Some GPS tests of satellite reception

Here we will describe some practical GPS tests about satellite reception. These GPS tests should not be considered scientific and all exclusive. However, the results will have practical value in the field.

A GPS receiver determines its Position (horizontal and vertical), its Velocity and the Time from the signals of at least four satellites by means of triangulation. The precision of the computations by triangulation depends on the constellation of all satellites of which the signals are taken into account (four or more). As the number and position of satellites will seldom be ideal, the maximum obtainable precision will be diluted in practice. Here we present the different terms of dilution of precision.

Dilution of precision (DOP) is a measure of the quality of the GPS data being received from the satellites. DOP is a mathematical representation for the quality of the GPS position solution. The main factors affecting DOP are the number of satellites being tracked and where these satellites are positioned in the sky. The effect of DOP can be resolved into HDOP, VDOP, PDOP and TDOP.

HDOP (Horizontal Dilution Of Precision) is a measure of how well the positions of the satellites, used to generate the Latitude and Longitude solutions, are arranged. PDOP less than 4 gives the best accuracy, between 4 and 8 gives acceptable accuracy and greater than 8 gives unacceptable poor accuracy. Higher HDOP values can be caused if the satellites are at high elevations.

VDOP (Vertical Dilution Of Precision) is a measure of how well the positions of the satellites, used to generate the vertical component of a solution, are arranged. Higher VDOP values mean less certainty in the solutions and can be caused if the satellites are at low elevations.

TDOP (Time Dilution Of Precision) is a measure of how the satellite geometry is affecting the ability of the GPS receiver to determine time.

PDOP (Positional Dilution OF Precision) is a measure of overall uncertainty in a GPS position solution with TDOP not included in the estimated uncertainty. The best PDOP (lowest value) would occur with one satellite directly overhead and three others evenly spaced about the horizon.
PDOP = SQRT(HDOP^2 + VDOP^2).

GDOP (Geometric Dilution Of Precision) is a measure of the overall uncertainty in a GPS position solution.
GDOP = SQRT(TDOP^2 + HDOP^2 + VDOP^2) or in another form
GDOP = SQRT(PDOP^2 + TDOP^2). GDOP value should be less than 5.

The Position Accuracy = Dilution Of Precision (DOP) times Measurement Precision. So, if the Measurement Precision = 1m and the DOP = 5, then the best position accuracy will be 5m.

PDOP, HDOP and VDOP are calculated in the GPS receiver and output in the NMEA sentence $GPGSA. The actual elevation and azimuth of the satellites in view are output in the NMEA sentence $GPGSV with a maximum of 4 per sentence. If there are more satellites in view, the sentence is simply repeated till all positions are transmitted.

In Test 1 we compare the performance of 3 different systems under different field conditions.

The Test 1 Page can be found here

We hope to do more GPS tests in the future.

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