The difference between those two alignments is the drift angle — Page 69, Lesson 64

Right, so we’ve just covered how the Doppler beams are aligned — the fore and aft beams along the aircraft’s heading axis, and the port and starboard beams aligned to track. The difference between those two alignments is the drift angle. That’s the geometric heart of the system. Now let’s look at what the Doppler system actually does with that information. The key sentence here: The Doppler continuously updates the values of aircraft drift and ground speed. So it’s not a one-shot measurement — it’s a continuous, real-time update of two quantities: drift and ground speed. Now, in early systems, here’s how the navigation worked. The aircraft’s departure point was loaded into a navigation computer. So you tell the computer where you started. Then, the values of drift and ground speed — the ones Doppler is continuously measuring — together with an input of aircraft heading, were fed into that same computer. So the computer receives three inputs: drift, ground speed, and heading. The computer then converted them into aircraft position. So it takes those three inputs and works out where the aircraft is. And the calculated position was then displayed as latitude and longitude, or alternatively as distances in nautical miles along and across track. So you have two display options: either a lat/long position, or a cross-track and along-track distance in nautical miles. Now, Figure 5.3 shows the Control and Display Unit (CDU) — that’s the CDU — for the B-52 system mentioned earlier. That’s the Racal RNS 252 Navigation Computer Unit. So this is the physical unit the crew uses to load the departure point and read the computed position. Let me make sure the flow is clear: Doppler measures drift and ground speed continuously. Those two, plus heading, go into the navigation computer. The computer, knowing where you departed from, converts all that into a position — either lat/long or along/cross-track distance in nautical miles. And the CDU is the interface for that. That’s the complete Doppler navigation loop as described here.