Summary
Highlights
The video introduces the Microwave Landing System (MLS) and immediately addresses why it is rarely seen in use today despite being an advanced piece of equipment. It was intended to replace the Instrument Landing System (ILS) but was superseded by GPS technology, which became more accurate and reliable without requiring expensive ground equipment upgrades. MLS suffered from unfortunate timing, emerging just before GPS became dominant, making it an unviable upgrade path from ILS.
MLS operates using microwaves in the super high frequency range (3-30 GHz), specifically between 5.031 and 5.090 GHz. It uses two beams: an elevation beam for vertical guidance and an azimuth beam for horizontal guidance, both using the same frequency but distinguishable through multiplexing. A precision DME (Distance Measuring Equipment) provides highly accurate distance-to-runway information. This combination allows for complex 3D approach procedures, including curved routes, which were a significant improvement over standard ILS, but a capability later matched by GPS.
Horizontal guidance is provided by a fan-shaped vertical beam that sweeps back and forth within a 40-degree coverage range, extending 20 nautical miles. The aircraft receives two distinct signals as the beam sweeps over it. By measuring the time gap between these two hits and knowing the precise sweep timing, the aircraft's azimuthal position is accurately determined. A smaller gap indicates the aircraft is closer to the right side of the range, while a larger gap indicates it is further to the left.
The vertical or elevation transmitter works on the same principle as the azimuth, but with a horizontal fan-shaped beam sweeping up and down. By measuring the gaps between passes and the precise timing of the sweep, the aircraft's vertical position is determined. The vertical coverage ranges from 0.9 degrees to 20 degrees up, out to 20 nautical miles. This combined with the azimuth creates a pizza-slice-shaped 3D wedge for approaches.
When a precision DME, accurate to within plus or minus 100 feet, is integrated, MLS can support curved and multiple waypoint approaches. Without a precision DME, only straight-in or slightly offset approaches are possible. MLS shares similar issues with ILS regarding beam reflections from vehicles or terrain, requiring a 'safe range' around transmitters. However, MLS has an advantage: its sweeping wave can be temporarily switched off to avoid reflections from obstructions like mountains, making the signal more reliable. Despite its advanced features, GPS proved to be superior, cheaper, and more prevalent, leading to MLS's limited adoption.