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Causes of Precision Error

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Causes of Precision Error

There are several phenomenon which can cause poor precision. For example, when satellite radio signals are transmitted, they are distorted by the troposphere and especially the ionosphere. In fact, satellites very low on the horizon are not good for getting a fix because the signals travel through so much of the atmosphere. Some GPS devices may even exclude these satellites from a fix to avoid the precision problems they would cause.

Figure 2-3: Satellite 1's radio signal travels through less of the atmosphere, resulting in less distortion. Satellite 2 is low on the horizon, however, resulting in significant atmospheric distortion.

Fortunately, atmospheric distortion can be measured and corrected for the most part. This is achieved by the use of GPS ground stations, fixed locations which constantly measure distortions in satellite radio signals. Calculated corrections are then broadcast by radio which, when combined with the actual satellite signal, gives a GPS receiver the ability to correct distortions in real-time.

Figure 2-4: Distortions in radio signals are corrected by combining satellite signals (1 and 2) with correction information transmitted via DGPS ground stations (3 and 4).

Precision errors can be compounded by slight inaccuracies in each satellite’s “ephemeris.” Ephemeris is a table giving the coordinates of a celestial body over time. If the satellite's actual course deviates from its ephemeris, precision can be further diluted. This sort of error can only be corrected by firing small rockets on the satellites themselves. Adjustments are transmitted from the GPS Master Control Station at Schriever Air Force Base in Colorado Springs, Colorado.

Figure 2-5: Deviations in a satellite's actual orbit path can also cause loss in precision.

As I covered in Part One of this article, each GPS satellite has four on-board atomic clocks: two cesium atomic clocks and two rubidium atomic clocks which are accurate to 1 second per 300,000 years! Still, even microfractions of a second error in these clocks can cause positional error because distance is measured at the speed of light. The Master Control Station keeps these errors at a minimum by uploading corrective information to satellites twice a day, every day.

The last detriment to GPS precision is an effect called "multipath," which is an effect caused when a receiver receives not only the satellite's signal, but additional signals which bounced off buildings and other obstacles. Deflected signals take a longer path to the receiver and are thus delayed. If they are used by the receiver, the measured distance to a satellite is overestimated, resulting in inaccurate multilateration. More advanced receivers solve multipath problems by utilizing only the first signal detected (which is the most direct path from the satellite), then discarding any delayed signals.

Figure 2-6: A receiver is confused by "multipath," where several reflected signals are received (red) along with the direct radio signal (green).

Solving all of these precision problems is done by using more sophisticated GPS receivers which use real-time correction data such as WAAS (for North America) and EGNOS (for Europe). Yet, these problems cause relatively small inaccuracies when compared with Geometric Dilution of Precision, which can cause a receiver to be inaccurate by more than an American football field. Fortunately, Geometric DOP is the easiest to manage with the right programming techniques.
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