The Microwave Landing System (MLS) provides multi approach paths with one ground installation which gives flexibility in approaches that reduces possible conflicts in the air especially around aerodromes with heavy traffic.

There are several forms of MLS. But Time Reference Scanning Beam (TRSB) is the one MLS technique which accomplishes this best. TRSB works this way:

• A narrow microwave beam generated by the azimuth antenna scans rapidly “TO” and “FRO”, through the entire coverage volume.
• Another beam, generated by the elevation antenna, scans down and then up.
• When an aircraft enters this broad coverage volume, its receiver detects two pulses from each complete beam scan from each antenna in the system.
• In the elevation, for example, the first scan sweeps down. As it passes the aircraft, the pulse is received and a timer is started. When the scan returns, a second pulse is received and the timer is stopped.

This is the key feature of TRSB. Instruments in the aircraft derive the airplane’s precise position from the time lapse between the two scan pulses of each beam. The aircraft position is updated right in the cockpit, several times per second.

Because TRSB generates each of these scanning beams at precise times, MLS onboard receivers can distinguish between direct and reflected signals. If the azimuth beam begins its scan and is reflected by an obstacle, it will send a pulse out of time sequence and the aircraft receiver will reject it.