If you wish to contribute or participate in the discussions about articles you are invited to join Navipedia as a registered user

# Positioning Error

Fundamentals
Title Positioning Error
Author(s) J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain.
Year of Publication 2011

The formal, predicted and measured accuracy parameters are discussed in this article. The first one is a measure of the uncertainty of the estimates, according to the statistical characterisation of the errors and the linear model used for the position estimate.

The predicted accuracy, provides the expected position accuracy based on a simple statistical description of the measurement errors. Its computation does not require the measurements, but only its standard deviation , and the approximated satellites and user location coordinates. Thence, it can be computed at any point and at any time with the almanac, without needing the measurements.

The measured accuracy is the measure of the true error and must be assessed with the actual measurements.

## Formal Accuracy

Denoting as , , , the diagonal elements of matrix  of (see Best Linear Unbiased Minimum-Variance Estimator (BLUE))



we call formal errors of the estimated components of vector  to the standard deviations:



The previous expressions provides a characterisation of the quality of the coordinates and clock estimates (linked to the measurement error model assumed). Thence, they are not the actual error, but only a measure of the uncertainty of the error estimates.

Previous expressions give the errors in the ECEF XYZ coordinates. Nevertheless, it is usually more meaningful to a user to think in terms of horizontal and vertical position error, or East, North and Up (ENU) coordinates.

Let  the transformation matrix (3) of ENU coordinates to XYZ, (i.e., which columns are the unit orthogonal vectors  as expressed in the XYZ coordinates system [footnotes 1] at a point of longitude  and latitude ) (see Transformations between ECEF and ENU coordinates):



thence,  is given, where  is the sub-matrix of  containing solely the geometric components [footnotes 2]. From this covariance matrix, they can be defined:



For the horizontal error, there are usually also defined:



It is usual to express the Horizontal error in two main directions where the covariance matrix  is diagonal. They define two orthogonal axis, associated to the minor and major axis of the error ellipse. The major axis is given by:



## Predicted Accuracy: Dilution of Precision

Assuming the simple statistical model:



the covariance matrix of  estimate is given by (see Parameters adjustment)



This expression depends upon two factors: 1) the measurements (prefit-residuals) variance , and 2) the geometry matrix (), which is linked only to the receiver-satellite rays geometry, (see Code Based Positioning (SPS)).

The Root Mean Square Error (RMSE) is given by:



what means that, the trace of matrix  is a scale factor on  for the RMSE.

Notice that, as the matrix  does not depend on the measurements, but only on the geometry, it can be computed from the almanac (because not accurate satellite positions are needed), i.e., does not require receiver measurements.

On the basis of this simple approach, the following Dilution of Precision (DOP) parameters are defined:



• Geometric Dilution of Precision:

• Position Dilution of Precision:

• Time Dilution of Precision:


Using (3) the sub-matrix matrix  of  can be transformed to ENU coordinates as , in order to define:

• Horizontal Dilution of Precision:


• Vertical Dilution of Precision:


Thence, estimations of the expected accuracy are given by:

• GDOP,  ... geometric precision in position and time
• PDOP,  ... precision in position
• TDOP ,  ... precision in time
• HDOP ,  ... precision in horizontal positioning
• VDOP ,  ... precision in vertical positioning

where, basically, DOP represents an approximate ratio factor between the precision in the measurements () and in positioning. This ratio is computed only from the satellites-receiver geometry.

## Measured Accuracy

Let ,  and  the errors in the East, North and Vertical components of the -th position estimate sample. The RMS vertical, horizontal (2-D) and 3-D errors are defined as [footnotes 3]:





Other measures of the quality of the position estimates are:

• 50th or 95th percentiles of horizontal, vertical and 3-D errors.
• CEP: Circular Error Probable, as the 50th percentile of horizontal error.
• SEP: Spherical Error Probable, as the 50th percentile of 3-D error.
• 2drms: calculated as twice the 2-D rms horizontal error given by (12).

Assuming that the position estimates follows a multivariate normal distribution centred at the true position and the errors in north, east and up are uncorrelated, with  and , thence, the following relations can be considered ( [Misra and Enge, 2001] [1]):

rms vertical error  vertical error (95%)
 2drms
0.9  3-D rms error
2-D rms error  2drms
 horizontal error (95%)
1.2  CEP
3-D rms error  2-D rms error
1.2  horizontal error (95%)
1.3  SEP

A wide discussion on such accuracy equivalences can be found in [Diggelen, 2007] [2].

## Notes

1. ^ Note  are the ellipsoidal coordinates and, thence, the vector  is orthogonal to the tangent plane to the ellipsoid, which is defined by . If  are taken as the spherical latitude and longitude, thence, the vector  is in the radial direction and  defines the tangent plane to the sphere.
2. ^ The matrix  can be obtained directly in the ENU coordinates, using the geometry matrix  computed in the ENU coordinates. In this local system the rows of matrix  (see Code Based Positioning (SPS)) are , where  and  are the elevation and azimuth angle of the satellite  observed from the receiver position, (see Transformations between ECEF and ENU coordinates).
3. ^ Notice that the  equals to the standard derivation  only when the mean position error is zero. Indeed, let 

## References

1. ^ [Misra and Enge, 2001] Misra, P. and Enge, P., 2001. Global Positioning System. Signals, Measurements and Performance.. Ganga-Jamuna Press, Massachusetts, U.S.
2. ^ [Diggelen, 2007] Diggelen, F. v., 2007. GNSS Accuracy: Lies, Damn Lies, and Statistics. GPS World. 18(1), pp. 26-32.