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A Look at Differential GPS
In the early days of GPS the accuracy was good to within 100 meters for a given location. This was due to the US Government’s policy of “Selective Availability." This introduced a deliberate timing error in the GPS signal. By the turn of the century, the US Government dropped this policy of “Selective Availability" and since then the accuracy of GPS units have improved tremendously. Most units today are accurate to within 5-10 meters. This is good enough for most user applications such as road navigation, etc. For certain more advanced applications, such as aircraft navigation and maritime navigation, even more accuracy (1-2 meters) is desirable. For this a more accurate GPS system is required and one of the solutions is Differential GPS or DGPS.
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Why DGPS or What’s Wrong with GPS?
So the question is can the accuracy be improved, if required? The answer to that is yes, it can be done. In most of the normal usse of GPS the accuracy of 10 meters/30 ft or less is actually pretty good. For example, even if you arrive at 30 feet away from the local Wal-Mart’s door you are fine. However, in certain applications high accuracy is critical such as in aircraft or maritime navigation. Geographic Information Systems or GIS is another application where a high accuracy is desirable.
So how do we derive better accuracy from the system that is up there in the sky? Let us look at the factors that limit the accuracy first, it then would be easier to understand how to work around them. The signal from the GPS satellites are low to start with. Then there are effects that further interfere with the reception. Multipath distortion or when the signal is received from several directions at the same time, rather than directly from the satellite is a major factor. When driving on a city street the GPS signal may be reflected from several tall buildings at your in-car unit. There are also certain technical limitations such as clock errors and an inherent inaccuracy in the internal calculations. In addition to all this, GPS is also affected by electromagnetic interference (EMI) and radio frequency interference (RFI).
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Differential GPS - How Does DGPS Work?
Differential GPS (DGPS) gets around the accuracy problem by relying on a number of fixed points on the earth whose GPS coordinates are well-known. Base stations or reference stations are located at these fixed points. The DGPS system works as follows:
- Base stations constantly measure the accuracy of the GPS signal received from each of the satellites that it has a Line-of-Sight (LoS) or direct visibility to.
- The base station calculates how much of a correction (compared to the known accurate position) is required per satellite and broadcasts this information to DGPS units in its vicinity.
- Using the original signal received from the satellite and the correction information received from the nearest fixed point (or points) the DGPS unit can then establish a more accurate estimate of its current position.
The basic assumption is that GPS receivers that are close to each other suffer from similar atmospheric effects, and so in all likelihood the base station and the closest DGPS unit will experience the same amount of error. Clearly, the accuracy of DGPS will depend on the distance from the closest base station. A DGPS unit, thus, needs to have the additional capability of receiving the signal from base stations) and carrying out the post-processing. The hardware and software for a DGPS unit are different from a traditional GPS units You are able to achieve sub-meter, sometimes in centimeters, accuracy with DGPS measurements.