TYPES OF GPS TECHNOLOGY M. Seedat, S. Thusi, A. Gogobala, M. Zaca, L. Mnembe – Survey and Land Information Dept. (April 2011) Introduction GPS or Global Positioning Systems are used extensively by surveyors as they provide accurate latitude and longitude positions. The Global Positioning System (GPS) is a technical marvel made possible by a group of satellites in earth orbit that transmit precise signals, allowing GPS receivers to calculate and display accurate location, speed, and time information to the user. It is hard to believe that something that was developed for military use and in missile guidance is now being used and accessed by the average person anywhere in the world. GPS works accurately in all weather conditions, day or night, around the clock, around the globe and has since become a multi billion rand industry with a wide array of products and services. With the wide selection of GPS products available I am sure one of us owns one, be it on a cell phone or in their car. This article will discuss how they work, their accuracies and their different applications. Page 1 Satellites – a history Satellites are useful in the surveying operation as they are the driving force for the operation of GPS systems. The first artificial satellite was successfully launched by the Soviet Union in October 1957; this satellite was called Sputnik 1. Sputnik 1 weighed 183 pounds, was about size of a cricket ball and took 98 minutes to orbit the Earth. The launch of this satellite Sputnik 1 has been named as the start of the space age and start of the US‐USSR space race which spanned over the years of 1960’s. In July 1958 the National Aeronautics and Space Act was passed by congress, more commonly known as the space Act; creating NASA (The National Aeronautics and Space Administration) from the National Advisory Committee for Aeronautics (NACA) and other government agencies. NASA was official as of October 1958. MIT researchers noticed that as Sputnik orbited the planet, its radio signal varied in strength. As it approached their position, the signal strength increased. When the satellite departed the MIT researchers' position, the signal strength decreased. From this increase and decrease of the radio signal, the MIT researchers could determine Sputnik's exact orbit. This recognition that radio signals from a satellite, or "artificial star" could determine distinct positions on the ground was truly the launching pad of the GPS system. The GPS satellite system The 24 satellites that make up the GPS space segment are orbiting the earth about 20200 kilometres above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling at speeds of roughly 4,000 kilometres an hour. GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.
Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. GPS operations depend on a very accurate time reference, which is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board. Page 2 Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions. The Global Navigation Satellite System (GLONASS) is based on a constellation of active satellites which continuously transmit coded signals in two frequency bands, which can be received by users anywhere on the Earth's surface to identify their position and velocity in real time based on ranging measurements. The system is a counterpart to the United States Global Positioning System (GPS) and both systems share the same principles in the data transmission and positioning methods. GLONASS is managed for the Russian Federation Government by the Russian Space Forces and the system is operated by the Coordination Scientific Information Centre (KNITs) of the Ministry of Defence of the Russian Federation. Post Processing Post processing is used in Differential GPS to obtain precise positions of unknown points by relating them to known points (normally two) such as Trigonometric beacons, Town survey marks, etc. Post processing is one of the most reliable GPS survey styles in terms of accuracy and results thereof. This is due to the occupation time for each point being surveyed which ranges from 8‐20 minutes for each point, depending on the number of satellites observed (in orbit) which should be a minimum of four satellites at that particular setup. The Survey and Land Information Department uses this survey style mainly to fix control points for different projects throughout the municipality. These coordinates then form part of our database of control points which are then used to carry out detail surveys. Page 3 VRS (Virtual Reference Stations) A VRS service uses data from several (permanent) reference stations which form a network to compute corrections that are generally more accurate than corrections from a single reference station. These corrections are then broadcast over the internet through a cell phone to the user. Operating in VRS does not require setting up a base from a known point, but the surveyor receives corrections from the internet on the cell phone through Bluetooth connection which provides centimetre level accuracy. Use of VRS corrections helps ensure the accuracy of the GPS data, independent of the distance to the nearest reference station. In KwaZulu Natal there are seven active reference stations from which the EThekwini Municipality can obtain these corrections, the closest being the one situated on the roof of the City Engineers building. The Survey and Land Information Department utilizes this VRS service to carry out detail surveys, beacon relocations, setting out of graves, etc. Page 4 Handheld GPS Receivers These units have taken the place of conventional magnetic compasses, instead of using the compass you will use the GPS device to navigate to a specific geographic position on the map. Handheld GPS units have about 5m to 30m accuracy; the more expensive units have more than one receiver in them and can track more satellites thus providing greater accuracies of 2m to 5m. These handheld devices allow you to download detail maps and colour photography to be stored in their memory. Their applications can range from GIS data capturing of the municipalities assets such as light poles, fire hydrants, water meters, manholes, etc. Automobile and Cell phone Navigation Systems As highlighted earlier, the automobile and cell phone navigation systems use the same principle of global positioning systems, however their accuracies are around 15 meters, especially for an automobile navigator. These were developed in the early 80’s. The automobile navigators are commonly used for finding destinations if you are travelling in an unfamiliar territory. These systems use the preloaded digital maps of the particular country like South Africa to turn‐by‐turn navigate to a particular place at the request of an operator/driver. This can be achieved by punching either the address or the latitude and longitude of that particular area or location. Page 5 As for the cell phone GPS’s, the same principles apply, however the difference is that these are more like hand held devices. They can be used in a number of operations i.e. location tracking, turn‐by‐turn directions, outdoor location services such as hiking trail. In America, all phones are built with the capacity to receive GPS signals; this allows the phone to transmit a person’s location to a Public Safety Answering Point after they have dialled 911, interesting. Automobile and cell phone navigation systems do not require any special or technical training; they are user friendly and can be used by almost anyone. Conclusion In the EThekwini Municipality the Survey & Land Information Department has been using GPS technology for the past 13 years and will continue to do so as it is an essential and productive tool ensuring fast and accurate information to all our clients. From topographical surveys, to locating services and setting out data there are numerous other applications this tool is used for. With the municipality’s boundary covering a vast area including rural communities GPS technology has assisted greatly in accessing these areas and carrying out timeous surveys to improve roads, infrastructure, etc. It is confident to suggest that most of the completed projects around our municipality be it a new road, stormwater pipeline, sidewalks, substations, reservoirs, pylons and ensuring maximum use of space for graves in cemeteries GPS technology has played a part in contributing to its completion and success. As GPS technology is always changing and improving, our department also needs to ensure that we keep up and continue providing a key role in maintaining these services well into the future. Page 6