Automatic Direction Finder (ADF) — Page 107, Lesson 91

Right, let's get into the ADF summary. This is the consolidation page for the whole Automatic Direction Finder chapter, so we're pulling everything together into one operational picture. First, the headline accuracy figure. The ADF is accurate to plus or minus 5 degrees within its designated operational coverage, and that's day only. Two things to lock in there. The accuracy figure refers to the measured bearing itself, so it does not include any compass error. And the day-only limitation is the big one — it's a direct consequence of the night effect, which we'll come back to in the errors list. Now let's build the system from the ground up, starting with the ground element, the NDB. There are two types. The Locator, abbreviated L, is used for an airfield let-down, and its range is 10 to 25 nautical miles. The En Route NDB is a navigation aid with a range of 50 nautical miles or more. Both are ground transmitters operating in the LF or MF band, 190 to 1750 kilohertz. Here's the range formula, and I want you to note the contrast carefully. The range in nautical miles is 3 times the square root of P, where P is the transmitter power in watts, and that's over water. Over land, the range is 2 times the square root of P. So the same transmitter gives you more range over water than over land — that's the coastal effect in action, and it's why the formula changes. Now the airborne side. The ADF equipment consists of aerials, a receiver, a control unit, and an indicator, which is either an RBI or an RMI. The principle of operation is that it derives a relative bearing by switched cardioids. A cardioid is a heart-shaped polar pattern, and by switching between cardioid patterns the receiver can determine the direction of the incoming signal relative to the aircraft's heading. That's the relative bearing. The frequencies match the ground side: 190 to 1750 kilohertz, LF and MF. Now the emission characteristics, and this is examinable detail. There are two modes. N0NA1A means BFO ON for tuning, identification and monitoring. N0NA2A means BFO ON for tuning, but OFF otherwise. BFO is the beat frequency oscillator — it makes the Morse identification audible. So in the A1A mode you keep the BFO on throughout, and in the A2A mode you switch it on only to tune, then switch it off. Presentation is on the RBI, the radio bearing indicator, or the RMI, the radio magnetic indicator. The difference is that the RBI shows the relative bearing, while the RMI combines it with the magnetic heading to show a magnetic bearing directly. The uses of the NDB are fourfold: homing, holding, approach, and as an en route navigation aid. Homing is flying directly towards the station, holding is the racetrack pattern, approach is the let-down, and en route is the long-range navigation. Now the errors, and this is the list you must know cold. Static interference from precipitation and thunderstorms. Station interference. Night effect — this is the big one, and it's why the accuracy is day only; at night the ionosphere reflects the signal and causes bearing errors. Mountain effect. Coastal refraction — the signal bends when it crosses a coastline. Quadrantal error — that's caused by the aircraft's own structure distorting the received signal. Bank angle, also called dip — when the aircraft banks, the loop aerial tilts and the bearing shifts. And finally, lack of failure warning — the ADF can fail without the pilot knowing, which is a serious operational limitation. So the summary is: ground NDB, airborne ADF, relative bearing by switched cardioids, day-only accuracy of plus or minus 5 degrees within the DOC, and a long list of errors you must manage. That figure pulls the whole summary together visually — the NDB types, the range formula, the airborne equipment, the principle, the frequencies, the emissions, the presentation, the uses, and the errors all in one diagram.