I am quite familiar with the general concept of bistatic FMCW radar. The issue that I was trying to understand relates to an HF bistatic FMCW radar and what techniques are available that allow detection of zero Doppler targets in the presence of the direct radiated signal. This radiated signal will results in a zero Doppler response at all ranges in the direction of the transmitter that in general will dominate and hence mask returns from these stationary targets if not removed.
I have searched but can not find a reference as to how this issue is addressed.
Hi,
I had a similar problem some years ago in designing a CW RCS test range range in the VHF band, and solved it by setting up a coherent co-location RF canceller between the transmit and receive rf feed lines to cancel the direct RF coupling between the two antennas.I took a small bleed signal from the Tx feed line , added a varialbe attenuator and phase shifter to the line then a combiner to add it to the receive line. The attenuator and phase shifter could then be manually adjusted to obtain 30 to 40 dB stable cancellation for this CW scenario.
With FMCW the bandwidth will be an issue to deal with (and keep you busy) and if so broadband cancellation techniqnes are available to sort that too, as are automated adjustment algorithms.
See my book reviews on research gate for references on the subject.
regs,
phil
I guess one should have bistiatic separation distance much smaller then the sum of distance to the targets - this way the beat frequency range will be different enough.
The better solution for this task is to use a spatial selection of targets and nulling transmitter signals on the basis of digital beamforming in a digital antenna array:
https://en.wikipedia.org/wiki/Digital_antenna_array
Thanks Vadym - yes I have looked at this as a solution and it would be effective when the vessel is at a different bearing to the direct transmit signal. The problem that I am trying to address is when the vessel is at the same bearing as the direct transmit signal. At this point the target is at the same Bearing, Doppler and range as the direct TX signal and the direct TX signal will tend to dominate. The only difference is in the shape of the ambiguity function. Any thoughts would be appreciated.
hi, it would be great if you could clarify your problem with a little more detail. It is unclear to me at present what your problem is from the inputs to date And your original description. I made a guess on my original response.
if you have time that would be very interesting.
Dear A.M. Ponsford , the current digital signal processing technics for digital antenna array have the capability to resolution and spatial selection of transmitter signal and target signals by angle interval between bearing to the direct transmit signal and target only 0.01 - 0.001 of the main beamwidth. Of course, such resolution is possible without an overload of Analog-to-Digital converters in receiver channels and by enough SNR of target signals. If you need spatial selection better than 0.001 of the main beamwidth it will be a problem.
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