A Method to Reduce the Need for Large Antennas in Microwave Landing Systems (MLS)

The replacement of the present Instrument Landing System (ILS) by the new Microwave Landing System (MLS) which is supposed to begin about 1980 is caused by increasing requirements for capacity, accuracy and coverage which ILS cannot meet. The increased accuracy is largely achieved by the use of microwave frequencies which implies larger antennas in wavelengths in spite of reduced physical dimensions. However, in many cases an unsatisfactory compromise has to be reached between requirements for high accuracy and demands for small antenna dimensions. A method to obtain good performance with relatively small antennas is to utilize a larger bandwidth for the transfer of signals as space and frequency are interrelated quantities. The transmitted signals are then modulated with a frequency much higher than the information bandwidth and demodulated in the receiver. In this way, a compressed pulse (for pulsed signals) or an efficient delay discrimination (for continuous signals) can be obtained by means of a correlating receiver. By exploiting the whole frequency band allocated for MLS use at C-band, the angle measurement accuracy can be improved as much as 20 - 80 times.