semi-passive radar

Discuss KerberosSDR - 4x Coherent RTL-SDR
User avatar
kb3cs
Posts: 34
Joined: Thu Apr 04, 2019 2:15 pm

semi-passive radar

Post by kb3cs » Thu Apr 04, 2019 2:31 pm

i'd like to invite comments on a possibly not-so-crazy idea for how to use the passive radar capability.

the idea is to use my own source of RF (a FRS walkie-talkie, for instance [no modulation FM, 0.5W, UHF band]) to illuminate the field of interest.

i suspect a simple sampling probe is enough to pick up the reference signal - a bit of wire on the 'hot' end of an exposed [unshielded] 50 ohm resistor on the end of a coax cable. is this sensible?

next, locate the RF illumination source behind the directional surveillance antenna in the relative null of the antenna pattern. this still making good sense?

use physical separation between the RF source and the surveillance antenna to control how much direct signal 'leaks' into the surveillance input port.

good idea or kinda crazy (as in: might be difficult to get good results)?

kuhny1
Posts: 15
Joined: Tue Mar 26, 2019 7:44 pm

Re: semi-passive radar

Post by kuhny1 » Thu Apr 04, 2019 8:59 pm

I would say your main limitation would be the TX power. 500mW wont really be enough to get any range out of it. The main point for using a passive radar, aleast recreationally, is because it allows you to use another transmitter running at >100W, legally. Its worth a shot, but I wouldnt expect more than a quarter mile.

rtlsdrblog
Site Admin
Posts: 2183
Joined: Mon Nov 19, 2012 11:54 pm

Re: semi-passive radar

Post by rtlsdrblog » Thu Apr 04, 2019 10:00 pm

If the TX is within coax distance of the KerberoSDR you can even just connect it directly via a splitter and sufficient attenuation. The better the quality of the reference signal RX the better the results.

But as the other poster mentioned, the low TX power might not be helpful unless you're looking to detect objects a few meters away. Also make sure you stay legal

User avatar
kb3cs
Posts: 34
Joined: Thu Apr 04, 2019 2:15 pm

Re: semi-passive radar

Post by kb3cs » Fri Apr 05, 2019 7:15 pm

i used FRS as an example because it is consumer gear and unlicensed. no problem staying legal.

the terminated but unshielded end of coax serves well as an RF probe and attenuation is adjustable by the distance to the transmitter's antenna.

1/4 mi is plenty of range for what i have in mind to measure: vehicle traffic. especially big rigs. i don't need to illuminate a whole county, just a stretch of road.

as i also hold an Amateur Radio license, i have higher power options legally available.

it looks like the general idea/plan passes the sanity check. thanks for being a sounding board!
:geek:

n3jli
Posts: 1
Joined: Sat Apr 06, 2019 4:48 pm

Re: semi-passive radar

Post by n3jli » Sat Apr 06, 2019 4:51 pm

Down a similar path:

I was thinking about ground penetrating radar.

User avatar
kb3cs
Posts: 34
Joined: Thu Apr 04, 2019 2:15 pm

Re: semi-passive radar

Post by kb3cs » Sat Apr 06, 2019 9:01 pm

i think you're going to need specially crafted modulation from the illumination source and pin diode T/R switches.
your echoes won't be doppler-shifted, of course, therefore your measurements will be time delays.

jsalsburg
Posts: 4
Joined: Wed May 15, 2019 9:56 pm

Re: semi-passive radar

Post by jsalsburg » Thu May 16, 2019 5:44 am

About Ground Penetrating RADAR. Requires a high voltage (Deadly 1000 Volts) 400 MHz Pulsed Transmitter driving a bow-tie antenna inside a large box facing the ground on wheels. Using an array of receiving antennas connected to the KerberosSDR is possible but would be large and cumbersome, the size of a car or larger. You may have seen a variant of this that scans streets and freeways for defects in the rebar inside the concrete. Also there is a version for railroads. The problem with this as a single user is that it takes a supercomputer to analyze the data which is massive. A Ground Penetrating RADAR array with 4 receivers would be a massive undertaking from the signal processing standpoint and may not provide an advantage over a single receiver which are available off the shelf.

jsalsburg
Posts: 4
Joined: Wed May 15, 2019 9:56 pm

Re: semi-passive radar

Post by jsalsburg » Thu May 16, 2019 6:14 am

About detecting traffic. The only reason to use Direction Finding/Passive RADAR for traffic activity is to determine 1. direction. 2. Range and 3.Velocity which can produce imaging of the traffic. Simple low cost RADAR modules are available online for less than $100 that will detect large truck's velocity for 1/4 mile distance with a fetch width of 30 degrees. If you are trying to cover a large omni-directional area, then use more than one. But if you want an image of the traffic movement as seen from above, Active back-scatter pulsed RADAR is needed not Passive RADAR. Typically RADAR for Traffic monitoring is 24 and 35 GHz because these higher illumination frequencies supply higher Doppler velocity frequencies and therefore greater velocity resolution of traffic movement because vehicles are moving so slowly; below 60 mph. Lower illumination frequencies are optimal for faster moving objects like aircraft and rockets; greater than 100 mph, but still are 1-4 GHz. NEXRAD also uses the 1-4 GHz frequencies but supply much lower Doppler resolution than higher frequency RADAR, the reason is that higher frequencies will not penetrate rain as well so the NEXRAD RADAR can image through the rain/hail/snow. Space RADAR uses 200 to 400 MHz RADAR to allow detection of space objects at much greater distance and velocities greater than 17,000 mph and distances as great as 10,000-1,000,000 miles.
http://radar.salsburg.com/

rtlsdrblog
Site Admin
Posts: 2183
Joined: Mon Nov 19, 2012 11:54 pm

Re: semi-passive radar

Post by rtlsdrblog » Thu May 16, 2019 10:45 am

jsalsburg wrote:
Thu May 16, 2019 6:14 am
About detecting traffic. The only reason to use Direction Finding/Passive RADAR for traffic activity is to determine 1. direction. 2. Range and 3.Velocity which can produce imaging of the traffic. Simple low cost RADAR modules are available online for less than $100 that will detect large truck's velocity for 1/4 mile distance with a fetch width of 30 degrees. If you are trying to cover a large omni-directional area, then use more than one. But if you want an image of the traffic movement as seen from above, Active back-scatter pulsed RADAR is needed not Passive RADAR. Typically RADAR for Traffic monitoring is 24 and 35 GHz because these higher illumination frequencies supply higher Doppler velocity frequencies and therefore greater velocity resolution of traffic movement because vehicles are moving so slowly; below 60 mph. Lower illumination frequencies are optimal for faster moving objects like aircraft and rockets; greater than 100 mph, but still are 1-4 GHz. NEXRAD also uses the 1-4 GHz frequencies but supply much lower Doppler resolution than higher frequency RADAR, the reason is that higher frequencies will not penetrate rain as well so the NEXRAD RADAR can image through the rain/hail/snow. Space RADAR uses 200 to 400 MHz RADAR to allow detection of space objects at much greater distance and velocities greater than 17,000 mph and distances as great as 10,000-1,000,000 miles.
http://radar.salsburg.com/
Regarding passive traffic radar I believe there have been a few academic papers on this topic. e.g. https://sci-hub.tw/10.1109/maes.2017.150271
Other advantages of passive radars relate to
the possibilities of stealth target detection and gap filling in areas not
covered by active radars [10], [11]. Another huge advantage of the
passive radar lies in the fact that there is no need for requirements for
spectrum allocation or powerful transmitters. Consequently, as the
aforementioned points make passive radars great representatives of
“green” technology, one can install them in “sanctuaries”–specific or
specialized places such as high-density urban areas, sites located in
the vicinities of hospitals, and/or other places where electromagnetic
emissions are limited by law because of the risk of interference or
simply through the public's fear of electromagnetic fields. Also, it is
worth mentioning that PCL systems can be cheaper than active ones
as the passive radar does not have its own transmitter and typically
consists of a simple radio receiver with commercial off-the-shelf
(COTS) hardware, and uses a commercially available PC
Higher bandwidth than the RTL-SDRs can provide is probably required for any real applications, but for testing the Kerberos should give some idea of the possibility.

snn47
Posts: 147
Joined: Tue Dec 27, 2016 11:00 pm

Re: semi-passive radar

Post by snn47 » Thu May 16, 2019 11:42 am

What do you want to detect, position (distance and angle to a target) and/or speed?

RADAR (Radio Detection and Ranging)is either primary (meaning you process passive echos on a target, for this you need transmitter(s) to illuminate a target to receive an echo(s))
or active using interrogations of transponder like those used for SSR, IFF, MLAT, ACS(TCAS) (1030/1090MHz) where the RADAR interrogator Rx receives active replies by a target.

Primary RADAR transmitter(s) and antenna(s) are normally at the RADAR location or within a few meter of each other.
- direction is derived from high gain, narrow beamwidth antenna (ATC Radar 36 dBi or more) which rotates 360°, alternatively scan in a given sector or follow a target. The antenna direction provides the direction relative to the RADAR antenna to the target
- distance is measured from the time elapsed between transmission and reception of the RADAR echo(s) in pulsed RADARS or from difference of frequency change e.g. for FM-RADAR, which is why without some form of modulation you cannot measure distance
- speed either through some form of detection of Doppler shift in a MTI/MTD circuit, or through calculation from the movement of target positions.

Semi passive RADAR requires RF source(s) that illuminate the target(s) (independent if it is your own Tx or if you use radiation from BC, TV or any other Tx that work). These illuminator are at separate locations than RADAR Rx’s. At least 3 receiver are required, that are distributed in a geometry around the area you want to detect targets to allow Multilateration derived by amplitude/frequency/phase modulation from target echo’s.

With a single source without modulation e.g. in form of some kind of pulse(compressed) or FM modulation it will be hard separate echo’s the desired targets vs. multipath reflections, backbeam and sidelobe radiation, which are likely to be stronger than target reflections.

Frequency selection is always a tradeoff between range, target resolution, antenna size, transmit power to overcome path loss.
Most MM- and RADARs for detection of aircraft used to be for initially well below 1000 MHz using e.g. Yagi antennas, until enough power could be generated at higher frequencies. There are all kinds of RADAR e.g. Weather, radio astronomy.
Then there are over the horizon radars below 30 MHz (e.g. the famous wood pecker from the sound of it’s signals).
Automotive RADAR components are now focusing around 70 GHz, while speeding radar are for size restriction >10 GHz.
Wall probing and ground penetrating RADAR use to my knowledge mostly UWB signals, which will severly interferey any other RADAR or reduce/interfere with

Post Reply