An improved bistatic Doppler measurement system with reduced contamination by sidelobe echoes

The bistatic Doppler velocity measurement, with one traditional transmitting radar associated with one or more passive radar receivers, is a useful way to retrieve the 2D or 3D wind field with low costs. On the other hand, the sidelobe contamination of the bistatic Doppler velocity measurement is the serious problem because low-gain wide beamwidth receiving antennas are usually used. We propose a new measurement system by which the sidelobe itself is reduced with an array receiving antenna. The simulated results to verify this system and preliminary results of its field test are presented.

We use low-gain wide beamwidth receiving antennas that are supposed to be fixed in azimuth and elevation from viewpoint of costs and easy to install. These are just same as the ordinal bistatic receiving antennas. But we compose an array with these antennas as elements. An array (multiple elements) leads to the improvement of receiving gain.

Furthermore, we dare to use grating lobes. In case of usual array antenna, the spacing between elements is selected short (less than one wavelength) to form only one strong main lobe and to avoid forming grating lobes. But it is difficult to form a narrow beam enough to reduce the sidelobe contamination with limited number of elements. We dare to select long spacing (e.g. 10 wavelengths) and to form many sharp grating robes (beams) simultaneously. Sidelobe contaminations near around the strong echoes are expected to be reduced with these sharp beams. Echoes from multiple beams can be separated (identified) by receiving time difference because the path lengths of transmitter-target-receiver are different among every receiving beam.

Once the number of elements and their positions are determined, the directions of receiving beams (namely observable area) are also fixed. To extend the observable area, we introduce the digital beam forming (DBF) technique. By shifting phases of receiving signals from each element, we can change the directions of receiving beams and extend the observable area.

We, National Institute of Information and Communications Technology (NICT), have a full polarimetric Doppler weather radar named COBRA (5.34 GHz, wavelength ë is about 5.6 cm) in Okinawa, Japan. Supposing to apply this new bsitatic system to COBRA, realistic conditions for COBRA are introduced to numerical simulations. Due to the array receiving antenna, the calculated received power in almost all observational area is increased compared with the ordinal system. Furthermore calculated time series of received signals show that the sidelobe contamination can be largely reduced with this new system. Simulated and preliminary experimental results will be presented.