Testing of ILS Receivers in a Multipath Propagation Environment

ILS receivers used for conventional flight inspection purposes are normally not dedicated measurement equipment. Some parts of a receiver are sensitive to dynamic signal changes caused by multipath propagation effects, e.g. those components which apply timedependent parameters such as gain control or filters. Measurement results may suffer from degradation effects and tend to be unreproducible. Real conditions of a multipath-affected RF environment cannot be obtained and interpreted from the DDM curve alone but must be derived from the complex bandpass signal of a receiver being placed in that RF environment. Useful information about the scatterer such as exposure time and reflection coefficient can then be derived from both time and frequency domain analysis. Having this information, it is possible to synthesize an RF signal close to real conditions. Since conventional ILS generators can only produce static DDM without reflections they are unable to generate complex signals. Hence, an experimental generator was designed to generate an arbitrary ILS signal on the carrier frequency containing dynamic multipath effects. The real reflective scenario of an A380 aircraft at Frankfurt airport was taken as an example to reproduce a corresponding RF signal that was then fed into a state-ofthe-art ILS measurement receiver to evaluate and to optimize its performance under these conditions. INTRODUCTION In the past ILS receivers were developed mainly to provide instruments aboard an aircraft that are able to generate the necessary flight guidance information for the pilot during landing. These devices used to have an analogue or semi-analogue (digital signal processing only in the baseband) receiver concept. However, the analogue components like diode demodulators or crystal filters on the intermediate frequency (IF) often cause problems due to non-linearities and temperature drift. Additionally, parts of these receivers such as filtering and AGC are sensitive to fast dynamic signal changes, caused for example by multipath effects. Normally, flight inspection today is still performed with ILS receivers designed for flight guidance, although they are clearly not dedicated measurement equipment and lack the required accuracy of all necessary parameters (e.g. RF power) and suffer from the effects mentioned above. The main objective of this paper is to describe a suitable method for improving even dedicated ILS measurement receivers by using the scientific findings obtained in the RF reflective scenario. The new ILS/VOR Analyzer R&S®EVS300 from Rohde&Schwarz was selected as the device under test.