| GISdevelopment.net ---> Asian GPS --->Proceedings --->2002 ---> GPS: Emerging Trends |
GPS technology and its applications
Sudhir Mathur
Area Coordinator, Central Road Research Institute, New Delhi
Pankaj Gupta, Neelam Jain
Scientists, Central Road research Institute, New Delhi
Sudhir Mathur
Area Coordinator, Central Road Research Institute, New Delhi
Pankaj Gupta, Neelam Jain
Scientists, Central Road research Institute, New Delhi
Abstract
Global Positioning System, GPS based on a constellation of 24 satellites orbiting the earth at a very high altitude. In other words GPS is a space based all whether radio navigation system. It broadcasts precise, synchronized timing signals to provide precise estimates of position, velocity, and time. It offers advantages of accuracy, speed, versatility and economy while in use as an aid for position-based data collection. Thus, a great technological revolution is taking place at the development of GPS as it can be used in any conceivable problem under the sky, where the exact position of any object or phenomena involved. The paper describes the applications, GPS architecture, basic principle of GPS and methods of observations.
Introduction
The global Positioning System (GPS) is a modern technology, which provides precise positioning of points (x, y and z) for navigation, surveying and Geographic Information System (GIS) data capture. Precise navigation based on the global positioning (GPS) has becomes a valuable auxiliary to photogrammetry, gravimetric, and topographic mapping. Accurate navigation is important when surveys are repeated to follow small changes in terrain height due to subsidence, or to track the flow of ice in a glacier. Initially, GPS were developed and use for military application only. Since the release to the civilian sector, global positioning systems have rapidly grown for ship and aircraft navigation as well as precise surveying and geological studies. With GPS, rates and relative motion of continental drift have been measured in centimeters per year as well as real time tracking of hazardous or sensitive material shipments.
The GPS provides continuous three-dimensional positioning 24 hours a day throughout the world. The technology seems to be beneficiary to the GPS user community in terms of obtaining accurate data up to about 100 meters for navigation, meter level for mapping, and down to millimeter level for geodetic positioning. The GPS technology has tremendous amount of applications in GIS data collection, surveying and mapping.
GPS Applications
GPS application is limitless in today’s scenario. GPS receivers are fast becoming small and cheap enough to be carried by any one. One of the most significant and unique features of the GPS is the fact that the positioning signal is available to users in any position worldwide at any time. With a fully operational GPS system, it can be generated to a large community of likely to grow as there are multiple applications, ranging from surveying, mapping, and navigation to GIS data capture. The GPS will soon be a part of the overall utility of technology. Few important GPS applications such as surveying & mapping, navigation, geodesy, military, remote sensing & GIS etc are as follows:
Navigation using GPS can save lot of time in the field. Any feature, even if it is under water, can be located up to one hundred meters simply by scaling coordinates from a map, entering way points, and going directly to the site. Examples include road intersections, corner posts, plot canters, accident sites, geological formations, and so on. GPS navigation in helicopters, in vehicles, or in a ship can provide an easy means of navigation with substantial savings.
Geodetic mapping and other control surveys can be carried out effectively using high grade GPS equipment. Especially when helicopters were used or when the line of sight is not possible, GPS can set new standards of accuracy and productivity.
The GPS was primarily developed for real time military positioning. Military applications include airborne, marine, and land navigation. Even GPS can be used to guide the missile and bombs to pin point attack on the enemy targets.
It is also possible to integrate GPS positioning into remote-sensing methods such as photogrammetry and aerial scanning, magnetometry, and video technology. Using DGPS or kinematic techniques, depending upon the accuracy required, real time or post-processing will provide positions for the sensor which can e projected to the ground, instead of having ground control projected to an image. GPS are becoming very effective tools for GIS data capture. The GIS user community benefits from the use of GPS for locational data capture in various GIS applications. The GPS data can easily be down loaded to a laptop computers in field it self by using any suitable software. This data can be available on the common base for their other purposes. Thus GPS can help in several aspects of construction of accurate and timely GIS databases.
GIS is a powerful tool for interpreting, analyzing, storing, displaying and retrieval of information collected from maps, Ground survey, GPS, aerial photographs, satellite images. It is used to locate the areas, which may be derived from the logical analysis or overlay analysis of two or more themes.
Use of GPS in GIS is particularly important and useful because of its capacity to provide both spatial and non-spatial attributes of themes. It handles data from diverse sources and forms links and interact connection between them. GIS can serve as a common platform and interface that permits data exchange and collaborate decisions. GIS allows the user to interact with the simulated environment and recreate the sensations that may be felt in interaction with the real world. Common applications of GIS include, as follows, where GPS can provide three-dimensional information about the features:
The system of GPS consists of three segments: (Fig.1) the space segment comprises the satellites: the control segment deals with the management of the operations of satellites and the user segment covers the activities related to GPS users. However, the system description, its components and mode of operations of different segments depend on the type of radio navigation system in use such as Navstar GPS, Glonass (Global Navigational Satellite System) etc.
The basic principle of determining the position by using GPS satellites is based on measurement of distances from the point of observations to the satellite. This is done by comparing the reading of transmitter antenna time with the receiver antenna time. It cannot be assumed that the two clocks will be strictly in synchronization since the clocks used in the present type of receivers are quartz clocks to reduce the cost of the receiver. The observed signal time will have a systematic synchronization error. Since the measured range has got this systematic error in it, the computed distances will also be biased, and therefore, these are called pseudo-range. To compute the position based on this pseudo-range, the error due to time bias has to be corrected and therefore, this is also taken as an unknown and determined before deriving the true range. As we know from the simple formulate of distance computation that
R = Ö ((X – Xi) 2 + (Y –Yi) 2 + (Z –Zi)) 2
Where X, Y & Z are the co-ordinate of the station, therefore unknown and Xi, Yi & Zi are the co-ordinates of the satellite, which is broadcast information.
To find the true range the time bias t is also has to be considering, thereforeR = ((X – Xi) 2 + (Y –Yi) 2 + (Z –Zi)) 2 + Tc
Where C is the velocity of light, R is pseudo-range and t is travel time.
Now in this equation, there are four unknown therefore, to solve this at least 4 satellites will have to be observed. The minimum requirement in this case is
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.
GPS is becoming a very popular and effective tool for the precise mapping and monitoring purpose. Many and many organizations are inducting GPS for the survey and other various purposes. Though, many factors affect the accuracy of the GPS survey e.g. number of satellite observed, observation time, surroundings at the place of observation, distance of each observation points etc. Accuracy also depends upon the interacted satellite’s geometry. The ultimate accuracy of a GPS determined by sum of several sources of error. The contribution of each source may vary depending on atmospheric and equipment conditions. Precision of observation such as landslide by the GPS depends on how above said errors can be eliminate of minimize. GPS cannot be used for the underground mapping as site of satellite is blocked. If such sites are falls in the continuous mapping, GPS survey can be switchover to electronic Total Station.
GPS industry is likely to continue to develop in the civilian community besides the specific use in defense. Now, GPS trend is continuing to be towards smaller, less expensive, and more easily operated devices for its wider use in the many aspects of community.
Acknowledgement
The paper is published with the kind approval of the Director of the Central Road Research Institute (CRRI), New Delhi. The guidance received from Prof. P. K. Sikdar, Director, CRRI is thankfully acknowledged.
References
Global Positioning System, GPS based on a constellation of 24 satellites orbiting the earth at a very high altitude. In other words GPS is a space based all whether radio navigation system. It broadcasts precise, synchronized timing signals to provide precise estimates of position, velocity, and time. It offers advantages of accuracy, speed, versatility and economy while in use as an aid for position-based data collection. Thus, a great technological revolution is taking place at the development of GPS as it can be used in any conceivable problem under the sky, where the exact position of any object or phenomena involved. The paper describes the applications, GPS architecture, basic principle of GPS and methods of observations.
Introduction
The global Positioning System (GPS) is a modern technology, which provides precise positioning of points (x, y and z) for navigation, surveying and Geographic Information System (GIS) data capture. Precise navigation based on the global positioning (GPS) has becomes a valuable auxiliary to photogrammetry, gravimetric, and topographic mapping. Accurate navigation is important when surveys are repeated to follow small changes in terrain height due to subsidence, or to track the flow of ice in a glacier. Initially, GPS were developed and use for military application only. Since the release to the civilian sector, global positioning systems have rapidly grown for ship and aircraft navigation as well as precise surveying and geological studies. With GPS, rates and relative motion of continental drift have been measured in centimeters per year as well as real time tracking of hazardous or sensitive material shipments.
The GPS provides continuous three-dimensional positioning 24 hours a day throughout the world. The technology seems to be beneficiary to the GPS user community in terms of obtaining accurate data up to about 100 meters for navigation, meter level for mapping, and down to millimeter level for geodetic positioning. The GPS technology has tremendous amount of applications in GIS data collection, surveying and mapping.
GPS Applications
GPS application is limitless in today’s scenario. GPS receivers are fast becoming small and cheap enough to be carried by any one. One of the most significant and unique features of the GPS is the fact that the positioning signal is available to users in any position worldwide at any time. With a fully operational GPS system, it can be generated to a large community of likely to grow as there are multiple applications, ranging from surveying, mapping, and navigation to GIS data capture. The GPS will soon be a part of the overall utility of technology. Few important GPS applications such as surveying & mapping, navigation, geodesy, military, remote sensing & GIS etc are as follows:
- Surveying and Mapping
- Navigation
- Geodesy
- Military
- Remote Sensing and GIS
Navigation using GPS can save lot of time in the field. Any feature, even if it is under water, can be located up to one hundred meters simply by scaling coordinates from a map, entering way points, and going directly to the site. Examples include road intersections, corner posts, plot canters, accident sites, geological formations, and so on. GPS navigation in helicopters, in vehicles, or in a ship can provide an easy means of navigation with substantial savings.
Geodetic mapping and other control surveys can be carried out effectively using high grade GPS equipment. Especially when helicopters were used or when the line of sight is not possible, GPS can set new standards of accuracy and productivity.
The GPS was primarily developed for real time military positioning. Military applications include airborne, marine, and land navigation. Even GPS can be used to guide the missile and bombs to pin point attack on the enemy targets.
It is also possible to integrate GPS positioning into remote-sensing methods such as photogrammetry and aerial scanning, magnetometry, and video technology. Using DGPS or kinematic techniques, depending upon the accuracy required, real time or post-processing will provide positions for the sensor which can e projected to the ground, instead of having ground control projected to an image. GPS are becoming very effective tools for GIS data capture. The GIS user community benefits from the use of GPS for locational data capture in various GIS applications. The GPS data can easily be down loaded to a laptop computers in field it self by using any suitable software. This data can be available on the common base for their other purposes. Thus GPS can help in several aspects of construction of accurate and timely GIS databases.
GIS is a powerful tool for interpreting, analyzing, storing, displaying and retrieval of information collected from maps, Ground survey, GPS, aerial photographs, satellite images. It is used to locate the areas, which may be derived from the logical analysis or overlay analysis of two or more themes.
Use of GPS in GIS is particularly important and useful because of its capacity to provide both spatial and non-spatial attributes of themes. It handles data from diverse sources and forms links and interact connection between them. GIS can serve as a common platform and interface that permits data exchange and collaborate decisions. GIS allows the user to interact with the simulated environment and recreate the sensations that may be felt in interaction with the real world. Common applications of GIS include, as follows, where GPS can provide three-dimensional information about the features:
- Engineering mapping
- Automated photogrammetry
- Sub-division design (cut/fill, street lay out, parcel layout)
- Cadastral mapping
- Highway mapping
- Utility/facility mapping and management
- Surface water mapping
- Event mapping (accidents, crime, fire, facility breakage, etc.)
- Census and related statistical mapping
- Watershed prioritization
- Land use planning and management
- Environmental impact studies
- E-business/E-commerce
- E-governance
- Mobile mapping/ WAP application
- 3D/4D GIS integration
The system of GPS consists of three segments: (Fig.1) the space segment comprises the satellites: the control segment deals with the management of the operations of satellites and the user segment covers the activities related to GPS users. However, the system description, its components and mode of operations of different segments depend on the type of radio navigation system in use such as Navstar GPS, Glonass (Global Navigational Satellite System) etc.
-
Space Segment:
The space segment of GPS consists of 24 satellites fielded in nearly circular orbits with a radius of 26,560 km, period of nearly 12 hours and stationary ground tracks. The satellites are arranged in sis orbital planes inclined at 550 relative to the equatorial plane, with four satellites distributed in each orbit. With this constellation, almost all users with a clear view of the sky have a minimum of four satellites in view. Each satellite receives and stores information from the control segment; maintain very accurate time through on board precise atomic clocks.
Figure 1: The major components of GPS - Control Segment:
The control segment of GPS consists of five tracking stations distributed around the earth of which one, located in Colorado Springs, is a Master Control Station. The control segment tracks all satellites, ensures they are operating properly and computes their position in space. The computed positions of the satellites are used to predict where the satellite will be later in time. These parameters are uploaded from the control segment to the satellites and referred to as broadcast ephemeredes. - User Segment:
The GPS user segment consists of the GPS receivers and the user community. Almost all GPS tracking equipment have the same basic components: an antenna, an RF (Radio Frequency) sections a microprocessor, a control and display unit (CDU), recording device and a power supply. Usually all component, with the exception of the antenna, are grouped together and referred to as a receiver. 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 time. GPS receivers are used for navigation, positioning, time dissemination, and other research. - GPS Signals:
Each GPS satellite continuously broadcasts ranging signals containing wealth of information. The information contained in GPS signals includes the carrier frequencies (L1 & L2), codes (coarse acquisition [C/A] & Precise [P]) and the navigational message. These allow users to measure their pseudo ranges and to estimate their positions in passive, listen only mode.
The basic principle of determining the position by using GPS satellites is based on measurement of distances from the point of observations to the satellite. This is done by comparing the reading of transmitter antenna time with the receiver antenna time. It cannot be assumed that the two clocks will be strictly in synchronization since the clocks used in the present type of receivers are quartz clocks to reduce the cost of the receiver. The observed signal time will have a systematic synchronization error. Since the measured range has got this systematic error in it, the computed distances will also be biased, and therefore, these are called pseudo-range. To compute the position based on this pseudo-range, the error due to time bias has to be corrected and therefore, this is also taken as an unknown and determined before deriving the true range. As we know from the simple formulate of distance computation that
To find the true range the time bias t is also has to be considering, therefore
Now in this equation, there are four unknown therefore, to solve this at least 4 satellites will have to be observed. The minimum requirement in this case is
- To know the co-ordinates of satellite antenna
- To know the satellite time at the time of emission of the signal
- Minimum 4 satellites, 4th one required to determine the time bias
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.
-
Absolute Positioning:
The pseudo ranges (the satellite antenna range, contaminated by the receiver block bias) from minimum four satellites are observed at the given epoch, from which the four unknown parameters – the 3D 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 absolute positioning in real-time is limited to about 100 meters, which can be improved to a few2 meters 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 satellite. - Relative Positioning:
In the translocation mode with tow or more GPS receivers observing the same satellites simultaneously many common errors, including the major effect of SA (selective availability) get cancelled out, yielding the relative positions of the two or more stations with a very high accuracy. The length of the base line between two stations, and also the absolute position of one of the stations, if accurate position of the other station is known, can be obtained to cm-level accuracy, using carrier phase observations. In differencing mode of observation, using single difference (difference of carrier phase observations from two receivers to the same satellite), double difference (between observations from two receivers to two satellites) and triple difference (difference of double differences over two time epochs), effect of many errors such as receiver and satellite clock errors etc., can be minimized.
Use of dual frequency observations (both L1 and L2 frequencies) eliminates the major part of ionosphere effect on the signal, thus improving the accuracy of positioning. With accurate satellite orbit and use of such refined procedures cm-level accuracy is possible even in regional and global scale surveys. - Differential GPS:
A modification of the relative positioning method is the differential GPS technique, where one of the two receivers can receive the messages given by this transmitter. The transmitting receiver is kept fixed on a point whose location is known to high degree of accuracy. Based upon this position. The receiver computes observations to the range/phase observations from GPS observations. Such as system is suited for applications such as vehicle guidance system, location-fishing boats close to the seashore, etc. The limited range of the transmitter restricts the use of such system to 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 or ship or aircraft. This technique has a number of important applications, including ship and aircraft navigation, photogrammetric survey control etc. - Selection of Point:
In planning a GPS survey, there are only two basic considerations in choosing a point i) Its location in an area of good sky visibility, and ii) Its proximity to road. The first requirement is of primary importance.
Following is a list of the desirable GPS site characteristics:- Clear view of the sky above 200 elevations
- Mark not likely to be disturbed
- Clear site for visible azimuth mark
- Space for parked vehicle
- Placed on publicly owned land
- Should be easily accessible
GPS is becoming a very popular and effective tool for the precise mapping and monitoring purpose. Many and many organizations are inducting GPS for the survey and other various purposes. Though, many factors affect the accuracy of the GPS survey e.g. number of satellite observed, observation time, surroundings at the place of observation, distance of each observation points etc. Accuracy also depends upon the interacted satellite’s geometry. The ultimate accuracy of a GPS determined by sum of several sources of error. The contribution of each source may vary depending on atmospheric and equipment conditions. Precision of observation such as landslide by the GPS depends on how above said errors can be eliminate of minimize. GPS cannot be used for the underground mapping as site of satellite is blocked. If such sites are falls in the continuous mapping, GPS survey can be switchover to electronic Total Station.
GPS industry is likely to continue to develop in the civilian community besides the specific use in defense. Now, GPS trend is continuing to be towards smaller, less expensive, and more easily operated devices for its wider use in the many aspects of community.
Acknowledgement
The paper is published with the kind approval of the Director of the Central Road Research Institute (CRRI), New Delhi. The guidance received from Prof. P. K. Sikdar, Director, CRRI is thankfully acknowledged.
References
- Brunner, F. K., Hartinger, H., Troyer, L., GPS signal diffraction modeling: the stochastic SIGMA-model, Journal of Geodesy 73: pp 259-267, 1999.
- Brunner, F. K., Hartinger, H, Richter, B., Continuous Monitoring of Landslides using GPS: A progress report; Geophysical Aspects of Mass Movements, Austrian Academy of Sciences, Vienna; pp 75-87, 2000.
- Garg, P. K., Ghosh S. K., Training Programme on Geographic Information System, Center for Remote Sensing, IIT Roorkee, pp 25-35, 2001.