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Xaviers Latest Nonsense

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ka9q:

--- Quote from: smartcooky on October 05, 2020, 02:31:56 PM ---Yep. If I correctly recall my time working on TACAN ground stations, we used range gating to help detect interrogation pulses and separating them from things such as reference pulses, squitter pulses and reply pulse-pairs.

--- End quote ---
Right. My background is in radio communication rather than radar or optical ranging, but the underlying principles are the same. In radio, you tune a receiver to the transmitted signal, with filters excluding all frequencies other than those produced by the transmitter. Most radio signals are continuous in time but limited in bandwidth. Radar and optical signals are usually limited in time (for range resolution) which forces them to be wider (but still limited) in bandwidth. The famous uncertainty principle in physics is also fundamental to communications where it expresses the inverse relationship between frequency and time. Narrow in time means wide in frequency, and narrow in frequency means wide in time. You cannot have narrow in frequency and narrow in time, though numerous cranks have tried.

The basic aspects of any signal -- its structure, bandwidth and timing -- are usually at least partly known a priori -- before receiving it. Every communications engineer is taught to take advantage of as much a priori information as you possibly can when building and operating a receiver. If there's a radio program you want to hear (I know, pretty retro, but...) you tune into its advertised frequency at the proper time and with the right type of receiver. As Shannon explained, communication is all about reducing the receiver's uncertainty, so you eliminate as much of that uncertainty as you can at the start.

The technical term for all this is matched filtering, and there are well-established mathematical proofs that it is optimal. Yet that didn't keep one professor I know from claiming to research "mis-matched filtering", claiming it might work better. I guess the system for selecting professors isn't perfect.

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