Global Navigation Satellite Systems (GNSS) — Page 324, Lesson 316

Let’s start with the idea of Dilution of Precision, or DOP. This is the term we use to describe how the geometry of the satellites—the angle at which their position lines cut across each other—and any error in the pseudo-ranges, which is the time synchronization between the satellite clock and the receiver clock, will degrade the accuracy of the calculated position. So, DOP is a measure of how much the satellite geometry and timing errors weaken the precision of your fix. Now, DOP is divided into several specific types, and I want you to know each one by name and what it refers to. First, Horizontal Dilution of Precision, or HDOP. This refers to errors in the X and Y coordinates—that’s your horizontal position, your latitude and longitude. Next, Vertical Dilution of Precision, or VDOP. This refers to errors in the Z coordinate—that’s your altitude. Then we have Position Dilution of Precision, or PDOP, which is simply a combination of HDOP and VDOP. So PDOP covers both horizontal and vertical errors together. After that, Time Dilution of Precision, or TDOP, which refers to timing errors—the synchronization errors in the clocks. Finally, Geometric Dilution of Precision, or GDOP, which is a combination of PDOP and TDOP. So GDOP is the overall measure that includes position and time errors together. Now, how do we minimize the errors caused by PDOP? The excerpt tells us that PDOP errors are minimized by two things: first, the geometry of the positioning of the satellites, which we call SVs—that’s short for Space Vehicles—in their orbits, and second, by the receiver selecting the four best SVs to determine position. So the receiver is actively choosing which four satellites to use based on their geometry. The satellite geometry that provides the most accurate fixing information is one SV directly overhead the receiver, and the other three SVs close to the horizon, spaced 120° apart. That’s the ideal arrangement—one overhead, three spread evenly around the horizon. Let’s move on to the effect of aircraft manoeuvre. When the aircraft manoeuvres, part of the aircraft may shadow one or more of the in-use SVs. That means the aircraft’s structure blocks the signal from a satellite. There are two possible outcomes of this. First, while the SV is shadowed, the signal may be lost, resulting in degradation of accuracy. Second, the receiver may lock onto reflections from other parts of the aircraft, again with a reduction in accuracy. So either way, you lose accuracy. The effect of manoeuvre can be minimized by careful positioning of the aerial on the aircraft. The optimum position for the antenna is on top of the fuselage, close to the aircraft’s centre of gravity. That placement minimizes the chance of the aircraft shadowing the satellites. Finally, let’s look at Selective Availability, or SA. SA was introduced into GPS by the US DOD—that’s the US Department of Defense—in about 1995. It deliberately degraded the accuracy of the fixing on the C/A code, which is the Coarse/Acquisition code, the signal used by civilian users. So SA was a deliberate degradation of accuracy for civilian users. The USA withdrew SA at 0000 on 01 May 2000, and President Clinton stated that it would never be reintroduced. SA downgraded the accuracy of position derived from the C/A code to the order of 100 metres spherical error. So that’s the magnitude of the degradation—about 100 metres. SA was achieved by introducing random errors in the SV clock time, which is known as dithering the SV clock time. So the mechanism was to randomly perturb the satellite clock, which then degraded the pseudo-range measurements and thus the position accuracy. That covers the key points of this section: the types of DOP, the ideal satellite geometry, the effect of aircraft manoeuvre and antenna placement, and the history and mechanism of Selective Availability.