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

We’re starting a brand-new topic now: the Microwave Landing System, or MLS. This is the system that was designed to overcome the limitations of the older ILS, and it works on a completely different principle. Let me walk you through how it operates. The core idea is Time Division Multiplexing, often abbreviated as TDM. Here’s what that means in plain terms: on a single MLS channel, only one frequency is used. But the ground equipment—the various angle sensors and the data transmitters—all share that one frequency by transmitting at different, synchronized time slots. Because they’re synchronized, they don’t interfere with each other. So instead of each function having its own frequency like ILS, they take turns on the same one. That’s the whole essence of time division multiplexing. Now, the heart of the angle measurement is something called the Time Referenced Scanning Beam, or TRSB. This is used for both azimuth and elevation. Let’s start with azimuth location—that’s your horizontal position relative to the runway centre line. Here’s how the aircraft figures out where it is. The ground station sweeps a beam. It starts its ‘to’ sweep at one extremity of its total scan range and travels at a uniform speed to the other extremity. Then it immediately starts its ‘fro’ scan, sweeping back to its starting position. The aircraft measures the time interval in microseconds between when it receives the ‘to’ beam and when it receives the ‘fro’ beam. That time difference is proportional to the angular position of the aircraft relative to the runway on-course line. So the bigger the time gap, the further off to one side you are; when you’re dead on the centre line, the timing is balanced. Now, a key advantage here: the pilot isn’t forced to fly the runway centre line. You can choose to fly the runway on-course line—that’s the QDM, the magnetic bearing to the runway—or you can select an approach path that is a pre-determined number of degrees ± the runway direction. In other words, you can offset your approach by a few degrees to the left or right if you want, and the system will guide you along that selected path. That’s a flexibility ILS never gave you. Now let’s look at glide slope location. The principle is identical, but now the beam scans up and down at a uniform speed within its elevation limits. The aircraft measures the time difference between the reception of the pulses from the up sweep and the down sweep. From that timing, it calculates its position relative to your selected glide slope angle. So just like azimuth, the vertical position is derived from the time difference between the two sweeps. And here’s where the time multiplexing really shows its value: the transmissions from these two scanning beams, and the transmissions from all the other components of the MLS system, are sent at different time intervals. That’s the time multiplexing in action—each component gets its own time slot on that single frequency. Now, let me cover the other components of the system, because MLS is more than just azimuth and elevation. First, there’s Flare. The standard was developed to provide for flare elevation guidance—that’s the automatic rounding-out just before touchdown. But here’s the important limitation: this function is not intended for future implementation. So don’t expect to see it in service; it’s in the standard but not planned for actual use. Second, there’s Back azimuth. This gives you go-around and departure guidance. It covers ±20° of runway direction and extends up to 15° in elevation. So if you have to go around, or you’re departing, you have guidance behind the runway too. Third, there’s DME—Distance Measuring Equipment. This is a big difference from ILS. Range along the MLS course is provided not by markers but by a DME. And here’s a critical precision requirement: for Category II and Category III approaches, you must have a precision DME, designated DME/P, that is accurate to within 100 feet. That’s the tolerance you need for those low-visibility approaches. Finally, there’s the transmission of auxiliary data. This is the data that gets broadcast to the aircraft, and it consists of four items: station identification, system condition, runway condition, and weather information. So the system tells you who it is, whether it’s healthy, the state of the runway, and the weather—all on that same multiplexed data stream. So to tie it all together: MLS uses one frequency per channel, with time division multiplexing to keep all the transmissions from interfering. Azimuth and elevation are both measured by the time-referenced scanning beam technique—measuring the time between the ‘to’ and ‘fro’ sweeps. And the system adds flare, back azimuth, DME for range, and auxiliary data transmission. That’s the complete picture of how MLS works.