On Sun, Apr 23, 2000 at 09:28:06PM -0400, Matthew R. Potter wrote:
> THis may be a little off topic but i think the topic is interesting. Has
> anyone heard of a type of passive radar that defeats stealth. i.e it relys
> on commercial radio and back ground radio as the source and the listening
> stations are totally passive. I assume the stealth objets look like black
> spots compared to the backrgound, or there are disturbances in the
> background relative to the rest of the atmosphere that are the targets.
> Obviously this is a very very good system as one dose not have to transmit
> any signal thus you cant target the sites based on radio signals. So you
> have to taget everythg that emits RF radation for a large radius. Or know
> where the sites are and drop a conventional or laser guided bomb on them. I
> think the chinese are developing a system like this. And conventional
> planes probably stick out like hell.
There has been quite a bit of public discussion of this passive
radar technology in the past 3 or 4 years. Apparently a prototype
has been built and tested by one of the big aerospace firms. And I
also recall recently reading that the US believes the mainland Chinese
have developed their own version and also have the technology.
It works by detecting energy from TV and FM radio (and presumably
any other useful VHF/UHF signals) that bounces off a target aircraft. As
people who live in the path of landing aircraft know, tv signals reflected
from aircraft can cause very noticable interference in over the air
reception (aircraft flutter). TVs are designed to reject this as much
as possible, but with suitable modern DSP technology it is quite possible
to identify the delayed signal components bounced off the aircraft
and determine what the time delay was for the reflected path versus
the direct path. Given that information from a whole network of ground
stations scattered about an area in accurately known locations it becomes
possible to mathematically construct an estimate of the position of the
scattering aircraft. And with the right clever signal processing math
this can be done simultaneuously for a bunch of aircraft each contributing
its own echo at slightly different delay from the direct path.
As for stealth objects, if they really don't reflect any energy
they are still invisible to such a multistatic radar. But some of the
secret of stealth design is based not on eliminating reflected energy
altogether, but directing it away from the source as much as possible
at odd angles so a traditional radar that uses the same antenna to receive
the reflection as it uses to transmit the signal gets as little back as
possible. And energy scattered from a myriad of TV and FM transmitters
at various angles to a stealth vehicle to a large network of receivers
at an equally diverse set of angles is not the simple case of a radar
looking at reflections directly back from an oncoming aircraft. And
if the stealth vehicle gets its stealth by reflecting stuff at odd
angles rather than absorbing it some signal from some TV or FM transmitter
may reach enough receivers located at the right odd angles (especially
if there are a lot of receiver sites) to make it detectable.
Another problem is that stealth absorptive techniques work
somewhat less well at low VHF frequencies such as near the FM broadcast
band and the geometry shaping tricks get harder to do with longer
wavelengths closer to the size of the vehicle, so stealth vehicles are
less stealthy at lowband TV and FM broadcast frequencies, where there
may be lots of signal sources.
Dave Emery N1PRE, die@...
DIE Consulting, Weston, Mass.
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2 5D 27 BD B0 24 88 C3 18