PDOP: Position Dilution of Precision (PDOP), directly translated as “precision strength ”, usually translated as “relative error ”. Specific meaning is: due to the good or bad observation results and the geometry of the measured satellite and receiver between the relevant and great influence, so the calculation of the amount of error caused by the above is known as the accuracy of the strength of the degree of weakness. The better the distribution of satellites in the sky, the higher the positioning accuracy (the smaller the value, the higher the accuracy).PDOP represents a parameter that relates the 3D positional accuracy to the geometric configuration of the navigation station. In the global positioning (GPS) system, it is equal to the ratio of the radial error (1°) of the user's position to the measurement error (1°) of the distance from the user to the satellite.

        PDOP takes values in the range 0.5 - 99.9, and is the value of the open root sign of the sum of the squares of the errors in latitude, longitude, and elevation, etc., so that the square of PDOP = the square of HDOP + the square of VDOP.

        Geometrically, PDOP is divided equally in proportion to the volume of the cone formed by the receiver and the lines of the four satellites that can be observed.1 For good localization, a small value of PDOP, for example, 3. A value larger than 7 is considered poorer. Thus, small PDOP values are associated with satellites that are farther apart.

        In GPS navigation and positioning, we use the geometric accuracy factor (DOP, dilution of precision, also translated as precision attenuation factor) to measure the effect of the spatial geometric distribution of the observed satellites on the positioning accuracy.DOP is categorized as follows:

        PDOP (position dilution of precision) is a three-dimensional positional precision factor: it is the open root of the sum of the squares of the errors of latitude, longitude and elevation.

        TDOP (time dilution of precision) clock difference precision factor: for the receiver within the time table offset error value.

        HDOP (horizontal dilution of precision) horizontal component precision factor: for latitude and longitude and other error sum of the square of the open root value.

        VDOP (vertical dilution of precision) vertical component accuracy factor

        The size of the DOP value is directly proportional to the GPS positioning error, the larger the DOP value, the larger the positioning error and the lower the positioning accuracy.

        PDOP directly reflects the distribution of GPS satellites, when the PDOP is larger, it indicates that the geometric distribution of the four GPS satellites in the air is not too ideal, and the perimeter of the graph they form is too short, and the positioning accuracy is low, and vice versa.

        The accuracy decay factor (DOP) is an indicator of position quality. It is a calculation that takes into account the position of each satellite relative to the other satellites in the constellation (geometric position) to anticipate the positional accuracy that can be obtained with that constellation. A small DOP value indicates a higher probability of strong satellite geometric position and accuracy. A high DOP value indicates a lower probability of weak satellite geometry and accuracy.

         A GPS receiver can get many satellites positioning information at the same time, but in the precision positioning, as long as four satellite signals that is already enough, a good receiver will be able to determine how to pick up these satellite signals to calculate the more reliable signals, if the receiver is selected by the signals, there are two satellites are very close to each other, the two satellites signals in the angle of the place will have an overlap of the area generated, as the distance gets closer, the two satellite signals in the angle will have an overlap of the area generated. The closer the distance, the larger this area will be, the greater the error affecting the accuracy. If the satellites are selected at a distance from each other, the area where the signals intersect will be more clearly defined, and the error will of course be reduced considerably.

        This also shows that although the satellites directly above our heads have better signals and are easier to lock on to, they are not as useful as satellites at a lower angle.