Microwave Landing Systems (MLS)

Introduction

• MLS were supposed to replace Instrument Landing Systems (ILS)
• Not widely used due to advancements in GPS technology
• Still taught for ATPL exams despite limited practical use

Reasons for Limited Use

• GPS offers accurate and reliable navigation
• GPS does not require ground equipment for approach
• High cost to upgrade from ILS to MLS
• MLS development was poorly timed, overshadowed by GPS

How MLS Works

• Uses microwaves in the super high frequency range (3 to 30 GHz)
  • Specifically, 5.031 to 5.090 GHz (5.31 to 5.09 GHz)
• Utilizes two beams:
  • Vertical Beam (Elev): Provides vertical guidance
  • Horizontal Beam (Aimus): Provides horizontal guidance
• Beams distinguished through multiplexing
  • Multiple signals sent with identifier codes to differentiate

Precision Distance Measuring Equipment (DME)

• Provides very accurate distance to runway
• Allows for creation of complex approach procedures
  • Enables curved routes and waypoints

Benefits of MLS

• 3D approach procedures using vertical and horizontal data
• Capable of providing guidance around obstacles (e.g., mountains)
• Less susceptible to signal bending or reflections
  • Signals can be interrupted to avoid reflection issues

Horizontal and Vertical Guidance Mechanism

• Horizontal Guidance

  • Fan-shaped vertical beam sweeps back and forth
  • Coverage range: 40° either side of centerline, up to 20 nautical miles
  • Aircraft position detected by measuring timing gaps

• Vertical Guidance

  • Horizontal fan-shaped beam sweeps up and down
  • Coverage range: 0.9° up to 20° vertical, up to 20 nautical miles
  • Position detected by timing gaps similar to horizontal beam

Limitations

• Suffers from similar issues as ILS, such as beam reflections
• Safe zones required around transmitters to avoid signal disruption
• Still more expensive and less practical compared to GPS

Conclusion

• MLS is rarely used today, overshadowed by the emergence of GPS
• Important to understand for academic purposes but not practically relevant
• Future focus will be on GPS systems, which are more widely used and studied