Sources of Error

There are numerous sources of error that will cause the rover unit’s position to be recorded erroneously.  These errors include, but are not limited to:

• Atmospheric Error (Ionosphere and Troposphere)
• “Multipath” Error (satellite signal is reflected from buildings, large signs, etc.)
• Vegetation
• Topography

All these will cause error and throw a position off.  Also, the U.S. Government had included an error source called Selective Availability that threw the satellite signal to the rover unit off up to another 100 meters (in addition to the error sources mentioned above). (Wood, 2000)   On May 2, 2000 selective availability was turned off, improving GPS accuracy a great deal.  The U.S. Government plans to leave selective availability off, unless there is a time of war or foreign conflict, at which time some form of selective availability may again commence.

Why then is GPS a valuable tool if error can creep into the process?  The answer lies in differential GPS, obtainable through the use of a base station.

Base Station

Figure 4: Base station antenna at JMU (SIC 2002)

A base station can be thought of as a GPS reference point[See Figure 4].  It is another GPS receiver that is positioned on an accurately surveyed location.  Consequently, the base station can be as simple as another rover unit placed over a surveyed location, such as a USGS benchmark.  The base station receives signals at the same times the rover unit receives them.  Since the base station is located on a known geographic point, error vectors can be obtained for every signal it receives.  If the base station received a signal that says its position is five meters northwest of its actual position, the reading the rover took (at that exact point in time) will also be five meters northwest of its position.  In other words, the base station is the means by which rover unit data is corrected (down to centimeter accuracy in some cases).  For reliable data, the rover unit data must be collected within 500 kilometers of the base station. This methodology is known as Differential GPS. As stated above, DGPS is a way of using the know error to the advantage of the GPS user.

Differential GPS: Real Time vs. Post Processing

Post-Processing
Post-processing is employed if the data does not need to be used immediately in the field.  For the deminer, post-processing is recommended for two reasons.  Post-processing is usually cheaper than real time DGPS and it enables the user to store files and stay organized.  This method involves bringing the rover unit back to the base station (or laboratory where base station files are stored, or can be obtained) and doing the post-processing there.

Figure 5: Example of a GPS Unit (SIC 2002)