Microwave Landing System (MLS) — Page 180, Lesson 169

We’re starting the Microwave Landing System, or MLS. This is the modern, more flexible successor to the ILS, and I want to walk you through how it actually works. The core principle is Time Division Multiplexing, which we abbreviate as TDM. Here’s the key idea: the whole MLS system uses only one radio frequency per channel. All the different ground equipment—the azimuth, the elevation, the data transmitters—share that single frequency. They don’t each get their own frequency like in older systems. Instead, they take turns. Their transmissions are synchronized in time so they don’t interfere with each other. That’s the “time division” part—each component gets its own time slot on the common frequency. Now, how does the aircraft figure out its position? It uses something called a Time Referenced Scanning Beam, or TRSB. This is used for both azimuth and elevation. Let’s start with azimuth, which is your horizontal position relative to the runway centre line. Here’s the mechanism. The ground transmitter sends out a beam that sweeps across the approach area. It starts at one extremity of its total scan and sweeps at a uniform speed all the way to the other extremity. That’s the “to” sweep. Then it immediately sweeps back to its start position—that’s the “fro” sweep. The aircraft receives both of these sweeps. It measures the time interval in microseconds between receiving the “to” beam and the “fro” beam. Because the beam sweeps at a constant speed, that time interval is directly proportional to the aircraft’s angular position relative to the runway on-course line. If you’re right on the centre line, the time between “to” and “fro” will be exactly half the total sweep time. If you’re off to one side, the interval shifts proportionally. That’s how the aircraft computes its azimuth position. Now, here’s a big advantage over the ILS. The pilot can choose to fly the runway on-course line, which is the QDM, or he can select an approach path that is a predetermined number of degrees ± the runway direction. So you’re not locked into one fixed path—you can offset your approach if you need to, say, to avoid terrain or traffic. Glide slope location works the same way, but vertically. Another beam scans up and down at a uniform speed within its elevation limits. The aircraft measures the time difference between receiving the “up” sweep and the “down” sweep, and from that it calculates its position relative to its selected glide slope angle. Notice—again, you select the angle. It’s not fixed at 3 degrees like a typical ILS. And because the azimuth beam, the elevation beam, and all the other components transmit at different time intervals, they all share that one frequency without interfering. That’s the time multiplexing in action. Now let’s look at the other components of the system. First, Flare. The standard has been developed to provide flare elevation guidance, but here’s the important limitation: this function is not intended for future implementation. So it exists in the standard, but don’t expect to see it in service. Second, Back azimuth. This gives go-around and departure guidance. It covers ±20° of runway direction, up to 15° in elevation. So if you have to go around or depart, you get guidance behind the runway too. Third, DME. Range along the MLS course is provided not by markers, but by a DME—Distance Measuring Equipment. And here’s a critical requirement: for Category II and Category III approaches, you must have a precision DME, called DME/P, that is accurate to within 100 feet. That’s a tight tolerance, and it’s mandatory for those low-visibility approaches. Finally, there’s the transmission of auxiliary data. This is the data that gets broadcast along with the guidance signals. It consists of four things: station identification, so you know which station you’re talking to; system condition, so you know if the system is healthy; runway condition; and weather information. So the system isn’t just giving you angular guidance—it’s also telling you about the state of the airport and the weather. Let me pull that together. MLS uses one frequency per channel, shared by time division multiplexing. Azimuth and elevation both use the time-referenced scanning beam, where the time between “to” and “fro” sweeps tells you your angular position. You can select your own approach path and glide slope angle. And you’ve got back azimuth for departures, a DME/P for range on Cat II and III, and auxiliary data for station, system, runway, and weather information. That figure shows you the approach coverage volume—how the beams cover the approach area. Take a look at it as you think through the sweep geometry.