A Study and Modelling of Chaff Clouds Affected by Wing-Tip Vortices

Abstract: Chaff are thin strips of either metal or metal coated fibres used as countermeasures against radar. When deployed from an airplane, the turbulence and wing-tip vortices from the airplane disperse the chaff into a chaff cloud, which may appear as a false target and jam the radar. A Doppler radar is able to discriminate between chaff and targets by using the Doppler spectra. Such signal processing methods already exist, and are evaluated with real data from flight tests. However, flight tests are expensive and a way to simulate a chaff cloud would therefore be preferred. This master thesis was a pilot study which investigated the current knowledge of chaff modelling with wing-tip vortices. Two models were created, an individual chaff model and a Gaussian model. The individual chaff model was based on physical phenomena and simulated the effect of the wing-tip vortices on a chaff cloud. The Gaussian model was based on distributions and trends, and should be adjusted to real data. Due to confidentiality, real data could not be used. Instead, two flight scenarios were used to evaluate the individual chaff model. The Gaussian model was then adapted to these results. The individual chaff model was able to simulate two separate flight scenarios with distinguishable results. Moreover, the model also showed some promising results when compared to RCS trends from previous published models. Statistical characteristics of the RCS values agreed with Swerling models. The RCS values were however low, due to insufficient diffusion simulation. This also resulted in inaccurately simulated cloud sizes. The Gaussian model was able to mimic the behaviour of the individual chaff model with reduced run-time. More work is necessary to fully understand the behaviour of wing-tip vortices, as well as the ability to use real data for evaluation. Theoretical work and more advanced simulation methods are required to understand and model the behaviour properly.