Positioning Modes of Operation Chapter 6OEM4 Family Installation and Operation User Manual Rev 12 67The position averaging function is useful for obtaining the WGS84 position of a point to a reasonableaccuracy without having to implement differential GPS. It is interesting to note that even a six houroccupation can improve single-point GPS accuracy from over 1.5 meters to better than a meter. Thisimproved accuracy is primarily due to the reductions of the multipath errors in the GPS signal.Again, it is necessary to keep in mind that the resulting standard deviations of the position averagingcan vary quite a bit, but improve over longer averaging times. To illustrate, the position averagingfunction was run for a period of 40 hours. The resulting standard deviation in latitude varied from0.152 to 1.5589 meters. Similarly, the variation in longitude and height were 0.117 to 0.819 metersand 0.275 to 2.71 meters respectively. This degree of variation becomes larger for averaging periodsof less than 12 hours due to changes in the satellite constellation. The graph provides some indicationof the accuracy one may expect from single-point position averaging.The next section deals with the type of GPS system errors that can affect accuracy in single-pointoperation.6.1.1 GPS System ErrorsIn general, GPS SPS C/A code single-point pseudorange positioning systems are capable of absoluteposition accuracies of about 1.8 meters or less. This level of accuracy is really only an estimation, andmay vary widely depending on numerous GPS system biases, environmental conditions, as well as theGPS receiver design and engineering quality.There are numerous factors which influence the single-point position accuracies of any GPS C/A codereceiving system. As the following list will show, a receiver’s performance can vary widely whenunder the influences of these combined system and environmental biases.• Ionospheric Group Delays – The earth’s ionospheric layers cause varying degrees of GPSsignal propagation delay. Ionization levels tend to be highest during daylight hours causingpropagation delay errors of up to 30 meters, whereas night time levels are much lower andmay be as low as 6 meters.• Tropospheric Refraction Delays – The earth’s tropospheric layer causes GPS signalpropagation delays. The amount of delay is at the minimum (about three metres) for satellitesignals arriving from 90 degrees above the horizon (overhead), and progressively increasesas the angle above the horizon is reduced to zero where delay errors may be as much as 50metres at the horizon.• Ephemeris Errors – Some degree of error always exists between the broadcast ephemeris’predicted satellite position and the actual orbit position of the satellites. These errors willdirectly affect the accuracy of the range measurement.• Satellite Clock Errors – Some degree of error also exists between the actual satellite clocktime and the clock time predicted by the broadcast data. This broadcast time error will causesome bias to the pseudorange measurements.• Receiver Clock Errors – Receiver clock error is the time difference between GPS receivertime and true GPS time. All GPS receivers have differing clock offsets from GPS time thatvary from receiver to receiver by an unknown amount depending on the oscillator type andquality (TCXO vs. OCXO, etc.). However, because a receiver makes all of its single-pointpseudorange measurements using the same common clock oscillator, all measurements will
