# Keep An Eye On The Neighborhood With This Passive Radar

If your neighborhood is anything like ours, walking across the street is like taking your life in your own hands. Drivers are increasingly unconcerned by such trivialities as speed limits or staying under control, and anything goes when they need to connect Point A to Point B in the least amount of time possible. Monitoring traffic with this passive radar will not do a thing to slow drivers down, but it’s a pretty cool hack that will at least yield some insights into traffic patterns.

The principle behind active radar – the kind police use to catch speeders in every neighborhood but yours – is simple: send a microwave signal towards a moving object, measure the frequency shift in the reflected signal, and do a little math to calculate the relative velocity. A passive radar like the one described in the RTL-SDR.com article linked above is quite different. Rather than painting a target with an RF signal, it relies on signals from other transmitters, such as terrestrial TV or radio outlets in the area. Two different receivers are used, both with directional antennas. One points to the area to be monitored, while the other points directly to the transmitter. By comparing signals reflected off moving objects received by the former against the reference signal from the latter, information about the distance and velocity of objects in the target area can be obtained.

The RTL-SDR test used a pair of cheap Yagi antennas for a nearby DVB-T channel to feed their KerberosSDR four-channel coherent SDR, a device we last looked at when it was still in beta. Essentially four SDR dongles on a common board, it’s available now for \$149. Using it to build a passive radar might not save the neighborhood, but it could be a lot of fun to try.

## 24 thoughts on “Keep An Eye On The Neighborhood With This Passive Radar”

1. Truth says:

It is like a 1D radar.
You have a number that can be considered an approximation of the distance from the delay between the two signals.
You have a number that can be considered an approximation of the speed (which would change based on relative angles) based on the doppler shift between the directly received reference signal and the reflected signal.

This begs the question if you used all four ports on the KerberosSDR, one for the reference and three extremely well spaced antennas (using equal lengths of cable, the object to be observed being at least ten times the size of the wavelength lightening up the object and the spacing of the antennas at least ten times the size of the object to be observed) at known relative positions in 3D space, would you have a crude radar by solving the position location of the intersecting hyperboloids.

1. I believe you could. This is exactly the idea behind the infamous Czech Vera/Tamara series of passive sensors:
https://en.wikipedia.org/wiki/VERA_passive_sensor

These are supposedly capable of detecting even stealth aircraft. Especially Tamara was feared for this capability during the late 80s – it was the only system capable of doing it in the world and it was made in the soviet bloc …

1. epitaxial says:

Some really brilliant work has come from soviet engineering. My favorite is the passive listening device invented by Leon Theremin.

2. Leithoa says:

I’d wager most of that capability has to do with the spectrum used. Stealth isn’t an invisibility cloak, It lowers your apparent cross-section to a given frequency band. You can (and they do) make a missile (or target drone) look like an B-52 by reversing the same principles that make a bomber look like a party balloon.

2. Truth says:

I wonder if you pointed the four antennas skyward and tuned them to 1090 MHz could you use ADS-B Mode S Airborne Positions transponder squawks ( https://mode-s.org/decode/adsb/airborne-position.html ) which contain Altitude, Latitude, Longitude and time to calculate the positions of the antennas. I suppose that it would be a kind of poor/smart persons GPS. One squawk would not provide an accurate position, but multiple squawks from multiple aircraft in multiple locations over time, could possibly average out to extremely accurate antenna locations.

Probably not useful for a handful of antennas, that you could walk about with a handheld GPS (phone) and tag them individually, but if you had a very large array.

1. Truth says:

Actually that message only has 1 bit allocated for the time, there must be another message type that contains the time, because 1 bit for time has me scratching my head. Time to dig up and start reading a very long and boring document.

2. Moryc says:

If police radars (the kind that takes pictures of speeding cars) were combined with rifles, the problem of speeding would die out…

1. Shannnon says:

Given the response of some people to being caught speeding I don’t think it would. For them the blame lies with the police, the speed camera, etc. not with their own driving.

1. yetihehe says:

Given that those people would be eliminated, I would say that this would solve many other problems with one (pardon the pun) shot.

2. Vinalon says:

Particularly on freeways without many lanes, sometimes it can be dangerous to go as slow as the speed limit. “Add 5-10mph when it’s >55mph” is the American way.

…Sorry, that’s “add ~10kph when it’s >=100kph”? When you want to take things slow without annoying people, you can just follow a big truck.

1. yetihehe says:

Following one big truck for longer periods of time may cause unnecessary stops by police (truckers will call police because they think someone wants to rob them).

2. Not too far from here at the border of the next US state south of us the speed limit transitions from 65 mph to 55 mph. At the same place, the average traffic speed jumps from 75 mph to 85 mph for reasons that are beyond me. Anyone doing the speed limit there is basically a rolling road block causing all kinds of near misses and traffic snarles.

1. Bill says:

I assume the state where people drive faster is less likely to enforce the speed limit.

3. cb88 says:

No, you merely need to increase the radar power output as well as increase the gain directionally targeted at the location of the excess of speed. Just fry whatever is going to fast… call it the ping of death.

4. Yes, but the scheme might be rather hard on cops (racing to a crime scene) and firefighters (racing to a fire), not to mention a hubby rushing to get his wife to the hospital when her birth pangs are three minutes apart.

3. Ren says:

I was thinking more along the lines of using a Hot Wheels Radar Gun strategically placed along the street and attached to a small solar panel. A 555 would trigger the radar at intervals.
It could be enough to slow down traffic in the neighborhood as radar detectors go off.
Relocate the radar gun after a week or so to keep the repeat offenders from thinking, “Oh, the detector lights up at this location all the time, so I can just ignore it.”

1. Bill says:

Just report your location as a speed trap on Waze

1. chester moore says:

Bill gets it.

Fun story. My wife is a cop, we have a patrol car in the driveway most of the time. She works a few towns over, so she can’t do anything about the traffic in our neighborhood. People have become accustomed to the patrol car that can’t pull them over and the near permanent Waze report, and they speed by our house and run the stop sign in front of our house. I offered the local cops the use of my driveway for a day. The footage from my driveway camera is priceless.

1. Jonathan says:

Oh I say… Well Played Sir!

2. thoriumbr says:

> The footage from my driveway camera is priceless.

1. Yeah, now that the famous 11-foot-8 bridge has been raised, we need a new Youtube channel to watch.

2. PFFT says:

And you get reported to the FCC for deliberately causing radio interference to legal radio receivers and a nice fat fine along with it… Let the real police do their jobs. Not that many people even run radar detectors to be worth your trouble.

But yeah, most people that are serious about radar detectors know that X-band is pretty much obsolete as enforcement radar and therefore have it locked out anyway rather than waste precious dwell time on it rather than scanning the much more wide and dangerous Ka band. And Escort has been making nice line of radar detectors with crowdsourcing built in for years now, which is effectively automatically tying their detectors to Waze and giving a heads up without even needing a sniff of RF… Cat and mouse.