The microwave landing system (MLS) is under development as a replacement for the VHF/UHF ILS. The time refference scanning beam system (TRSB) was selected as the preferred system for worldwide standardization by ICAO AWO Divisional Meeting recently.

This paper discribes experimental model of TRSB-MLS azimuth subsystem and experimental study of angle guidance error.

The azimuth subsystem consists of azimuth guidance antenna which includes a sector omni antenna and a scanning beam antenna, transmitter, and airborne receiver. The scanning beam antenna is linear phased array. This antenna has a 3 degrees azimuth beam width which is electronically scanned ±40 degrees in azimuth, and has a 0 to 20 degrees elevation coverage with a sharp underside cutoff for multipath reduction.

The transmitter employs TWT for the final stage of amplification, and radiates 10 watts of CW power at 5 GHz.

Effectiveness of reduction of angle guidance error by smoothing of received signal, and error due to threshold level variation in the receiver envelope processor are examined with the experiments using the azimuth subsystem. Experimental results show that the error can be reduced by decreasing the bandwidth of envelope processing circuit, and accuracy of 0.04 degrees is obtained over azimuth coverage.

Doppler VOR has an improvement with respect to siting effects compared to the conventional VOR, although the Doppler VOR is costly to install.

SSB Doppler VOR is preferred to be operated because of simple and low cost construction, but the SSB Doppler VOR has the systematic bearing error caused by undesired amplitude modulation which results from the rotating sideband antenna pattern distorted by the counterpoise.

This paper describes an analytical study on the systematic error sources and the error reduction methods by means of compensating signal radiation.

The summary flight test results are also presented and discussed in relation to the effectiveness of the error reduction method.