GPS Error Sources

GPS errors are a combination of noise, bias, blunders.

Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter).
Bias errors result from Selective Availability and other factors
Selective Availability (SA)
SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. The potential accuracy of the C/A code of around 30 meters is reduced to 100 meters (two standard deviations).
The SA bias on each satellite signal is different, and so the resulting position solution is a function of the combined SA bias from each SV used in the navigation solution. Because SA is a changing bias with low frequency terms in excess of a few hours, position solutions or individual SV pseudo-ranges cannot be effectively averaged over periods shorter than a few hours. Differential corrections must be updated at a rate less than the correlation time of SA (and other bias errors).
Other Bias Error sources;
SV clock errors uncorrected by Control Segment can result in one meter errors.
Ephemeris data errors: 1 meter
Tropospheric delays: 1 meter. The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters.
Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual.
Multipath: 0.5 meters. Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. Multipath is difficult to detect and sometime hard to avoid.
Blunders can result in errors of hundred of kilometers.
Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers.
User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters.
Receiver errors from software or hardware failures can cause blunder errors of any size.
Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution.

Geometric Dilution of Precision (GDOP) and Visibility
GPS ranging errors are magnified by the range vector differences between the receiver and the SVs. The volume of the shape described by the unit-vectors from the receiver to the SVs used in a position fix is inversely proportional to GDOP.
Poor GDOP, a large value representing a small unit vector-volume, results when angles from receiver to the set of SVs used are similar.
Poor GDOP

Good GDOP, a small value representing a large unit-vector-volume, results when angles from receiver to SVs are different.
Good GDOP
GDOP is computed from the geometric relationships between the receiver position and the positions of the satellites the receiver is using for navigation. For planning purposes GDOP is often computed from Almanacs and an estimated position. Estimated GDOP does not take into account obstacles that block the line-of-sight from the position to the satellites. Estimated GDOP may not be realizable in the field.
Good Computed GDOP and Bad Visibility Equals Poor GDOP
GDOP terms are usually computed using parameters from the navigation solution process.
Pseudo-Range Navigation Solution Example refer page
GDOP Computation Example
In general, ranging errors from the SV signals are multiplied by the appropriate GDOP term to estimate the resulting position or time error. Various GDOP terms can be computed from the navigation covariance matrix. ECEF XYZ DOP terms can be rotated into a North-East Down (NED) system to produce local horizontal and vertical DOP terms.
GDOP Components
PDOP = Position Dilution of Precision (3-D), sometimes the Spherical DOP.
HDOP = Horizontal Dilution of Precision (Latitude, Longitude).
VDOP = Vertical Dilution of Precision (Height).
TDOP = Time Dilution of Precision (Time).
While each of these GDOP terms can be individually computed, they are formed from covariances and so are not independent of each other. A high TDOP (time dilution of precision), for example, will cause receiver clock errors which will eventually result in increased position errors.

Differential GPS (DGPS) Techniques

The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal.
Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution, DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV. These corrections are passed to the remote, or rover, receiver which must be capable of applying these individual pseudo-range corrections to each SV used in the navigation solution. Applying a simple position correction from the reference receiver to the remote receiver has limited effect at useful ranges because both receivers would have to be using the same set of SVs in their navigation solutions and have identical GDOP terms (not possible at different locations) to be identically affected by bias errors.

Differential Code GPS (Navigation)

Differential corrections may be used in real-time or later, with post-processing techniques.
Real-time corrections can be transmitted by radio link. The U. S. Coast Guard maintains a network of differential monitors and transmits DGPS corrections over radiobeacons covering much of the U. S. coastline. DGPS corrections are often transmitted in a standard format specified by the Radio Technical Commission Marine (RTCM).
Corrections can be recorded for post processing. Many public and private agencies record DGPS corrections for distribution by electronic means.
Private DGPS services use leased FM sub-carrier broadcasts, satellite links, or private radio-beacons for real-time applications.
To remove Selective Availability (and other bias errors), differential corrections should be computed at the reference station and applied at the remote receiver at an update rate that is less than the correlation time of SA. Suggested DGPS update rates are usually less than twenty seconds.
DGPS removes common-mode errors, those errors common to both the reference and remote receivers (not multipath or receiver noise). Errors are more often common when receivers are close together (less than 100 km). Differential position accuracies of 1-10 meters are possible with DGPS based on C/A code SPS signals.

Differential Code-Phase Navigation
Errors Reduced by Differential Corrections

all about global positioning system page: 1,2,3,4,5,6,7,8.9.10,11,12

previous || Gps home || next