Microwave Landing System (MLS) — Page 180, Lesson 171

I want to walk you through the Microwave Landing System, or MLS, as it's presented in your Radio Navigation manual. This is a ground-based precision approach aid, and I'll teach it to you from the ground up, exactly as the book lays it out. Let's start with the core principle, which is the scanning beam. Look at Figure 10.4 in your materials. The MLS ground equipment generates a very narrow, fan-shaped beam that sweeps back and forth across the approach sector. The book shows you the beam scanning from one side to the other, and it labels these two directions. One sweep is the "TO" scan, and the other is the "FRO" scan. So the beam physically moves from the "TO" limit to the "FRO" limit and back again. Now, here's the key measurement. The aircraft's receiver measures the time difference between receiving the "TO" scan and the "FRO" scan. That time difference is directly related to the aircraft's angular position relative to the runway centre line. Think about it: if the aircraft is exactly on the centre line, the beam will take the same time to sweep from "TO" to the aircraft as it does from the aircraft to "FRO". If the aircraft is off to one side, one sweep will be shorter and the other longer. That time difference is the angular measurement. The figure also shows a "measurement threshold". This is the signal strength level that the receiver must detect before it starts timing the scan. The book notes that the time difference measurement is directly related to the received signal strength. So the receiver only begins its timing once the signal crosses that threshold, ensuring a consistent and accurate measurement. Now, let's look at the coverage volume, which is shown in Figure 10.2. The MLS coverage in the UK extends to a specific range, up to a specific height, and to a specific angle either side of the on-course line. The correct figures are: 20 nautical miles in range, up to a height of 20,000 feet, and 40 degrees either side of the on-course line. So you have a very wide angular coverage of plus and minus 40 degrees, and a substantial vertical and horizontal reach. Next, let's consider the system components. Figure 10.5 shows the MLS component site on the ground. The system consists of several ground-based elements. There's an azimuth station, which provides the horizontal guidance, and an elevation station, which provides the vertical glide path. There's also a DME-P, which is a precision Distance Measuring Equipment, and it provides the distance information. These are all sited on the ground to cover the approach path. Now, let's move to the airborne equipment. The book describes it as being designed to continuously display the aircraft's position in relation to the preselected course and glide path, along with distance information. This happens during approach and also during departure. So it's not just for landing; it gives you continuous position information on the way out as well. The display itself consists of two cross bars, similar to an ILS display. But there's a critical difference: the indications are given relative to the selected course, not a fixed localizer. Because the MLS is a scanning beam system, you can select any course within its coverage, and the display will show your deviation from that selected course. Furthermore, the book says it's possible to program the computer to give segmented approaches and curved approaches. This is a major MLS capability. For those curved or segmented approaches, a DME-P must be installed on the ground to provide the precise distance information needed to fly those paths. Finally, let's look at the control unit. To receive ILS, MLS, and GPS transmissions, aircraft are equipped with multi-mode receivers and a combined control unit. This is for ease of use by the flight crew. So instead of having separate boxes for each system, you have one unit that lets you select and tune whichever navaid you need. An example of such a control unit is shown in Figure 10.6. So, to summarize the whole picture: the ground station sweeps a narrow beam, the aircraft measures the time difference between the "TO" and "FRO" scans to determine its angular position, and that information, along with DME-P distance, is displayed on cross bars in the cockpit. The system offers wide coverage, selectable courses, and the ability to fly curved approaches. That's the MLS in its entirety.