Measurement of wideband fm-cw-radar characteristics during approach of aircraft

In this paper a method to measure and characterize wideband fm-cw-radars like the radio altimeter of aircraft is presented. The limitations of frequency domain measuring systems will be shown by comparing the results from frequency and time domain based measurements. The need of additional measuring equipment like a down converter and amplifier to improve the signal quality in time domain is described as well as the used software tools to calculate the spectrum. Finally it is described how the frequency ramp speed and shape of the frequency modulated signal can be determined in addition to the already calculated occupied bandwidth and spectral power density.

1. MOTIVATION

Investigations on characteristics of ultra wideband (UWB) transmission channels have been performed at the institute for electromagnetic compatibility and published in the papers [1] to [5]. In these papers it is shown how UWB channels could be characterized in different environments. The investigated fm-cw-radars in this paper are used as an essential navigation system for the final landing approach and determine the altitude above ground, hence the acronym LRRA (Low Range Radio Altimeter) or simply RA and belong to the group of UWB devices. It's quiet difficult to find technical specifications of the manufacturers of the fm-cw-radar systems mounted in aircrafts. In [6] it is shown that the RA work with a frequency sweep, but no data for the sweep time, used frequency band and the shape of the frequency ramp was available. The frequency allocation table of Germany [7] specifies a frequency range from 4.2 GHz to 4.4 GHz for these RA systems. So the question now was “How could we perform measurements on this 200 MHz wide UWB signal to reveal several EMC relevant aspects like the occupied bandwidth (OBW), the spectral power density and the frequency change over time (df/dt) of this radar signal?”.