Dilution of precision (navigation)

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Dilution of precision (DOP), or geometric dilution of precision (GDOP), is a term used in satellite navigation and geomatics engineering to specify the error propagation as a mathematical effect of navigation satellite geometry on positional measurement precision.

Understanding the Geometric Dilution of Precision (GDOP) with a simple example. In A someone has measured the distance to two landmarks, and plotted their point as the intersection of two circles with the measured radius. In B the measurement has some error bounds, and their true location will lie anywhere in the green area. In C the measurement error is the same, but the error on their position has grown considerably due to the arrangement of the landmarks.

Navigation satellites with poor geometry for Geometric Dilution of Precision (GDOP).

Navigation satellites with good geometry for Geometric Dilution of Precision (GDOP).

Introduction[]

The concept of dilution of precision (DOP) originated with users of the Loran-C navigation system.^[1] The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation. This can be defined as:^[2]

${\displaystyle GDOP={\frac {\Delta ({\rm {Output\ Location}})}{\Delta ({\rm {Measured\ Data}})}}}$

Conceptually you can geometrically imagine errors on a measurement resulting in the ${\displaystyle \Delta ({\rm {Measured\ Data}})}$ term changing. Ideally small changes in the measured data will not result in large changes in output location. The opposite of this ideal is the situation where the solution is very sensitive to measurement errors. The interpretation of this formula is shown in the figure to the right, showing two possible scenarios with acceptable and poor GDOP.

More recently, the term has come into much wider usage with the development and adoption of GPS. Neglecting ionospheric ^[3] and tropospheric^[4] effects, the signal from navigation satellites has a fixed precision. Therefore, the relative satellite-receiver geometry plays a major role in determining the precision of estimated positions and times. Due to the relative geometry of any given satellite to a receiver, the precision in the pseudorange of the satellite translates to a corresponding component in each of the four dimensions of position measured by the receiver (i.e., ${\displaystyle x}$, ${\displaystyle y}$, ${\displaystyle z}$, and ${\displaystyle t}$). The precision of multiple satellites in view of a receiver combine according to the relative position of the satellites to determine the level of precision in each dimension of the receiver measurement. When visible navigation satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Consider two overlapping rings, or annuli, of different centres. If they overlap at right angles, the greatest extent of the overlap is much smaller than if they overlap in near parallel. Thus a low DOP value represents a better positional precision due to the wider angular separation between the satellites used to calculate a unit's position. Other factors that can increase the effective DOP are obstructions such as nearby mountains or buildings.

DOP can be expressed as a number of separate measurements:

• HDOP – horizontal dilution of precision
• VDOP – vertical dilution of precision
• PDOP – position (3D) dilution of precision
• TDOP – time dilution of precision
• GDOP – geometric dilution of precision

These values follow mathematically from the positions of the usable satellites. Signal receivers allow the display of these positions (skyplot) as well as the DOP values.

The term can also be applied to other location systems that employ several geographical spaced sites. It can occur in electronic-counter-counter-measures (electronic warfare) when computing the location of enemy emitters (radar jammers and radio communications devices). Using such an interferometry technique can provide certain geometric layout where there are degrees of freedom that cannot be accounted for due to inadequate configurations.

The effect of geometry of the satellites on position error is called geometric dilution of precision (GDOP) and it is roughly interpreted as ratio of position error to the range error. Imagine that a square pyramid is formed by lines joining four satellites with the receiver at the tip of the pyramid. The larger the volume of the pyramid, the better (lower) the value of GDOP; the smaller its volume, the worse (higher) the value of GDOP will be. Similarly, the greater the number of satellites, the better the value of GDOP.

Interpretation[]

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The DOP factors are functions of the diagonal elements of the covariance matrix of the parameters, expressed either in a global or a local geodetic frame.

Computation[]

As a first step in computing DOP, consider the unit vectors from the receiver to satellite i:

${\displaystyle \left({\frac {(x_{i}-x)}{R_{i}}},{\frac {(y_{i}-y)}{R_{i}}},{\frac {(z_{i}-z)}{R_{i}}}\right)}$

${\displaystyle R_{i}={\sqrt {(x_{i}-x)^{2}+(y_{i}-y)^{2}+(z_{i}-z)^{2}}}}$

where ${\displaystyle x,y,z}$ denote the position of the receiver and ${\displaystyle x_{i},y_{i},z_{i}}$ denote the position of satellite i. Formulate the matrix, A, which (for 4 pseudorange measurement residual equations) is:

${\displaystyle A={\begin{bmatrix}{\frac {(x_{1}-x)}{R_{1}}}&{\frac {(y_{1}-y)}{R_{1}}}&{\frac {(z_{1}-z)}{R_{1}}}&-1\\{\frac {(x_{2}-x)}{R_{2}}}&{\frac {(y_{2}-y)}{R_{2}}}&{\frac {(z_{2}-z)}{R_{2}}}&-1\\{\frac {(x_{3}-x)}{R_{3}}}&{\frac {(y_{3}-y)}{R_{3}}}&{\frac {(z_{3}-z)}{R_{3}}}&-1\\{\frac {(x_{4}-x)}{R_{4}}}&{\frac {(y_{4}-y)}{R_{4}}}&{\frac {(z_{4}-z)}{R_{4}}}&-1\end{bmatrix}}}$

The first three elements of each row of A are the components of a unit vector from the receiver to the indicated satellite. The last element of each row refers to the partial derivative of pseudorange w.r.t. receiver's clock bias. Formulate the matrix, Q, as the covariance matrix resulting from the least-squares normal matrix:

${\displaystyle Q=\left(A^{T}A\right)^{-1}}$

The elements of Q are designated as:

${\displaystyle Q={\begin{bmatrix}\sigma _{x}^{2}&\sigma _{xy}&\sigma _{xz}&\sigma _{xt}\\\sigma _{xy}&\sigma _{y}^{2}&\sigma _{yz}&\sigma _{yt}\\\sigma _{xz}&\sigma _{yz}&\sigma _{z}^{2}&\sigma _{zt}\\\sigma _{xt}&\sigma _{yt}&\sigma _{zt}&\sigma _{t}^{2}\end{bmatrix}}}$

PDOP, TDOP and GDOP are given by:^[6]

${\displaystyle {\begin{aligned}PDOP&={\sqrt {\sigma _{x}^{2}+\sigma _{y}^{2}+\sigma _{z}^{2}}}\\TDOP&={\sqrt {\sigma _{t}^{2}}}\\GDOP&={\sqrt {PDOP^{2}+TDOP^{2}}}\\\end{aligned}}}$

Notice GDOP is the square root of the trace of the ${\displaystyle Q}$ matrix.

The horizontal and vertical dilution of precision,

${\displaystyle HDOP={\sqrt {\sigma _{n}^{2}+\sigma _{e}^{2}}}}$

${\displaystyle VDOP={\sqrt {\sigma _{u}^{2}}}}$,

are both dependent on the coordinate system used. To correspond to the local horizon plane and the local vertical in either a north, east, up coordinate system.

See also[]

• Circular error probable
• GNSS positioning calculation

References[]

Further reading[]

• DOP Factors
• Writing Your Own GPS Applications: Part 2 - Causes of Precision Error
• manually calculating GDOP
• HDOP AND GPS HORIZONTAL POSITION ERRORS

• Article on DOP and Trimble's program: Determining Local GPS Satellite Geometry Effects On Position Accuracy.
• Notes & GIF image on manually calculating GDOP: Geographer's Craft
• GPS Errors & Estimating Your Receiver's Accuracy: Sam Wormley's GPS Accuracy Web Page
• GPS Accuracy, Errors & Precision: Radio-Electronics.com