Flight Experiments on MLS Elevation Subsystem

Abstract

MLS (Microwave Landing System) has been configured to replace present ILS (Instrument Landing System) which cannot fulfil new needs for aircraft operation. MLS can be operated with high accuracy, wide coverage and is suitable for precision area navigation in approach and landing phase. However, this wide coverage of MLS signals degrades the accuracy due to reflection and diffraction by buildings in the vicinity of the antennas. These undesirable propagation problems are more remarkable in the elevation than in the azimuth guidance.

Flight experiments for MLS elevation subsystem were carried out to study and analyze these effects, where an aircraft equipped with an MLS receiver, a telemetry system and a Television tracking system were used.

The results of the flight experiments and analyses are as follows:

(1) Even under severe siting conditions with buildings and hangars close to the MLS antenna, a high guidance accuracy in the centerline approach, at about 1/3 of the SARPs (ICAO Standards And Recommended Practices) limit could be obtained. And an adequate margin in the received electric field strength, could be obtained at the 20 nautical miles of the SARPs requirements.

(2) Scalloping phenomena were observed and analyzed. These phenomena were interpreted as interference due to the reflection by hangars near by.

(3) An error, approximately +1 degree or more, was measured in shadow region of the hangars. There is enough field strength in deep diffraction region, therefore the angle error is so large that the aircraft will be informed of an elevation angle much higher than the actual one.

In addition, the diffraction characteristics can be estimated by the geographical conditions in the vicinity of the antennas are shown.

These results shown here are expected to be the basic data to give MLS specification, siting criteria and operational procedures.