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Detecting these kinds of signal is an area of active military research. The key is that if the signal comes from one direction, you can use multiple antennas an
by sudosysgen 5y ago
Detecting these kinds of signal is an area of active military research. The key is that if the signal comes from one direction, you can use multiple antennas and filter by time-delay/phase to find it even if it is below the noise floor.
To this extent, there are methods to reliably detect signals 12dB under the noise floor, and as I said this is an area of active research and militaries are probably even further ahead.
- dognotdog 5y agoThat is quite interesting, and makes sense! The spread-spectrum signal is a psuedo-random, but same sequence for all receivers, while the noise varies and should cancel out. Though, if I am not mistaken, the laws of super-sampling hold in this case, and SNR is only improved by O(sqrt(N)) for N samples, so this approach will have practical limits as one has to place a very large number of receivers in range of the emitter (or do auto-correlations over a very long time window), with enough separation to make sure the noise isn't correlated anymore, if looking for a signal deep below the noise floor.
- sudosysgen 5y agoI'm rusty on this but I do believe such a limit exists. However thankfully for this research it's about detecting radars, so thanks to the inverse square law there's a limit to how deep beneath the noise floor you need to look.