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Global Positioning System
 
 
What is Global Positioning System (GPS)
Global Positioning System or GPS is a constellation of 27 satellites orbiting the earth at about 12000 miles. These satellites are continuously transmitting signals and anyone with a GPS receiver on earth can receive these transmissions. By measuring the travel time of signals transmitted from each satellite, a GPS receiver can calculate its distance from the satellite. Satellite positions are used by receivers as precise reference points to determine the location of the GPS receiver. If a receiver can receive signals from at least 4 satellites, it can determine latitude, longitude, altitude and time. If it can receive signals from 3 satellites, it can determine latitude, longitude and time. The satellites are in orbits such that at any time anywhere on the planet one should be able to receive signals from at least 4 satellites. The basic GPS service provides commercial users with an accuracy of about 10 to 15 meters.
How GPS works
  1. The basis of GPS is "triangulation" from satellites.
  2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
  3. To measure travel time, GPS needs very accurate timing.
  4. Along with distance, we should know exactly where the satellites are in space.
  5. Finally we must correct for any delays the signal experiences as it travels through space.
Triangulating: At a Glance
  • Position is calculated from distance measurements (ranges) to satellites.
  • Mathematically we need four satellite ranges to determine exact position.
  • Three ranges are enough if we reject ridiculous answers.
  • Another range is required for technical reasons.
Pseudo Random Code
The Pseudo Random Code (PRC) is a fundamental part of GPS. The signal is so complicated that it almost looks like random electrical noise, hence the name "Pseudo-Random." There are several good reasons for that complexity: First, the complex pattern helps make sure that the receiver doesn't accidentally sync up to some other signal. The patterns are so complex that it's highly unlikely that a stray signal will have exactly the same shape. Since each satellite has its own unique Pseudo-Random Code this complexity also guarantees that the receiver won't accidentally pick up another satellite's signal. Thus, all the satellites can use the same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system. The code also makes it possible to use "information theory" to "amplify" the GPS signal. And that's why GPS receivers don't need big satellite dishes to receive the GPS signals. It assumes that we can guarantee that both the satellite and the receiver start generating their codes at exactly the same time. On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board. But we have to remember that both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work. To compensate for the less accurate clocks in our receivers, we make an extra satellite measurement. If three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.
How a GPS Receiver works
At any given moment at any point on the planet there are between 6 and 9 satellites above the horizon. Once the GPS receiver has locked on to 3 satellites, it can display your longitude and latitude to about 100 foot accuracy. If the receiver can see 4 satellites it can also tell you your altitude. Most modern GPS receivers are able to store your track.
All the GPS receivers helps the user to:
  • See exactly where the user currently is
  • See exactly what path the user has followed using tracks
  • Store and then get back to a place the user has visited using waypoints
  • Get from point A to point B using waypoints and routes

GPS receivers are especially useful in environments where it is easy to get lost: on the ocean, in the woods, in the air flying at night, etc.

Differential GPS

In order to achieve on-line positioning with high accuracies, Differential GPS (DGPS) is used. Differential positioning uses the point position derived from satellite signals and applies correction to that position. These corrections, difference of determined position and the known position, are generated by a reference receiver, whose position is known and is fed to the instrument, and are used by the second receiver to correct its internally generated position.

The principal of DGPS is simple. If 2 receivers are placed close to one another, around 100-200 kms they will be subject to the same amount of errors and travel through the same atmospheric conditions. Therefore, one uses 2 receivers- one at a known point (base) while the other receiver is collecting the data in the field (rover). The base receiver at the known point stores the position data in the memory or on a PC, while the rover stores the data from the field in its onboard or external memory. The computer compares the second by second data from GPS unit at the base with the actual known point data at the base station and determines the amount of error. When the data from the rover is downloaded in the PC, the software applies the corrections to the rover data and corrects the rover readings. This method is called the post processing method. Instead of using the post processing method, one can now utilize the real time correction method. In this case instead of storing the base station data and processing on the PC, the error is calculated in the receiver at the base and broadcast.
Uses of GPS
GPS receivers are used for navigation, positioning, time dissemination, and other research.
  • Navigation in three dimensions is the primary function of GPS.
  • Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers. Surveying, geodetic control, and plate tectonic studies are examples.
  • Time and frequency dissemination, based on the precise clocks on board the SVs and controlled by the monitor stations, is another use for GPS.
  • Research projects have used GPS signals to measure atmospheric parameters.
  • Georeferencing: that is assigning correct latitude and longitude to the control points of satellite imageries and topographic maps.
Integration – GPS/GIS Technology
GPS is a powerful tool providing a unique position of a specific feature. With this information, one can navigate back to it. However, one cannot relate this “feature position” to any other “feature position” unless one is standing at the site and other features are visible. GIS by itself provides great analysis capability but to achieve that one needs plenty of good data. As explained earlier, some data is available but a lot of other data needs to be collected to allow the full capabilities of GIS to be utilized. Combining the GPS data with GIS allows for greater capabilities than what GPS and GIS can provide individually. With the combination of two technologies one is able to display the “FIELD/ACTUAL SITE” on a PC and make informed decisions. There is no need to make specific site visits or review several documents/drawings. Also, data can be shared by unlimited users in various departments for their own specific needs and analysis.
Another important advance in this technology has been the introduction of software which allows bringing into GIS not only GPS position information but a digital picture. With this software, one can study relationships between features but also view actual photographs of the features right on his PC.
   
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