Let’s pick this up right where the transponder’s pulse rate left off — Page 257, Lesson 248

Let’s pick this up right where the transponder’s pulse rate left off. I told you that as the aircraft gets closer to the beacon, the interrogation rate climbs — and at lock-on, that rate is further reduced to about 25 pulse pairs per second. That’s the final, settled interrogation rate once the system has locked on to you. Now, here’s the danger that rate reduction is designed to prevent: beacon saturation. This happens at about 2700 pulse pairs per second — which is roughly 100 aircraft interrogating simultaneously. When the transponder is being hammered that hard, it can’t answer everyone properly. So the receiver’s gain is reduced, and it responds only to the strongest pulses. That means weaker signals — aircraft farther away or at a poorer angle — simply get ignored. The system sacrifices weaker targets to keep serving the strong ones. Next, the station ident. Every DME beacon transmits a three-letter identifier in Morse code. Here’s the catch: during the ident period, range information is not available. The transponder is busy sending the identifier, so it can’t answer range interrogations at that moment. You lose range data briefly while the ident is being sent. Now let’s talk about how a DME is paired with a VOR — this is the VOR/DME frequency pairing concept. There are three possible relationships. First, associated. This means the DME is co-located with the VOR, or within 100 feet of it in a TMA — that’s a Terminal Control Area — or within 2000 feet outside a TMA. When they’re associated, the call signs are the same and the frequencies are paired. You tune the VOR frequency and the DME channel is automatically selected. Second, not associated. This is when the DME serves the same location as the VOR but isn’t co-located. In that case, the third letter of the DME call sign is ‘Z’ — that’s your clue that it’s not physically with the VOR. The frequencies are still paired, but the call signs differ by that final letter. Third, separated. This is when the DME is more than 6 nautical miles from the VOR. Here the call signs are different entirely — no shared identity at all. Now, coverage. DME is line of sight — it needs a clear path between you and the beacon. That range is reduced by intervening high ground and by your bank angle — if you’re banking steeply, the aircraft structure can block the signal. The DOC — that’s the Document of Compliance, the published protected range — gives you the guaranteed coverage. And there’s an echo protection circuit that eliminates reflections — it filters out signals that have bounced off terrain or buildings so you only see the direct path. Finally, accuracy. The DME is accurate to ±0.25 nautical miles plus ±1.25% of the range. For precision systems, that tightens to ±0.2 nautical miles. Now, two important errors. First, slant range error — this becomes significant when your range is less than 3 times your height. If you’re close to the station and high up, the measured slant distance is noticeably longer than the true horizontal distance. Second, ground speed error — this increases as you go higher and closer to the station. The geometry of the slant range distorts your computed ground speed more severely in those conditions. So to tie it together: the system locks on at 25 ppps, saturates at 2700 ppps, identifies itself with a three-letter code that briefly blanks range, pairs with VORs in three distinct ways, works line-of-sight with echo protection, and gives you accuracy that degrades with slant range and height. That’s the complete DME picture.