Radio Altimeter — Page 270, Lesson 315

Let's start with the radio altimeter. This is the instrument that tells you exactly how high you are above the ground directly below you — not above sea level, but above the terrain. That's a crucial difference, and it's why this instrument is so important for approaches and for terrain awareness. First, the frequencies. Two frequency bands have been used in the past, but only one is used at present: the SHF band. That stands for Super High Frequency, and it runs from 4200 MHz to 4400 MHz. The older band was the UHF band, Ultra High Frequency, from 1600 MHz to 1660 MHz — that one is no longer used. Now, here's the clever part. The radio altimeter doesn't just send out a single fixed frequency. The carrier frequency is swept — it's varied automatically by ±50 MHz, and it does this 300 times a second. So the transmitted signal is continuously changing frequency, sweeping up and down. Why sweep at all? Because of the way the height is measured. At very low altitude, the reflection returns almost instantaneously. If the frequency weren't changing, there'd be no measurable difference between the transmitted and received signal. But because the frequency is sweeping, the returned signal has a slightly different frequency than what's being transmitted at that moment. That frequency difference is what you measure, and it tells you the height. At very low altitude, you need a wide sweep to get a measurable frequency difference — that's why the sweep is ±50 MHz. The signal is transmitted downwards from a flush mounted horn antenna. "Flush mounted" means the antenna sits level with the aircraft skin, not sticking out. The beam it generates is conical or elliptical, and it's wide enough that even with pitch angles of ±30° and roll angles of ±60°, some portion of the beam still travels vertically. That's essential, because the height is determined from the shortest path to the ground — and the shortest path is always vertically below the aircraft. So even if you're banked or pitched, the instrument still measures the true vertical height. Because the transmission is continuous, you can't use the same antenna for transmitting and receiving — the transmitted signal would swamp the receiver. So there's a separate receiving antenna, similar to the transmitting one, positioned far enough away to avoid interference with the transmitted signal. The radiated power is of the order of one watt. That's not much — but it's enough, because the signal only has to travel down and back. Now let's look at the indicator. I want to walk you through the basic indicator, which is shown in Figure 20.2. The height scale is logarithmic. That means it's expanded at the low end — from zero to 500 feet, the scale is stretched out so small height changes are easy to read. From 500 to 2500 feet, it's a reduced, non-linear scale. So the scale is compressed as you go higher. There's a mask. The height pointer disappears behind a mask in three situations: when altitude exceeds 2500 feet, when there's any fault in the transmitted signal, and when the altimeter is switched off. So if you see the pointer gone, you know one of those three things is happening. There's also a failure warning flag. The flag appears when there's too much radio noise that corrupts 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 the flag is your warning that the reading may not be trustworthy. Then there's the press-to-test button, which is also the height selector. It's a dual-purpose device. When you press it, the height pointer swings round to a known pre-set altitude. That's a confidence check — it tells you the equipment is likely to operate satisfactorily. When you turn it, it selects the reference Low Altitude Index to the desired height. So pressing tests the system, turning sets your reference height. Finally, the low height warning. The warning light illuminates if the aircraft is flown below any pre-selected height set by the pilot. And there's an audible element too: an alert tone sounds with increasing loudness from approximately 100 feet above the decision height setting, and the warning is marked by the sudden cessation of that tone. So the tone gets louder as you approach your set height, and then it stops abruptly when you go below it — that sudden silence is the audible warning. There's also an EFIS indicator, the Boeing style, shown in Figure 20.3. That's the radio altimeter — the instrument that measures your true height above the ground, using a swept frequency, a wide beam, and a separate receiving antenna, and presents it to you on a logarithmic scale with a mask, a failure flag, a test button, and a low height warning.