We're starting a brand-new chapter now: the Radio Altimeter — Page 262, Lesson 312

We're starting a brand-new chapter now: the Radio Altimeter. This is a really satisfying instrument because it's a beautiful application of basic radio physics, and it's absolutely critical for modern operations. Let me walk you through it. First, the definition. The Radio Altimeter is a device capable of measuring the height of an aircraft above ground with a high degree of accuracy. Note the phrase "above ground" — that's the key difference from your barometric altimeter, which measures height above a pressure datum. The radio altimeter gives you true height above the terrain directly beneath you. Now, it does more than just give the pilots a display. It has two other important functions. First, it supplies the automatic flight system with data to affect automatic landings when used in association with the ILS or MLS — that's the Instrument Landing System and the Microwave Landing System. Second, it furnishes height information and rate of change of height to the Ground Proximity Warning System, the GPWS, and it is a crucial component of that system. So when you hear about GPWS warnings, the radio altimeter is the sensor feeding it that height and that rate of change. How does it work? It uses primary radio principles, transmitting a Frequency Modulated Continuous Wave — that's FMCW — in an elliptical pattern vertically below the aircraft. So the energy is sent straight down in a cone that's elliptical in shape. The principle is simple: the radio altimeter determines the time taken for a radio wave to travel from the aircraft to the ground directly beneath the aircraft and back again. That's a round trip. But here's the clever part — it doesn't measure time directly. During this time, the transmitted frequency changes at a known rate from its start level to +50 MHz and back again, completing what we call a "cycle." So the frequency is being swept up and down continuously. Why can't it just keep increasing? Because the carrier frequency cannot be increased indefinitely. So after half a wavelength, the change is reversed — the frequency is then decreased at a constant rate down to a specified value before being increased again. That complete "modulation cycle" or "frequency sweep" is what you see in Figure 20.1. Now here's the measurement logic. The equipment compares the frequencies of the transmitted and received signals. Since the rate of frequency change is known, the frequency difference becomes a measure of the time taken for the radio wave to travel to and from the surface. Think about it: the wave goes down, hits the ground, comes back. By the time it returns, the transmitter has already moved on to a different frequency. The bigger the height, the longer the delay, the bigger the frequency difference. So from that frequency difference, aircraft height may be determined. But there's a problem, and I want you to understand this because it's a real engineering subtlety. A breakdown of frequency difference occurs when the transmitter changes the direction of its frequency sweep. At the moment the sweep reverses — at the top and bottom of that triangular wave — the frequency relationship between transmitted and received signals gets confused. How is that overcome? By relating aircraft height to the average beat frequency — and that's the difference between transmitted and received frequency — observed over a short sampling period. The frequency changeover points are thereby ignored. So the system doesn't try to measure at the instant of reversal; it samples over a short window and averages, which neatly sidesteps those messy changeover points. So to summarise the whole chain: transmit FMCW downward, receive the echo, compare frequencies, the difference tells you the round-trip time, and that gives you height. And the averaging over a sampling period handles the sweep-reversal glitch. That's the core operating principle of the radio altimeter. Next we'll look at the frequencies it uses and then the indicators — both the basic one and the EFIS styles from Boeing and Airbus.