Let’s start the Radio Altimeter chapter — Page 262, Lesson 312

Let’s start the Radio Altimeter chapter. I want to walk you through what this instrument actually is and how it works, because it’s one of the most safety-critical devices on the flight deck. 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” — this is the key difference from a 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 three jobs. The first is obvious: it provides a flight deck display of height above ground level, which we abbreviate as AGL. But it has two other important functions. It supplies the automatic flight system with data to effect automatic landings when used in association with the ILS or MLS — that’s the Instrument Landing System and the Microwave Landing System. And 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 feeding it the height and how fast that height is changing. Now let’s get into the physics. The instrument makes use of primary radio principles, transmitting a Frequency Modulated Continuous Wave — FMCW — in an elliptical pattern vertically below the aircraft. So the antenna sends a continuous wave, and the frequency of that wave is being modulated, meaning it’s being changed in a controlled way over time. Here’s the operating principle. 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. During this time, the transmitted frequency changes at a known rate from its start level to +50 MHz and back again to complete a “cycle.” So the frequency sweeps up by 50 megahertz, then comes back down. Why does it do this? 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. The complete “modulation cycle” or “frequency sweep” is illustrated in Figure 20.1. Here’s the clever part. 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, and comes back. By the time it returns, the transmitter has already moved on to a different frequency. The difference between what was sent and what came back tells you how long the round trip took — and from that time, aircraft height may be determined. But there’s a problem, and I want you to understand it because it’s a real engineering limitation. A breakdown of frequency difference occurs when the transmitter changes the direction of its frequency sweep — that’s the moment it stops going up and starts going down, or vice versa. At that changeover point, the frequency relationship breaks down and you’d get an error. How is that overcome? The equipment relates aircraft height to the average beat frequency — and here “beat frequency” means the difference between transmitted and received frequency — observed over a short sampling period. The frequency changeover points are thereby ignored. So instead of relying on any single instant, the system averages the beat frequency over a short window, and that smooths out the glitches at the sweep reversal points. So to summarise the whole chain: transmit an FMCW signal downward, receive the echo, compare the frequencies, convert the frequency difference into a round-trip time, and from that time derive height. And the averaging over a sampling period handles the sweep-direction changeovers. That’s the core of the radio altimeter. Next we’ll look at the frequencies it uses and then the indicators — the basic one, then the Boeing and Airbus EFIS styles.