01 October 2011


Edited by Dr. K.Srinivasa Raju, Associate professor,Anna University, Chennai, India
Two major GNSS existing today namely GPS by USA and GLONASS by Russia have evolved from dedicated military systems into true dual-use systems. Satellite navigation technology is utilized in numerous civil and military applications, ranging from golf and leisure hiking to spacecraft navigation. The application of GNSS can be of two categories namely differential applications and autonomous applications. Most of those applications can be broken down into three broad areas: Navigation, Surveying, GIS and Mapping. These three broad classifications can further be broken down into real-time (RT) and post-processed (PP) applications depending upon the requirements of each area.
Geographic Information Systems and Mapping applications generally deal with products that are not immediately time-dependent. Therefore, there is usually little need for real-time correction of GPS spatial data. The information requirements of GIS/Mapping applications can be met by post-processing and include the point, line, and area. Precision surveying applications also do not normally require real time data correction. Beyond just simple map-making, typical GPS/GIS applications include utilities management, police incident reporting, postal addressing, a wide range of demographics, resource management, and much more.
Navigation requires where the navigator is at that exact moment since the position is constantly changing. Navigation applications, therefore, are more interested in instantaneous position, velocity and heading.
Navigation is concerned with determination of instantaneous position, velocity, and heading.  GPS is as well suited to these tasks as for point, line, and area determination. Real-time navigation applications involving air travel, using augmentation systems such as the WAAS and ILS are gaining wide acceptance and large systems are currently either being planned or installed. Precise sea navigation through the use of differential broadcast beacons already exist.

2.1 Aviation, Maritime Navigation
The aviation community has propelled the use of GNSS. These systems provide guidance for en route through precision approach phases of flight. Incorporation of a data link with a GNSS receiver enables the transmission of aircraft location to other aircraft and/or to air traffic control. This function is called automatic dependent surveillance. Key benefits are air traffic monitoring for collision avoidance and optimized routing to reduce travel time and fuel consumption. Similarly, The Space Shuttle utilizes GPS for guidance in all phases of its operation (e.g., ground launch, on-orbit and reentry, and landing). GNSS has been used by both commercial and recreational maritime communities. Navigation is enhanced on all bodies of water, from oceanic travel to river ways, especially in inclement weather.
2.2 Land Navigation
One of the real applications for GNSS that directly impact the common civilian is inland navigation. This includes rail, trucking, emergency (police, fire, ambulance, etc.), and private vehicles. Similar techniques are used in farming, surface mining, and grading for real-time control of vehicles and in the railroad community to obtain train locations with respect to adjacent tracks. GNSS is a key component in intelligent transport system (ITS). In vehicle applications, GNSS is used for route guidance, tracking, and fleet management. Combining a cellular phone or data link function with this system enables vehicle tracing and/or emergency messaging. AVL, or Automated Vehicle Location is a technique that uses GNSS for determination of vehicle location and helps in Vehicle tracking or Vehicle navigation applications. Monitoring public transport services for providing information on their arrival times at different locations is gaining importance even in Indian cities. In-vehicle navigation system to help the drivers to travel in non-familiar areas with the help of GNSS integrated with GIS has become a inbuilt functionality in high end cars of today.

GNSS can be applied in mapping in two ways. The first is simply applying a controlling reference to some other source of data like providing control points for georeferencing and other is to use GNSS to acquire the location information of features directly.
3.1 Remote Sensing and Photogrammetry
GNSS is applied in Remote Sensing and Photogrammetry at two levels. The most obvious is that aerial photographs and satellite images must e registered to the Earth to give it geographic meaning. GNSS can be used to control, or geo-reference Aerial Photographs and satellite imagery. Once properly registered, the GIS information can accurately be extracted from the image.
The other level where GNSS is being applied to satellites and Aircrafts in space itself. More and more, GPS is being used to position the satellites in their orbits. In fact, many new satellites are equipped with on-board GPS receivers as part of their own internal navigation and position holding systems. Even the space shuttle now flies by GPS positioning.
3.2 Utility and Asset Mapping
Typical applications of GNSS include utility and asset mapping and automated airborne mapping, with remote sensing and photogrammetry. The features such as trees, telephone poles and fire hydrants, are very often not clearly visible on aerial photographs or maps hence cannot be mapped from remote sensing. GNSS Receivers with software designed for GIS data collection can be taken to that representative telephone pole, tire hydrant, or whatever, and, with only a few keystrokes, record all of the necessary information about them such as number of cross-pieces, number of transformers, condition, number of spigots, serial number and, of course, their precise geographic locations. With this technique, hundreds of such features can be recorded in a single day, impossible with traditional survey methods.
3.3 Geographic Information Systems (GIS)
Integration of GNSS and GIS has widened the scope of applications of GNSS. The locations determined through GNSS can be stored and analysed in GIS to derive solutions and make decisions in many organizations like police, postal, courier services.
By recording the GNSS positions of any number of types of incidents in a GIS, police can instantly identify “hot spots” of criminal activity that might need extra patrols. Knowing the types of crimes that occur at a particular geographic location is invaluable and easily available through their GIS. Their GIS can tell them where a high number of car accidents occur, suggesting that there might be a problem with the road or with the lights or signs there, or perhaps the speed limit needs to be changed.
The Post Office or Courier Company needs to know the locations of all of the postal residents in a given area so that the most efficient carrier routes can be calculated by their GIS.
3.4 Agriculture
Recently, GIS, GPS, and remote sensing have matured enough to be used in agriculture. GIS companies such as the Environmental System Research Institute (ESRI) have developed software applications that enable growers to assess field conditions and their relationship to yield. Real time kinematic and differential GNSS applications for precision farming are being developed. This includes soil sampling, yield monitoring, chemical, and fertilizer applications.
The other development in the field of Agriculture is Precision farming. It uses GNSS to guide tractors in real-time at sub-meter levels of precision, accurate enough to be able to keep aligned with individual furrows. Experiments are even currently ongoing with fully automated farming vehicles that essentially drive themselves around and through the fields. Several GNSS manufacturing firms offer products to guide airborne applicators of pesticides. These systems involve customized mapping routines to direct the pilot of crop duster swath by swath over a particular field.
The huge economic advantage of using GPS in surveying applications drove the development of very sophisticated GPS equipment and tools to predict GPS coverage and derive position with centimeter accuracy. Extreme accuracy is possible by applying information on satellite positions available after the fact to the data obtained in the field. The value of the GNSS technology in the surveying stems from the availability of absolute positions with respect to a universal coordinate system (WGS-84) and from the fact that they can be determined with a much smaller survey crew. The all weather capabilities of GNSS system will help improving the efficiency of survey team with reference to time. A single surveyor can collect data in the field, where it would take a two- or three-person crew to achieve the same results using some conventional methods. Collected data can be processed to the required accuracy using inexpensive computing facilities, and the GPS equipment in the field can be used by the surveyor for rough surveys or the location of benchmarks or other features. Differential and kinematic techniques can provide accurate real-time information in the field and obviate the need for postprocessing the data, further reducing the cost of surveying operations
Diversified GNSS applications are seen in the recent years where the technology is widely used in recreation, tourism and similar applications. Some obvious recreational applications of global positioning technology include hiking and orienteering. Several organizations are developing positioning systems for golfing applications. Differential corrections are required, as accuracy of a few feet is needed. Course managers are attracted to the idea as a method to speed up play and improve the utilization of an existing resource. Receivers have been put on golf carts to display from a database the distance to the green, to the pin, and to any hazards that may be of interest from a given location.
At outdoor parks, tourists can use a GNSS device that will allow them to conduct a self-guided tour without any external signs or references. At each designated spot, vignettes can be automatically triggered by the GNSS unit and direct tourists based on their present position and knowledge of where they have been previously. Recently, recreational and commercial users were able to display digital maps on their GNSS-equipped cell phones via a service provided by Cellular Telephone Industry Association members.
GPS receivers are small enough to be worn on the wrist. This has opened up applications for joggers to keep track of their location, speed, and distance, as well as for keeping track of children and for blind people to navigate.
GNSS Time Transfer Application
The fact that GNSS is based on accurate time references implies that the signals can be used for the synchronization of very accurate clocks and timing standards. Each satellite has multiple atomic clocks on board, and each is frequently updated to system time. A prime application of this accurate timing capability is in the control of data communications networks like the Internet, where data packets time-share the same communications bandwidth. Receivers and transmitters can be synchronized, reducing the data overhead required of the system. Other applications include synchronized switching of power grids and timing of racecars. There are several manufacturers of equipment dedicated solely to the extraction of accurate time from the GPS signal.
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