained to cm level accuracy, using carrier phase observations. In a differencing mode of observations, using single difference (difference of carrier phase observations from two receivers to the same satellite), a double difference (between observations from two receivers to two satellites), and triple difference (difference of double differences over two-time epochs), the effect of any errors such as receiver and satellite clock errors, etc., can be minimized. The use of dual-frequency observations (both L1 and L2 frequencies) eliminates the major part of the ionosphere effect on the signal, thus improving the accuracy of positioning. With accurate satellite orbit information and the use of such refined data-processing and modeling techniques, few mm to cm level accuracy is possible even in regional or global scale surveys.
Differential GPS: A modification of the relative positioning method is the differential GPS (DGPS) technique, where one of the two receivers observing simultaneously is equipped with a transmitter and another receiver (s) can receive the messages given by this transmitter. The transmitting receiver is kept fixed on a point whose location is known to a high degree of accuracy. Based upon this position, the receiver computes corrections to the range/phase observations from a GPS satellite and transmits them to the other receiver, which can apply these corrections to improve the accuracy of its own position computed from GPS observations. Such a system is suited for applications such as vehicle guidance system, locating fishing boats close to the seashore, etc. The limited range of the transmitter restricts the use of such a system to a few km.
Kinematic GPS: In the Kinematic GPS technique, one of the receivers is in relative motion with respect to the other receiver, having been mounted either on a vehicle, ship or aircraft. Even with the difficulties encountered in obtaining the constantly changing position of the moving receiver, the method also offers some advantages over static surveying, including the ease with which the ambiguity resolution (estimating the whole number of wavelengths in the phase observable) can be done. This technique has a number of important applications, including ship and aircraft navigation, photogrammetric survey control, etc.
GPS Segments The Global Positioning System basically consists of three segments:
- Space Segment
- Control Segment
- User Segment.