First, let's anchor the geometry — Page 69, Lesson 64

Let me walk you through the Doppler Navigation System, because this is where the aircraft's own motion becomes the navigation sensor. First, let's anchor the geometry. The aircraft has a fore/aft axis — that's the line running from nose to tail, and it represents heading. Then we have the port and starboard beams — the beams pointing out to the left and right sides of the aircraft — and their alignment represents track. The difference between heading and track is drift. That's the angle between where the nose points and where the aircraft actually travels over the ground. Now, the Doppler system itself. The Doppler continuously updates the values of aircraft drift and ground speed. So it's not a one-shot measurement — it keeps refreshing these two quantities as you fly. Here's how the early systems worked. The aircraft's departure point was loaded into a navigation computer. So you told the computer where you started. Then the values of drift and ground speed — which the Doppler was continuously producing — together with an input of aircraft heading, were also fed into that computer. The computer took all three inputs — drift, ground speed, and heading — and converted them into aircraft position. The calculated position was then displayed in one of two ways. Either as latitude and longitude, or as distances in nautical miles along and across track. So you could see your position as a coordinate pair, or as how far you'd travelled along your intended track and how far you'd drifted off it, measured in nautical miles. Now, Figure 5.3 shows the Control and Display Unit — the CDU — for the B-52 system I mentioned earlier. That's the Racal RNS 252 Navigation Computer Unit. The CDU is the pilot's interface — it's where you load the departure point and where the computed position is displayed. So the whole chain is: the Doppler measures drift and ground speed continuously, you feed in heading, the navigation computer combines these with the departure point, and out comes a position — either as lat/long or as along-track and across-track distances in nautical miles. That's the core of Doppler navigation.