User's Manual Part 1

Positioning Modes of Operation Chapter 5
OEMV Family Installation and Operation User Manual Rev 5B 77
The position averaging function is useful for obtaining the WGS84 position of a point to a reasonable
accuracy without having to implement differential GPS. It is interesting to note that even a six hour
occupation can improve single-point GPS accuracy from over 1.5 meters to better than a meter. This
improved 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 averaging
can vary quite a bit, but improve over longer averaging times. To illustrate, the position averaging
function was run for a period of 40 hours. The resulting standard deviation in latitude varied from
0.152 to 1.5589 meters. Similarly, the variation in longitude and height were 0.117 to 0.819 meters
and 0.275 to 2.71 meters respectively. This degree of variation becomes larger for averaging periods
of less than 12 hours due to changes in the satellite constellation. The graph provides some indication
of 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-point
operation.
5.1.1 GPS System Errors
In general, GPS SPS C/A code single-point pseudorange positioning systems are capable of absolute
position accuracies of about 1.8 meters or less. This level of accuracy is really only an estimation, and
may vary widely depending on numerous GPS system biases, environmental conditions, as well as the
GPS receiver design and engineering quality.
There are numerous factors which influence the single-point position accuracies of any GPS C/A code
receiving system. As the following list shows, a receiver’s performance can vary widely when under
the influences of these combined system and environmental biases.
• Ionospheric Group Delays The earth’s ionospheric layers cause varying degrees of
GPS signal propagation delay. Ionization levels tend to
be highest during daylight hours causing propagation
delay errors of up to 30 meters, whereas night time
levels are much lower and may be as low as 6 meters.
• Tropospheric Refraction Delays The earth’s tropospheric layer causes GPS signal
propagation delays. The amount of delay is at the
minimum (about three metres) for satellite signals
arriving from 90 degrees above the horizon (overhead),
and progressively increases as the angle above the
horizon is reduced to zero where delay errors may be as
much as 50 metres 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
directly affect the accuracy of the range measurement.
• Satellite Clock Errors Some degree of error also exists between the actual
satellite clock time and the clock time predicted by the
broadcast data. This broadcast time error causes some
bias to the pseudorange measurements.
• Receiver Clock Errors Receiver clock error is the time difference between GPS
receiver time and true GPS time. All GPS receivers have
differing clock offsets from GPS time that vary from
receiver to receiver by an unknown amount depending
on the oscillator type and quality (TCXO versus OCXO,