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

Let’s pick up right where the geometry of the satellites starts to bite into your accuracy. I want to walk you through Dilution of Precision, or DOP. Here’s the core idea: the satellite geometry — that’s the angle of cut between your position lines — and any error in the pseudo-ranges, which comes from time synchronization, will degrade the accuracy of the calculated position. So even if every measurement were perfect, the way the satellites are arranged in the sky can stretch or shrink your error. That’s what DOP quantifies. Now, DOP is further divided into five specific types, and I want you to hold each one clearly. First, Horizontal Dilution of Precision, or HDOP. This refers to errors in the X and Y coordinates — that’s your horizontal plane, your latitude and longitude. Second, Vertical Dilution of Precision, VDOP. This refers to errors in the Z coordinate — that’s your altitude. Third, Position Dilution of Precision, PDOP. This is a combination of HDOP and VDOP — so it covers your three-dimensional position error. Fourth, Time Dilution of Precision, TDOP. This refers to timing errors — the clock errors in the system. And fifth, Geometric Dilution of Precision, GDOP. This is a combination of PDOP and TDOP — so it’s the overall geometric error including both position and time. Now, here’s the key operational point. Errors caused by PDOP are minimized by two things: the geometry of the positioning of the Space Vehicles, the SVs, in their orbits, and by the receiver selecting the four best SVs to determine position. So the receiver isn’t just grabbing any four satellites — it’s actively choosing the four that give the best geometry. What does the best geometry look like? The SV 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 up top, three spread evenly around the horizon. That gives you the tightest, most accurate fix. Let me show you that contrast visually — good geometry versus poor geometry. Now, let’s move to the effect of aircraft manoeuvre. When you manoeuvre the aircraft, part of the aircraft may shadow one or more of the in-use SVs. There are two possible outcomes. First, whilst 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 you lose the direct signal, or you get a reflected, multipath signal — both hurt you. 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 way, during manoeuvres, the antenna is least likely to be shadowed by the aircraft structure. Finally, let’s talk about Selective Availability, or SA. SA was introduced into GPS by the US Department of Defense, the US DOD, in about 1995. It deliberately degraded the accuracy of the fixing on the C/A code — that’s the Coarse/Acquisition code, the one used for civilian users. So civilian users got a deliberately degraded position. 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. And how was SA achieved? By introducing random errors in the SV clock time — known as dithering the SV clock time. So they deliberately jittered the satellite clock to ruin the pseudo-range measurements for civilian users. So to tie it together: DOP tells you how good your geometry is, the receiver picks the best four SVs to minimise PDOP, manoeuvre can shadow satellites and hurt accuracy unless the antenna is well placed, and SA was a deliberate military degradation that’s now been switched off. That’s the full picture on DOP, manoeuvre effects, and Selective Availability.