GPS Q&A
By Mike Etherington

Q. When using post-processing differential techniques, how accurate does your control point need to be? N.N. Carlsbad, Calif.

A. The fundamental theory behind differential is simple. At least two receivers must operate at the same time, with one unit located on a known point, and both receivers must be using enough common satellites within the same group to derive positions. In differential post-processing, data collected on the known point (also called the control point, benchmark, or reference point) is used to remove inaccuracies from the data taken on a field point. Because position coordinates are not absolute but relative, some error in the control point is permissible. However, the amount of allowable error depends on the method of post-processing being used.
      The most common method of post-processing uses pseudorange data (measured distances from satellite to receiver) to calculate corrections in the position data transmitted by the satellites. They're called pseudoranges because the effects of SA cause them to be inaccurate, or pseudomeasurements. These data (either three or four measurements) are used by the receiver to trilaterate its position on the Earth.
      Corrections are applied to the data collected by other GPS receivers in the area to determine the position of unknown sites. Because the position coordinates are relative to each other, the accuracy of the control point could vary from its true position by as much as several hundred meters.
      Control points must be held more tightly when employing carrier phase and ambiguity resolution processing methods which yield precise accuracies. In the case of ambiguity resolution, best results are obtained when the absolute control point error is less than 10 meters.
      However you choose to collect position fixes in a particular area, make sure you work with the most precise set of control points available.

Q. How long do you need to stay on site to achieve post-processed centimeter accuracy using low cost, L1-only GPS receivers? M.S. Englewood, Colo.

A. Like the answer to most GPS questions, the response is "it depends." Fortunately, many GPS receivers take the guess work out of the process with a quality indicator that lets users know when they have recorded enough data for reliable processing.
      In this case, post-processed centimeter accuracy is dependent upon the length of the baseline (distance between control and remote point) and the availability and geometry of the visible satellites at the time the measurements are being recorded. For best results, remember these three rules of thumb:
• The amount of data you are required to collect increases as the baseline gets longer.
• Data collected from satellites positioned low in the sky requires more processing than data collected from satellites with higher elevation angles.
• The more satellites you use, the quicker you will achieve a solution.

QUESTIONS ABOUT GPS? Send your GPS questions to us at: EOM, Inc. 4901 E. Dry Creek Rd., Suite 170, Littleton, CO 80122, fax to 303-713-9944, or send via e-mail: [email protected]

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