Dilution of precision (DOP) or geometric dilution of precision (GDOP) is a term, used in GPS and geomatics engineering, to specify the additional multiplicative effect of GPS satellite geometry on GPS precision.
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The concept of dilution of precision originated with users of the LoranC navigation system^{[1]}. More recently, the term has come into much wider usage with the development and adaptation of GPS.
The signal from each GPS satellite has level of precision; depending on the relative geometry of the satellites, these precisions can be combined to give amplified or greatly compressed precision. When visible GPS 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 GPS positional precision due to the wider angular separation between the satellites used to calculate a GPS 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, VDOP, PDOP, and TDOP are respectively horizontal, vertical, positional (3D), and temporal dilution of precision. They follow mathematically from the positions of the usable satellites. GPS 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 electroniccountercountermeasures (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 and it is roughly interpreted as ratio of position error to the range error. Now let's imagine that a square pyramid is formed by lines joining the four satellites and receiver at the point. The larger its volume, the better the value of GDOP; the smaller its volume, the worse the value of GDOP will be. Similarly, the greater the number of satellites, the better the value of GDOP.
DOP Value  Rating  Description 

1  Ideal  This is the highest possible confidence level to be used for applications demanding the highest possible precision at all times. 
12  Excellent  At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications. 
25  Good  Represents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable inroute navigation suggestions to the user. 
510  Moderate  Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended. 
1020  Fair  Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location. 
>20  Poor  At this level, measurements are inaccurate by as much as 300 meters with a 6 meter accurate device (50 DOP × 6 meters) and should be discarded. 
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.
As a first step in computing DOP, consider the unit vectors from the receiver to satellite i: where and where and denote the position of the receiver and and denote the position of satellite i. Formulate the matrix, A, as:
The first three elements of each row of A are the components of a unit vector from the receiver to the indicated satellite. The elements in the fourth column are c which denotes the speed of light. Formulate the matrix, Q, as:
This computation is in accordance with "Section 1.4.2 of PRINCIPLES OF SATELLITE POSITIONING" where the weighting matrix, P, has been set to the identity matrix.
The elements of Q are designated as:
The Greek letter is used quite often where we have used d. However the elements of Q do not represent variances and covariances as they are defined in probability and statistics. Instead, they are strictly geometric terms. Therefore, d, as in dilution of precision, is used. PDOP, TDOP and GDOP are given by:
in agreement with "Section 1.4.9 of PRINCIPLES OF SATELLITE POSITIONING".
The horizontal dilution of precision, , and the vertical dilution of precision, , are both dependent on the coordinate system used. To correspond to the local horizon plane and the local vertical, x, y, and z should denote positions in either a north, east, down coordinate system or a south, east, up coordinate system.
