Reference signal estimation for an airborne passive radar

Estimating the transmitted signal is a mandatory task in passive radar in order to perform target detection. For DVB-T-based ground passive radar where clutter echoes are located at zero-Doppler, the reference signal can be recovered either by focusing the reception array in the transmitter direction, or by decoding the received signal. However, this estimation becomes much more difficult in the case of an airborne receiver since the receiver motion induces clutter multipaths spread in the range-Doppler domain. In this chapter, we thus consider the problem of estimating the reference signal in airborne passive radar applications using DVB-T transmitters. We first briefly recall the principal methods for estimating the reference signal in classical passive applications, i.e. antenna beamforming toward the transmitter direction or OFDM decoding principle for terrestrial configuration. We then present the main features of the aeronautical channel, and several methods for coping with the time-varying channel, notably antenna diversity and basis expansion model (BEM). Finally we present and discuss the performance obtained with these reference signal estimation methods obtained on real airborne signals. It appears that combining antenna diversity together with a BEM representation of the aeronautical channel provides substantial improvement over classical demodulation and these methods considered separately.