How Passive Radar Works

kklisura an hour ago | parent | next [-]

> By listening to how broadcasts like FM radio and digital TV bounce off objects, it's possible to determine their positions and velocities.

Wasn't some Github repo ITAR'd, couple of years ago, due to having python code for some SDR doing this?

Edit: Found it. 3years ago https://www.reddit.com/r/RTLSDR/comments/yu9rei/krakenrf_pul...

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Geo_ge an hour ago | parent | prev | next [-]

There is a lot of talk of military uses here, but this technique is also used for environmental monitoring.

GNSS interferometric reflectometry (GNSS-IR) uses navigation satellites as the transmitter for a bi-static radar. The measurement device is any GNSS receiver (even your phone).

The technique can estimate environmental parameters like sea level, soil moisture, snow depth, and vegetation water content from systematic changes in the the multi-path around the antenna.

There is an open source Python package for this technique: https://github.com/kristinemlarson/gnssrefl

An introductory paper "The Accidental Tide Gauge": https://www.kristinelarson.net/wp-content/uploads/2015/10/La...

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amoshebb 2 hours ago | parent | prev | next [-]

I read a lot about passive radars trying to leech off of opportunistic waves, and lots about actual troops preferring to play hide-and-seem with anti-radiation weapons just to use active machines.

A config that strikes me as obvious but doesn’t seem to be popular would be just bistatic where you fire your own transmitter far away from yourself?

There’s got to be a reason, but it seems like best of both worlds.

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pixelesque 9 minutes ago | parent | next [-]

One reason is Low Probability of Intercept radars (and transmitters / datalinks) do exist, and are very difficult (but not impossible) to identify and locate.

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bob1029 an hour ago | parent | prev | next [-]

We've already got a really extreme version of this.

https://en.wikipedia.org/wiki/GNSS_reflectometry

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blittle 2 hours ago | parent | prev | next [-]

Probably more complicated to setup in a hostile environment because you'd need multiple transmitters, which also need to remain stationary, or at least you need to accurately know when they move.

	▲	twic an hour ago \| parent [-]
		Knowing where the transmitters are is vital. So wonder if you build in a positioning system to them. Each transmitter transmits a signal, but also rebroadcasts the signals it receives from the other transmitters on separate bands (these can be at lower power). If you can pick up a few transmitters, is that enough to build a model of where they are relative to each other, and then where they are relative to you? If each transmitter picks up the rebroadcasts if its own signals, then with some assumptions about the rebroadcast lag (or measurements of it added to the signal!), that's enough to know the range to each other transmitter, right? So maybe they do that and then just broadcast the ranges (tagged on to their main signal), then any remote receiver can work it all out from there.

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eskaytwo an hour ago | parent | prev [-]

Much more covert, and civilian infrastructure also less likely to be blown up.

	▲	thakoppno an hour ago \| parent [-]
		It seems to me this makes broadcast towers targets.

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quantum_state 4 hours ago | parent | prev | next [-]

If one visualizes the electromagnetic field in the environment, including all objects in it and any changes, one would naturally come across many applications in sensing the associated changes in the field. One classic example is the eavesdropping case at Moscow US embassy in the former Soviet Union.

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wpietri 2 hours ago | parent | prev | next [-]

What's this website? Who put it up?

Those have always been good questions to ask, but especially these days.

	▲	defrost an hour ago \| parent [-]
		The image screams Australia, the image title confirms ... https://www.silentiumdefence.com.au/our-solutions/space-doma... but whether they have any connection or are just getting image jacked ... couldn't say.

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smartscience 3 hours ago | parent | prev | next [-]

Are there any clever tricks for the data processing involved here, given that the delay is a shift in the time domain and the Doppler effect is a shift in the frequency domain? Maybe involving fractional Fourier transforms, or wavelets?

	▲	vscode-rest 2 hours ago \| parent [-]
		Lots of clever tricks, all ITAR controlled.

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sebasv_ 4 hours ago | parent | prev | next [-]

I want this, but I do not have the experience with radio signals to build this myself without more guidance. Is there a DIY proof of concept I could lean on? How much more challenging will this be if you are in an area with overlapping FM signals from 2 transmitters sending the same signal?

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lormayna 3 hours ago | parent | next [-]

This is a very good point to start: https://www.rtl-sdr.com/tag/passive-radar/

If you have more than one receiver, the main issue is time sincronisation between the receivers.

Using the two transmitters will complicate things a lot

	▲	willis936 2 hours ago \| parent [-]
		For this kind of thing you need to step up to the kraken.

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quantum_state 4 hours ago | parent | prev [-]

Would suggest to take a look at Jackson’s book on classical electrodynamics to get some intuition.

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aurizon 2 hours ago | parent | prev | next [-]

I wonder if the many Starlink satellites can be used for this? True, the signals are low and are steered, but the nature of steering creates many side lobes that will be useable in this manner. It would be a complex computational task with satellites in motion as well as ground stations transmitting on offset frequencies. I suspect various research/military labs are playing with this?

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blittle an hour ago | parent | next [-]

I did some searching and seems like an active area of research: https://ieeexplore.ieee.org/document/8768105/ and https://ieeexplore.ieee.org/document/9905046/

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eskaytwo an hour ago | parent | prev | next [-]

With their movement you might get some strange SAR like effects. Computationally complex but could add another dimension over a static tower.

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Aspos 2 hours ago | parent | prev [-]

Such radar would be a game changer.

	▲	g8oz 32 minutes ago \| parent [-]
		Indeed, full spectrum Western dominance over air space in the global South could come to an end. If that goes, the chains of hegemony loosen significantly.

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Chrisszz 4 hours ago | parent | prev [-]

a website for passive radars, cool xD