If you had a decent collection of antennas and enough digitizing and processing power, you could form a near-optical-quality 3D image of your surroundings, just by analysing the RF environment. This would be ultra-wideband radar with no transmitter.
Lots of people must be working very hard on this. The last thing a stealth aircraft needs is a radar transmitter.
John
There are obvious reasons why it can never be near optical quality imaging the wavelengths are orders of magnitude longer.
It has already been solved for the static case. Reflections of cellular phone masts from a stealth aircraft that never go back down to the source cause disruption to traffic in adjacent cells which can be used to infer the position of an otherwise stealth plane.
Regards, Martin Brown
I had a similar idea to get the location of other cars for smart vehicle-to-vehicle systems (anti-collision, lane departure warning, smart cruise-control, etc). Never put it in practise though. It shouldn't be hard to detect a car by the EMC emissions.
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
...and you can see inside a house by analysing the reflected light coming out of the windows.
Duh. They even write about it in the textbooks on radars. Try one.
The last thing the enemy would do is provide convenient RF illumination of their objects.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
If you figure that out, let me know. The new game in town, performed by 'illegals', is to hide in your blind spot... collide and collect insurance.
Had such an event Friday a week ago... "bumper bump" in a parking lot (only dirt transferred). Lots of Mexican yammering. I pulled out my cellphone and started snapping photos... they took off :-) ...Jim Thompson
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The only thing bipartisan in this country is hypocrisy
Not easily. The walls are opaque. That's not the case for radio waves in free space.
None of my radar books mentions it. Got any suggestions?
The world is full of illumination sources. There was talk of a Chinese system that passively analyzes longwave signals and can track stealth aircraft that are black at microwave frequencies.
One could also co-locate near something like an existing air traffic or weather radar and steal the transmit pulses, to make your own PPI receiver. With cheap 1 dB mmics and mixers and adcs, all you need is a lot of DSP. It's even a potential commercial product.
In the general case, some modest number if point e-field probes, and a lot of secondary processing, should visualize the environment.
John
Not necessarily. In any given band, this is probably true (although, for some reason, negative or complex index-of-refraction metamaterials are supposed to be able to make lenses that can see less than a wavelength), but after deconvolving over space *and* frequency, you get parts of some wavelengths filling in the corners of others, like building a square wave from harmonics. You get an artibrarily accurate representation.
RF extends to the THz band, which for most viewing distances is essentially optical in detail, so even if the above is not possible (let alone feasible), the statement ("ultra-wideband RF environment") is unqualified enough to be technically true, though when understood in those terms it becomes something of a vacuous truth (i.e., they've already done THz imaging).
BTW, you've already got a name for this product.
I recall reading an article about having solved a scattering matrix for a hunk of something white. That is, they took a white or translucent or frosted material and reconstructed the image transmitted through / reflected off it. Some very interesting things become possible when data collection and computation tend towards infinity.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
At long distances it could well be better. I've seen radar-derived images of satellites that were better than any conventional (non-adaptive optics) ground-based telescope could manage.
I meant "near-optical" to mean that you might resolve distant objects to image quality similar to what you could get from conventional optical instruments. From a moving platform, maybe better. The potential advantage of the RF thing is imaging everything in space in all directions simultaneously.
And maybe they are not stealthy at longer wavelengths.
John
Next time, pull out your buddies Smith & Wesson...
-- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.
For detecting cars and trucks, use the FM band where the approximate resonant frequency in the "cavity" that has the engine lies. Since the slot antenna is horizontal (the break between the hood and car body) it is obvious which polarization is dominant. All other frequencies radiated by the engine are way below JAE (i think that is the designation, it has been 30+ years) limits.
In what waveband? I have made large scale radio astronomy observations that were at the same resolution as the best optical telescopes. It is not a trivial exercise and would require something like the VLA and clever tracking to keep the satellite in view. And also powerful directed RF illumination of the target in the right waveband.
Modern optical scopes do surprisingly well at imaging satellites in space. Amateurs with webcams and customised tracking mounts do it for fun. BMEWS catalogues them when it is in its idle wait for ICBM threat mode and the orbital elements are made available to amateurs.
Synthetic aperture radar is relatively easy and gives extremely good results these days. But the results are only roughly comparable with what you would get with a standard camera lens as opposed to a large diffraction limited telescope. D/lambda really hurts in the RF band.
Jodrell Bank and Arecibo are two of the largest single dishes on the planet. Operating at 5GHz (6cm) even its 76m dish is just 1270 wavelengths across which translates roughly to same resolution in the optical that a 0.5mm webcam lens can acheive. Roughly 10 independent pixels across the moon. Arecibo after its refurb can do about 8x better. A linked interferometer array can do much better but it requires many simultaneous baselines for a decent image.
The need to keep the number of baselines high for VLBI has resulted in damage to some of the steerable big dish instruments in the past. When a big collaborative exeperiment is running the operators are loathe to drop out unless the weather conditions are really vile.
Crucial point is that some of their stealth comes from never reflecting anything back to the radar illuminator. That makes it hard for a missile to get a lockon at long range. One tactic for air defence is to have sacrificial radar emitters and sophisticated passive receivers operated as a distributed phased array. It is pretty certain that the radar emitters would quickly get zapped in a real war situation.
Regards, Martin Brown
In what waveband? I have made large scale radio astronomy observations that were at the same resolution as the best optical telescopes. It is not a trivial exercise and would require something like the VLA and clever tracking to keep the satellite in view. And also powerful directed RF illumination of the target in the right waveband.
Modern optical scopes do surprisingly well at imaging satellites in space. Amateurs with webcams and customised tracking mounts do it for fun. BMEWS catalogues them when it is in its idle wait for ICBM threat mode and the orbital elements are made available to amateurs.
Synthetic aperture radar is relatively easy and gives extremely good results these days. But the results are only roughly comparable with what you would get with a standard camera lens as opposed to a large diffraction limited telescope. D/lambda really hurts in the RF band.
Jodrell Bank and Arecibo are two of the largest single dishes on the planet. Operating at 5GHz (6cm) even its 76m dish is just 1270 wavelengths across which translates roughly to same resolution in the optical that a 0.5mm webcam lens can acheive. Roughly 10 independent pixels across the moon. Arecibo after its refurb can do about 8x better. A linked interferometer array can do much better but it requires many simultaneous baselines for a decent image.
The need to keep the number of baselines high for VLBI has resulted in damage to some of the steerable big dish instruments in the past. When a big collaborative exeperiment is running the operators are loathe to drop out unless the weather conditions are really vile.
Crucial point is that some of their stealth comes from never reflecting anything back to the radar illuminator. That makes it hard for a missile to get a lockon at long range. One tactic for air defence is to have sacrificial radar emitters and sophisticated passive receivers operated as a distributed phased array. It is pretty certain that the radar emitters would quickly get zapped in a real war situation.
Regards, Martin Brown
The Chinese have a distributed system that uses lots of low(ish) frequency transmitters and they look for "holes" and distortions that give away the location of stealth aircraft.
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK http://www.blogtalkradio.com/onetribe - Occult Talk Show
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