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The Global Positioning System (GPS)
— Former US President Bill Clinton [The US previously employed a technique called Selective Availability (SA) to globally degrade the civilian GPS signal. New technologies demonstrated by the military enable the U.S. to degrade the GPS signal on a regional basis. GPS users world-wide would not be affected by regional, security-motivated, GPS degradations, and businesses reliant on GPS could continue to operate at peak efficiency.]
How GPS Works?
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For a GPS receiver to find its user location, it has to determine two things:
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GPS satellites send out radio signals that a GPS receiver can detect. Fort the GPS receiver to know how far away the satellite is, it measures the amount of time it takes for the signal to travel from the satellite to the receiver. Since we know how fast radio signals travel — they are electromagnetic waves travelling at the speed of light, about 186,000 miles per second in a vacuum — we can figure out how far they have travelled by figuring out how long it took for them to arrive.
Measuring the time would be easy if we knew exactly what time the signal left the satellite and exactly what time it arrived at the receiver. Solving this problem is key to the Global Positioning System. One way to solve the problem would be to put extremely accurate and synchronised clocks in the satellites and the receivers. The satellite begins transmitting a long digital pattern, called a Pseudo Random Code, as part of its signal at a certain time, let's say midnight. The receiver begins running the same digital pattern, also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern. The length of the delay is equal to the time of the signal's travel. The receiver multiplies this time by the speed of light to determine how far the signal travelled. If the signal travelled in a straight line, this distance would be the distance to the satellite.
The only way to implement a system like this would require a level of accuracy only found in atomic clocks. This is because the time measured in these calculations amounts to nanoseconds. To make a GPS using only synchronised clocks, we would need to have atomic clocks not only on all the satellites, but also in the receiver itself. Atomic clocks usually cost somewhere between £30,000 and £60,000, which makes them too expensive for everyday consumer use.
The Global Positioning System has a very effective solution to this problem — a GPS receiver contains no atomic clock at all. It has a normal quartz clock. The receiver looks at all the signals it is receiving and uses calculations to find both the exact time and the exact location simultaneously. When we measure the distance to four located satellites, we can draw four spheres that all intersect at one point, as illustrated above. Four spheres of this sort will not intersect at one point if we have measured incorrectly. Since the receiver makes all of its time measurements, and therefore its distance measurements, using the clock it is equipped with, the distances will all be proportionally incorrect. The receiver can therefore easily calculate exactly what distance adjustment will cause the four spheres to intersect at one point. This allows it to adjust its clock to adjust its measure of distance. For this reason, a GPS receiver actually keeps extremely accurate time, on the order of the actual atomic clocks in the satellites.
One problem with this method is the measure of speed. As we saw earlier, electromagnetic signals travel through a vacuum at the speed of light. The earth, of course, is not a vacuum, and its atmosphere slows the transmission of the signal according to the particular conditions at that atmospheric location, the angle at which the signal enters it, and so on. A GPS receiver guesses the actual speed of the signal using complex mathematical models of a wide range of atmospheric conditions. The satellites can also transmit additional information to the receiver.
The other crucial component of GPS calculations is the knowledge of where the satellites are. This isn't difficult because the satellites travel in a very high, and predictable orbits. The satellites are far enough from the earth (11,000 miles) that they are not affected by our atmosphere. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the US Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.
The most essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, so that it can mathematically determine the receiver's position on Earth. The basic information a receiver provides, then, is the latitude, longitude and altitude (or some similar measurement) of its current position. Most receivers then combine this data with other information, such as maps, to make the receiver more user-friendly. We can use maps stored in the receiver's memory, connect the receiver up to a computer that can hold more detailed maps in its memory or simply buy a detailed map of our area and find our way using the receiver's latitude and longitude readouts.
Geographers have mapped every corner of the Earth, so we can certainly find maps with the desired level of detail. We can look at a GPS receiver as an extremely accurate way to get raw positional data, which can then be applied to geographic information that has been accumulated over the years. We can even use GPS to accurately update older maps and record new/changed features on them (see CDC field mapping work along Lake Victoria, Kenya below).
GPS receivers are an excellent navigation tool, with seemingly endless applications. GPS technology has enabled a revolution in the ways we move people, goods and information; build communities; manage the environment; predict the weather and natural disasters; and respond to emergencies. Combined with other geomatics technologies, GPS data can be used in a wide range of applications, including locating and tracking vehicles and other objects, managing infrastructures, time-stamping information and images, and navigating between points on the globe.
A standard GPS receiver will not only place us on a map at any particular location, but will also trace our path across a map as we move. It can tell us:
Most receivers have a certain amount of memory available for users to store their own navigation data. This greatly expands the functionality of the receiver, because it essentially lets its user make a record of specific points on Earth. The basic unit of user input is the waypoint. A waypoint is simply the co-ordinates for a particular location. We can save this in our receiver's memory in two ways:
We can tell the receiver to record its co-ordinates when we are at that location; or
We can find the location on a map (the internal map or another one) and enter its co-ordinates as a waypoint.
This capability lets us use our GPS receiver in a number of different ways. We can record any specific location that interests us, so that we will be able to find it again at a later time, e.g., record where we left our car.
We can also combine a series of different waypoints to form a route. One way to use this function is to periodically record waypoints as we make a trip so that we can backtrack, or follow the same route again on another trip. Route mapping also lets us plan ahead. When we have time to examine a map at home, we can record a series of waypoints along the roads or trails that lead to our destination. Then, when we are travelling, all we will need to find our way is our GPS receiver. As we travel, the receiver will show us which way to go and give us the distance to our next waypoint. All we need to do is follow its simple directions!
Receivers with route capabilities will let us save a certain number of waypoints to memory so that we can use them again and again. If the receiver has a data port, we can also download our routes to a computer, which has much more storage memory, and then upload them again when we plan to follow those routes. (Brain, 2000)
Because they have so much more storage capability, computers can do a lot more with GPS location data than our average receiver. A receiver with a data port can feed the raw location co-ordinates into a computer running more complicated software. There are a number of available software applications that can place us on detailed maps of particular areas. If we want to use our receiver for complicated navigation, down back-roads for example, this capability will help us out tremendously. We can also update our computer maps, so that they include any surveying adjustments or changes in an area (see CDC field mapping work along Lake Victoria, Kenya below), whereas a receiver's onboard map usually cannot be changed. When we use our receiver in conjunction with a computer, we increase the receiver's capabilities considerably. Also, our receiver will not be outdated as quickly, because in conjunction with a computer, all it needs to do is provide co-ordinates — the computer does the rest. (Brain, 2000)
GPS technologies are dramatically improving transportation on land, at sea and in the air. GPS has been called the most significant development in air navigation and control since radar — and with good reason. It has improved safety by allowing better management of the air corridors, and will reduce fuel consumption by helping establish more efficient routes and schedules. At sea, GPS also helps vessels reach their destinations safely and efficiently.
But the largest transportation market for GPS lies in land navigation and vehicle positioning. GPS technology is used to dispatch police, ambulances and fire fighters in emergency situations, reducing the response time and saving lives. A vehicle-mounted GPS system has been developed that records road conditions and roadside features, maps transportation corridors and performs other useful functions. The technology is even being used to locate stolen vehicles. (Geomatics Canada Web Site, 2000)
GPS equipment and software is being used to speed the infrastructure building process and to provide precise positions for the creation of databases of resources and the production of maps that can be used to evaluate the feasibility and impact of programs and projects and help implement them. For example, GPS can be used to improve fertilising and harvesting operations by directing the movement of farming equipment. Images from remote sensing satellites provide information about an area's fertilisation/harvesting needs, and GPS is then used to guide the application of the fertiliser or the harvesting equipment. GPS can be also used to support the investigation of hazardous waste sites, the mapping of ecosystems, the monitoring of oil spills and clean-up efforts, and the tracking and mapping of airborne pollutants. (Geomatics Canada Web Site, 2000)
In Kenya, the Division of Parasitic Diseases of the Centres for Disease Control and Prevention (CDC, Atlanta, Georgia, US) works with the Kenya Medical Research Institute to study malaria and to work to prevent it. Nearly three hundred researchers work on various projects near Lake Victoria and Kisumu, Kenya's third largest town. These researchers use Differential Global Positioning Systems (DGPS) to collect positions and data in the field, and then edit and analyse this data in ArcView GIS. One study region had its last map made in the late 1960s, and researchers needed an updated map for their study. Researchers began by creating an updated map of a 225 square mile region just outside of Kisumu along Lake Victoria using GPS. The DGPS mapping team hired local fishermen to row them in small fishing boats to map the shore of the lake. Roads were mapped by driving cars along them while a team member captured location data with the DGPS. Once they had an updated map of the region, they could begin using their GIS and create the maps that would help them understand the impact of bednets (treated with odourless insecticides) on malaria, childhood mortality, and mosquito populations. (Lang, 2000)
"The idea behind the initial version of Digital Angel is to build a microchip that can be worn close to the body. This microchip will include biosensors that will measure the biological parameters of the body and store this information. It will also have an antenna that will receive signals from GPS satellites. The geographical location of the chip can be derived from these signals. The antenna also communicates with ground stations. It will receive commands from the stations and will send the biological information and location data to the ground station. This could take the form of a distress signal sent to a monitoring facility when the unit detects a medical emergency."
Source: www.digitalangel.net
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"The decision to discontinue Selective
Availability is the latest measure in an ongoing effort to make GPS more
responsive to civil and commercial users world-wide. This increase in accuracy
will allow new GPS applications to emerge and continue to enhance the lives of
people around the world."
![Source: Lang L. GIS for Health Organisations. California: ESRI Press. 2000 [ISBN 1-879102-65-X]](images/gps_victoria.jpg)
