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| Tuesday, 28 December 2004 |
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Global Positioning Systems (GPS) - Sophistication made simple by Dr. Ranjith Premalal de Silva, President / Geo-Informatics Society of Sri Lanka and Senior Lecturer in Agricultural Engineering University of Peradeniya The use of Global Positions Systems is no longer limited to filed surveying or specialised navigation demanding a great deal of knowledge or expertise. It has become a technology of a common man today. From simple leisure tour navigation to sophisticated sub millimetre level accuracy mapping, GPS could provide the cost-effective solutions at varying degree of accuracy. Within the next decade, GPS would probably be an integral part of the automobiles, mobile phones, cameras and wrist watches. Field surveying and navigation demand a reliable and efficient predicting of ground information in three dimensions. However, the traditional methods of surveying and navigation need tedious field and astronomical observation as well as expertise for deriving positional and directional information. In addition, the effect of different climatic, diverse field conditions and many unavoidable circumstances create erroneous predictions. With the advancement of the science, various efficient and precise technique and instruments came into the picture for efficient and precise estimation of ground information. Successful attempts made by USA since early 1960s to use space based artificial satellites and establishing network of control points over a large region, rapid improvements in higher frequency transmission and precise clock signals along with advanced stable satellite technology, led to the development of an efficient and highly precise system called 'Global Positioning System' for predicting ground information in three dimensions. Global Positioning System is now becoming an integral component of the global information system as an economical, all weather, real time, continuously available and very precise information source for variety of applications ranging from mapping and surveying to international air traffic management and global research. Therefore, it is of immense important to be aware of this advanced technology and its scientific background in a rapidly developing trend of information technology. GPS concepts GPS which is a navigational system, is also called NAVASTAR GPS (Navigational System with Time and Ranging) was first introduced for military purposes as a navigational tool by the US Department of Defence. At present, it is being widely used for civil purposes as well. GPS provides a spatially coded satellite signals that can processed in a GPS receiver, enabling the receiver to compute position, velocity and time. The whole system can be categorised into three main components: * Space segment, * Control segment, * User segment. The space segment The nominal GPS Operational constellation consists 24 satellites that orbit the earth in 12 hours and are placed in six near circular orbits where each orbit occupied by 4 space vehicles at a height of 22,000 km. The satellite orbits repeat almost the same ground track once in each day. All the space vehicles are occupied with atomic clock with an accuracy range of 1*10-12 to 1*10-13 seconds. Each satellite transmit signals with two carrier frequencies; L1 (1575.42 MHz) and L2 (1227.6 MHz). The all satellites in constellation can be grouped into three. * Block I: Development or R and D satellite. Numbered from 1 to 11 and launched at the earlier stage for experimental purposes. * Block II: Production or Operating satellites. * Block III: Replenishment satellites. The control segment The segment consists of a system tracking five ground stations around the world. The master control station is located at Schriever Air Force Base in Colarado. The signals tracked by the ground stations all over the world are then transmitted to the master control station at Colarado where the satellites ephemeris and the behaviour of satellite clock are pre-computed to formulate satellite data. The formulated data are then transmitted to the ground antennas and up-linked via S-band to the satellites. The space vehicles then send subsets of the orbital ephemeris data to GPS receivers over radio signals. The function of the control segment are to monitor and control the satellite system continuously, determine the GPS system time, predict the satellite ephemeris and the behaviour of the satellite clocks and up-date the navigation message for each particular satellite periodically. The user segment The segment consists of appropriate GPS receiver with antenna and power supply unit. The receiver should contain enough channels to collect data from available satellites. GPS receivers convert satellite signals into position, velocity and time estimates. The main components of a GPS receiver are antenna with pre-amplifier, RF section with signal identification and signal processing, micro-processor for receiver control, data sampling and data processing, precision oscillator, power supply, user interface, command and display panel, memory, data storage. Most modern receiver has a keypad and a display for communication between the user and the receivers. The keypad is used to enter commands, external data like station number or antenna height or to select a menu operation. The display indicates computed coordinates, visible satellites, data quality indices and other suitable information. Current operation software packages are menu driven and very user friendly. GPS receivers are used for navigation in three dimensions in air crafts, ships, ground vehicles, and as well as for hand carrying individuals while those are used in positioning at reference locations and widely used in surveying, geodetic control and in plate tectonic studies. Time and frequency dissemination are important usage of the system where time dissemination based on the precise atomic clock. Then astronomical observations, telecommunication facilities and laboratory standards can beset to precise time signals or controlled to accurate frequencies by special purpose GPS receivers. Further, GPS signals are also used to measure the atmospheric parameters in research works. GPS applications The data generated by the GPS is efficiently and effectively used in wide range of fields such as Agriculture, Asset Management, Construction, Environment & Forestry, Factory Automation, Location Based Services, Mapping & GIS, Marine, Military, Mining, Mobile Resource Management, OEM Boards & Chipsets, Oil & Gas, Public Safety, Survey, Timing & Synchronization, Transportation & Vehicle Security, Urban & Municipal and Utility Management. Environment & Forestry The world is changing at a rapid pace, which affects the way resources are managed. That's why environmental scientists and foresters know that its important to monitor change, so that timely and informed management decisions can be made. People all over the world are using GPS to enhance the accuracy of their work whether it is species monitoring, ecological population density studies, soil run-off surveys, or flood analysis. With accurate data important decisions about our planet are made with more confidence. No matter what environment is being studied, GPS Technology adds value by providing with an easy-to-use, efficient, and cost-effective way of keeping your GIS current and accurate. Fertilizing and crop protection Optimizing fertilizer a crop protectant applications is critical to consistently attaining the yields you need. GPS Technology helps to fertilize and protect the crops more precisely and more productively, saving the time and money and getting better results. GPS can be used for applying straight, accurate swaths every time, with minimal overlap and skip and without the hassle of foam markers, row makers, or flaggers. Drive faster, more precisely, even in reduced light to get more work done more quickly. Here a less amount of fertilizer or protectant material should be used and applied only where it is needed. For an effective management of all aspects, making initial field measurements; generating detailed records; monitoring and managing field operations using prescription an application maps and reports; and generating more precise billing also a must. Water Management Accuracy is very important parameter in the field of water management. The record should be accurately maintained data to understand how the water system is changing imperative for public health and safety. There are some company makes GPS dedicated for water management such as Trimble GPS provides accurate, easy to use, and rugged products for the harsh field environment. Mapping, Cadastral, Planning & Land Management Surveyors in both the public and private sector need to accurately survey boundaries for legal and documentation purposes. GPS provides different survey methodologies allowing surveyors to meet the regulations of their country. GPS data can be combined with total station positioning technologies allowing for the collection and management of cadastral information. And with a seamless interface to a variety of CAD, design and GIS packages that can easily be transferred data and designed information between the field and office. Land survey One major advantage of GPS is that its capability of forming a powerful building block in integrated system, GPS together with a coordinate system and Geographic Information produces a map. The geometric location of objects can be entered in the GIS or GIS can be taken into the field with GPS direct entry. These facilitate efficient and accurate land surveying. Although, Global Positioning System is a highly advantageous satellite navigational system, there are some limitations due to noise, bias and blunders when using the system in various applications. One of major limitations is the signal shadow caused by buildings, trees, towers etc. Therefore the exclusive usage of GPS will be restricted into open areas. Our status and potentials In Sri Lanka, the use of GPS has generally been restricted to sophisticated mapping exercises and academic research. Practical applications of the technology have not been made known to the public. Although few vendors operate in the country, none has really made an attempt to introduce new application innovations to expand their prospective market share. However, it is timely for us to learn the basics of this useful technology and get the maximum benefit of the technology in our development endeavors. |
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