GPS/GIS integration


Accuracy Issues
There are several sources of error that reduce the accuracy of the GPS receivers. Some are random in nature and beyond our control but some errors can be controlled.

As the satellites are transmitting signals from an elevation of about 12000 miles, the radio signals pass through earth’s atmosphere before reaching the receiver. The signals have to pass through ionosphere and are delayed. This delay does affect the accuracy obtained by the receiver. Keep in mind that the signals travel at speed of light 186000 miles per second and even a 1/100th second delay can put one off by 1860 miles.



The satellite orbits get affected by gravitational pulls, which cause disturbance in their orbits and create errors in the position.

The receivers themselves can cause some errors. For example if there is faulty clock in the receiver, it will impact the readings.



The multipath error applies to the placement of the receiver. This happens when the receiver gets a direct signal from a satellite and also a reflected (bounced off) signal from a large object (trees, high rise buildings etc.) around the receiver.



How is accuracy improved - DGPS
Some receivers have the capability to automatically correct for some of the above errors. However even with these types of receivers one expects to get 5-10 meters accuracy. Some users feel that with the removal of SA, they can average the readings and get sub meter accurate readings. This is not true. The readings may improve but several inaccurate readings will not always give you an accurate reading. In order to improve the accuracy further, one needs to use a technique called differential GPS (DGPS). Note that this will increase the cost of the system.



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. So 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. This method, while providing good accuracy has some limitations and disadvantages:
  • One needs 2 receivers (thereby raising the cost) or access to some base station data from a location within 200 kms from the rover.
  • This method also does not provide you with real time navigation capabilities.
  • Frequently, if the satellites tracked by the base And rover units are different, the readings will not be corrected.
  • The other factor to consider in this method is the fact that for every hour spent in the field to collect the data, one needs to spend about an hour in the office post processing this data.
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. The U.S. Coast Guard offers one such system and if one has a GPS receiver with an appropriate beacon receiver, one can receive the corrections in real time and accurate GPS readings are displayed. While the service is free, it has a limited range. This range is heavily dependent on the topography of the area. Therefore, this is not a solution for everyone.



The second real time system is offered by private companies who are transmitting the correction signals from a satellite. This allows for far better coverage all over the world. But the service ranges around $800.00 to $5,000.00 per year for various areas in the world. Monthly service is also available.

The last 2 methods offer the capability of real time accurate readings and therefore good navigation capabilities. Also one does not have to spend time in the office to correct the field data.


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