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In busy and congested airplane areas (around large airports, for example), there could be dozens of commercial aircraft, all with their own weather radar transmitters/receivers actively transmitting and receiving radar signals. How are the radar receivers able to discern between their own radar's returns from those of other aircraft? I can't imagine them to all have different TX frequencies to prevent interference, but not sure how else the receivers could be set to reject other aircraft's radar transmissions.

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    $\begingroup$ Does this answer your question? What is garbling in the context of radar? $\endgroup$ – expeditedescent Mar 3 at 16:39
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    $\begingroup$ Not a dupe at all. This question is about the radar transmissions themselves; the suggested question on garbling is about transponder replies. $\endgroup$ – Ralph J Mar 3 at 17:31
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    $\begingroup$ Edited to hopefully clear up some of the confusion (although I find the question entirely clear - and a good one). As the OP notes in a comment below, this is NOT about ATC radar nor transponders whatsoever. It's all about why one aircraft's weather radar doesn't interfere with another aircraft's weather radar. $\endgroup$ – Ralph J Mar 4 at 4:47
  • $\begingroup$ My complete guess, without knowing anything about the situation, is that the radar in each plane may not be using the exact same frequency radio waves. It might be a set variation determined at the factory or they might be able to "listen" before a scan and automatically select a frequency that isn't in use. If I were designing the system I would attempt to create such a feature. $\endgroup$ – randomhead Mar 4 at 13:31
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"Radar to radar interference" / "Mutual radar interference" is an ongoing research topic, though much of the current research (based on my cursory Googling) seems focused on interference between radars on self-driving cars. That leads me to believe a certain degree of radar-to-radar interference is currently just tolerated.

The link shared by randomhead gives us some insight into how maritime radars mitigate mutual interference, both directly and indirectly. The same techniques should be applicable to aircraft weather radars, which tend to also be X-Band and also be pulsing radars.

Directly

  • The interference rejection capability of the radar essentially relies on timing differences between the radars. A pulse radar sends out many pulses, separated by around 1 millisecond. Microsecond-resolution timing between the transmission of the pulse and the receipt of its echo is used to determine range to a target.

    Interfering radar pulses will arrive somewhat randomly on that microsecond timescale, but actual target echoes (e.g. from weather) will be correlated across multiple pulses. The radar can be made to only acknowledge a return as real if enough echoes arrive. Related topics here seem like "m-out-of-n" detection, "binary integration", "coincidence detection", "dual threshold detection", and "double threshold detection".

    Activating the "interference rejection" mode on a radar (or having it always on, which I suspect is the case on many aviation weather radars) will cause a radar to reject certain weak real returns (more false negatives). Luckily, clouds full of precipitation tend to return radar fairly well.

Indirectly

  • A different section of the same link gives instructions for tuning radar. This implies that there is room in the band for more than one transmitter to be operating simultaneously, though it may not be organized formally into channels. This is reinforced later in the description of the Search and Rescue Transponder (SART) scanning the radar band for pulses to reply to, which wouldn't be necessary if they all operated on the same frequency.

    So, automatically or manually, weather radars should also be able to deconflict by frequency separation. This will, of course, run into limitations as the number of transmitters grows, but at least it reduces the amount of work the Interference Rejection mode has to do.

  • If the radar can change properties of its pulse, such as the pulse length or frequency modulation, it can potentially use those properties to distinguish between its pulses and pulses from other transmitters. The link doesn't really support that being used to reduce interference in maritime navigation radar, though, and I kind of doubt it's used for such in aviation weather radar either. I think I've heard of schemes like that being used for more warfare-oriented radars but that's even further outside my own expertise.

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    $\begingroup$ I doubt this answer is good, per se, but I hope it can be a nucleation point for many edits or attract the attention/ire of someone who knows better. $\endgroup$ – Erin Anne Mar 5 at 2:33
  • $\begingroup$ I agree, the current generation of aviation weather radars are extremely sophisticated. The only people that would know are the designers and coders and that knowledge would likely be intellectual property and possibly sensitive export controlled technology. $\endgroup$ – Jabirhee Mar 6 at 23:44
  • $\begingroup$ @Jabirhee uhhh there's tons of research papers out there discussing methods for interference rejections, so...I dunno about the export control status. I've just had difficulty finding aviation-weather-radar-specific sources among all the other related radar topics. $\endgroup$ – Erin Anne Mar 7 at 3:29
  • $\begingroup$ Yes, there are plenty of academic papers and textbooks on radar systems. I know of 2 companies that successfully manufacture modern weather radar systems for transport aircraft. They both probably have government contracts with the US government too. Anyone doing business with them will be subject to a non-disclosure agreement and the company will likely have a policy for adhering to ITAR regulations. A few of the people that work there would be able to accurately describe what factors are relevant and what methods are effective for achieving a good result. $\endgroup$ – Jabirhee Mar 7 at 6:48

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