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 (figure 2-1). Some GPS devices may even exclude these satellites from a fix to
avoid the precision problems they would cause.
Fortunately, atmospheric distortion can be measured and corrected for the most
part. This is achieved by the use of GPS ground stations (figure 2-2), fixed locations which
constantly measure distortions in satellite radio signals. Calculated corrections are then broadcasted by radio which, when combined with the
actual satellite signal, gives a GPS
receiver the ability to correct distortions in real-time.
Precision errors can be compounded by slight inaccuracies in each satellite’s
“ephemeris” (figure 2-3). 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.
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" (figure 2-4), 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.
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.