Radio Altimeter — Page 270, Lesson 315

Right, let's get into the Radio Altimeter. This is a fundamental piece of kit for low-altitude work, especially approaches and terrain avoidance. First, the frequencies. Historically, two bands were used, but today we only use one. The current band is the SHF band, which stands for Super High Frequency, operating between 4200 and 4400 MHz. The old band was the UHF band, Ultra High Frequency, between 1600 and 1660 MHz. You won't see UHF used for this anymore. Now, the clever bit — how it actually measures height. The carrier frequency doesn't stay fixed. It's swept automatically, varying by ±50 MHz, and it does this 300 times a second. Why? Because at very low altitude, the reflected signal returns almost instantly. A wide sweep is needed to create a measurable frequency difference between the transmitted and received signals. That difference is what we use to compute height. The signal is transmitted downwards from a flush-mounted horn antenna. The beam it generates is conical or elliptical, and it's deliberately wide. It has to be wide enough so that even with pitch angles of ±30° and roll angles of ±60°, some portion of the beam still travels vertically. The height is determined from the shortest path to the ground, which will always be vertically below the aircraft. That's a key point — it's measuring the true vertical distance, not slant range. Because transmission is continuous, we can't use a single antenna for both sending and receiving. We need a separate receiving antenna, similar to the transmitting one. And it must be positioned far enough away from the transmitter to avoid interference with the transmitted signal. The radiated power is on the order of one watt. Now, the indicator. Let's look at the basic indicator first. The height scale is logarithmic. It's expanded from zero to 500 feet, giving you fine resolution at low altitude, and then it's a reduced, non-linear scale from 500 up to 2500 feet. So the scale is compressed as you go higher. There's a mask. The height pointer disappears behind it under three conditions: when altitude exceeds 2500 feet, when there's any fault in the transmitted signal, or when the altimeter is switched off. So if you don't see the pointer, one of those three things is happening. Then there's the failure warning flag. This appears when there's too much radio noise corrupting the returning signal, or if local reflections are received from the airframe itself, or in the event of a loss of power to the equipment. So it's your cue that the reading is unreliable. Next, the Press to Test button, which is also the Height Selector. It's a dual-purpose device. When pressed, the height pointer swings round to a known pre-set altitude. That's your confidence check — it tells you the equipment is likely to operate satisfactorily. When turned, it selects the reference Low Altitude Index to the desired height. So you set your decision height there. Finally, the Low Height Warning. The warning light illuminates if the aircraft is flown below any pre-selected height set by the pilot. And this is also marked audibly — there's an alert tone that sounds with increasing loudness from approximately 100 feet above the decision height setting, and it suddenly ceases when you go below. So you get both a visual and an aural cue. There's also an EFIS indicator, the Boeing style, which we'll look at separately. But that's the core of the radio altimeter — the frequency, the sweep, the antenna geometry, and the indicator functions.