Penetrating Radar

Started by January 6, 2005
I've read somewhere that UWB-based Radar has been used by the military
because of its ability to detect objects through walls. It's not
intuitively obvious to me why a wideband system would have this
capability. Can someone explain it?
-- 
Randy Yates
Sony Ericsson Mobile Communications
Research Triangle Park, NC, USA
randy.yates@sonyericsson.com, 919-472-1124

Randy Yates wrote:

> I've read somewhere that UWB-based Radar has been used by the military > because of its ability to detect objects through walls. It's not > intuitively obvious to me why a wideband system would have this > capability. Can someone explain it?
I was going to guess that it had to do with resonances in walls, but it seems that UWB means very short pulses, which are needed to close objects. The pulse length of 1ns means only one or a few cycles at radar frequencies, and so a wide bandwidth. http://www.uwbgroup.ru/eng/ourworks/uwbradar_2/chapt3.htm -- glen
I can see two possible explanations:

The physical dimensions of walls are such (~0.05 - 0.1 m thick) that
you need extremely
short pulse lengths to resolve objects that distance apart. The
"ususal" methods of
frequency-sweep type pulses followed by a matched filter might not work
in-doors,
because of resonant effects in rooms. So one might have to resort to
physical pulse
lengths to get these things to work.

The other possible explanation for UWB to be needed, is that the waves
that
penetrate the walls are affected by quite complex propagation effects
internal
to the walls. One might have to use a broad frequency band in order to
estimate
what effects are due to internal propagation in the wall, and what
effects are
due to an object being located on the other side.

Either way, obstructed wave propagation and a source possibly located
in a
resonant cavity makes this to be a non-trivial remote sensing problem.
It's not 
intuitively obvious to me that this works at all.

Rune

Randy Yates wrote:

> I've read somewhere that UWB-based Radar has been used by the military > because of its ability to detect objects through walls. It's not > intuitively obvious to me why a wideband system would have this > capability. Can someone explain it?
Is it possibly the radar equivalent of a gated-laser fog penetration system? I read of demos years ago, but never knew of any that were actually deployed. jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Jerry Avins <jya@ieee.org> writes:

> Randy Yates wrote: > > > I've read somewhere that UWB-based Radar has been used by the military > > because of its ability to detect objects through walls. It's not > > intuitively obvious to me why a wideband system would have this > > capability. Can someone explain it? > > Is it possibly the radar equivalent of a gated-laser fog penetration > system? I read of demos years ago, but never knew of any that were > actually deployed.
I dunno. Maybe gating the laser is spreading the spectrum, so in some ways it's similar? -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
On 06 Jan 2005 19:04:32 -0500, Randy Yates
<randy.yates@sonyericsson.com> wrote:

>I've read somewhere that UWB-based Radar has been used by the military >because of its ability to detect objects through walls. It's not >intuitively obvious to me why a wideband system would have this >capability. Can someone explain it?
I think it's kinda practical. As others have mentioned, in order to resolve very close distances you need LOTS of bandwidth. The usual well-known radar analyses show pretty easily that resolution in the range dimension is a function of bandwidth, so if you want good resolution on small objects you need buttloads of bandwidth (a technical term). There was a lot of noise being made about UWB about five years ago and at the time I saw some interesting images made by commercial UWB wall-imaging devices. The idea was to find things like pipes and wiring, but another obvious application is to find covert bugs and things like that. Ground penetrating radars are also UWB for the same reason. I would think most decent radar texts would cover the analysis for why resolution is a function of bandwidth. It's the fundamental reason why most radars chirp instead of pulse. Most UWBs pulse instead of chirp, though. I think that's because it's hard to make a system that sweeps that much bandwidth and still stays reasonably linear doing so. The electronics are apparently much easier to build for pulses than for sweeps that wide. You don't need as much sophisticated signal processing (which is really, really hard and requires hiring expensive geniuses to figure out) for pulsed systems as you do for swept systems. Since the range is much shorter the power issues aren't as bad. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Randy Yates wrote:
> Jerry Avins <jya@ieee.org> writes: > > > Randy Yates wrote: > > > > > I've read somewhere that UWB-based Radar has been used by the
military
> > > because of its ability to detect objects through walls. It's not > > > intuitively obvious to me why a wideband system would have this > > > capability. Can someone explain it? > > > > Is it possibly the radar equivalent of a gated-laser fog
penetration
> > system? I read of demos years ago, but never knew of any that were > > actually deployed. > > I dunno. Maybe gating the laser is spreading the spectrum, so in some > ways it's similar?
Now, this is interesting. I you guys made me realize I don't know much (nothing at all, actually) about optical signal processing, as implemented in various laser-based devices. Does anybody know of a decent antology of such techniques? I've seen the book by Scolnik (sp?) referenced (but haven't seen the book itself) as such an antology for radar. Rune
Eric Jacobsen wrote:

> you need buttloads of bandwidth (a technical term).
Often abbreviated "bob". Ciao, Peter K.
Peter K. wrote:
> Eric Jacobsen wrote: > > >>you need buttloads of bandwidth (a technical term). > > Often abbreviated "bob". > > Ciao, > > Peter K. >
Great ... just great .... yet another acronym I have to memorize!! :)
Eric Jacobsen wrote:
> On 06 Jan 2005 19:04:32 -0500, Randy Yates > <randy.yates@sonyericsson.com> wrote: > > >>I've read somewhere that UWB-based Radar has been used by the military >>because of its ability to detect objects through walls. It's not >>intuitively obvious to me why a wideband system would have this >>capability. Can someone explain it? > > > I think it's kinda practical. > > As others have mentioned, in order to resolve very close distances you > need LOTS of bandwidth. The usual well-known radar analyses show > pretty easily that resolution in the range dimension is a function of > bandwidth, so if you want good resolution on small objects you need > buttloads of bandwidth (a technical term). > > There was a lot of noise being made about UWB about five years ago and > at the time I saw some interesting images made by commercial UWB > wall-imaging devices. The idea was to find things like pipes and > wiring, but another obvious application is to find covert bugs and > things like that. > > Ground penetrating radars are also UWB for the same reason. > > I would think most decent radar texts would cover the analysis for why > resolution is a function of bandwidth. It's the fundamental reason > why most radars chirp instead of pulse. Most UWBs pulse instead of > chirp, though. I think that's because it's hard to make a system that > sweeps that much bandwidth and still stays reasonably linear doing so. > The electronics are apparently much easier to build for pulses than > for sweeps that wide. You don't need as much sophisticated signal > processing (which is really, really hard and requires hiring expensive > geniuses to figure out) for pulsed systems as you do for swept > systems. Since the range is much shorter the power issues aren't as > bad. > > > Eric Jacobsen > Minister of Algorithms, Intel Corp. > My opinions may not be Intel's opinions. > http://www.ericjacobsen.org
Yes, most of the radar texts do cover this, it's usually discussed under "ambiguity function" analysis. In Radar you can often get away with an assumption that the doppler shift is a frequency shifted version of your pulse, unfortunately in Sonar systems you don't have that luxury. BTW - I'm neither highly paid nor a genius. Is there something I'm missing?