On 7/22/13 6:30 AM, Tom Van Baak wrote:
For what it's worth, the application is a radar that detects
buried victims in disaster rubble, so the data we are collecting is
basically heartbeats and breathing. the "when was the data taken"
is a "where were we when the data was collected" need. The "sync"
requirement comes from being able to find the same heartbeat in
multiple data streams.
Jim,
That's a fascinating application. Ok, one last comment then. As much
as GPS is an obvious solution, did you consider the use of multiple
homebrew timing pulse pseudolites instead? If one placed a couple of
them around the vicinity of the disaster area you could triangulate
for your ranging and timing information. Since the transmitters would
then be less than 1 km away instead of more than 20,000 km away, you
avoid all the limitations of GPS. Moreover, the solution would work
if GPS were functional or not, something perhaps important for a
disaster situation. Using some clever waveform (like what Loran-C
did) you could better penetrate into all sorts of rubble and overcome
multi-path at the same time.
Great minds think along similar paths.. Yes, in the long term plan,
that's essentially what we'll do: multilateration among the multiple
units. But for the mean time, we need to mark the locations with GPS
derived coordinates. The search and rescue folks currently use Garmin
handhelds, so they're aware of where it works and where it doesn't.
I'm pretty sure there are a number of papers on local ranging and
navigation based on these technique (e.g., UAV, robotics). The
advantage is that it's a local solution, it can work indoors,
underground, underwater, even on the moon; in general, it doesn't
rely on a functioning GPS constellation with its inherently weak or
jammable signals.
Interestingly, there's a whole lot of "laboratory demos" of various and
sundry techniques, but the GPS-denied time/frequency/position problem is
very non-trivial. For instance propagation through rubble is a fairly
harsh multipath/scattering environment. The delay spread is about 10%
of the distance traveled, and knowing what the average "epsilon" is, so
you can turn propagation time into distance, is not easy to determine.
Anyone who claims that they can do centimeter level position
determination at a range of 10 meters in rubble (or can do microwave
imaging/SAR/etc, which implies the same thing) is, I think, blowing
smoke, unless they have a VERY sophisticated measurement setup and a
huge amount of computational horsepower to solve the "inversion
problem". (essentially, look at the first returns in time, use that to
solve for the EM parameters of the first layer; then move the analysis
front forward a step, do it again, etc., etc.,etc.)
Fortunately, unlike seismic data processing, for the most part the
medium is linear, albeit somewhat anisotropic.
Of course all this from the guy who once tried to use seismic sensors
and GPSDO to triangulate moles in his backyard...
A valiant effort I'm sure.. And, I'll bet you learned all about the
propagation anisotropy of soil<grin>
But yes, in general there are general strategies to do multilateration
(and multiangulation using direction of arrival as well) among multiple
nodes that are cooperating. Taking a general strategy to something that
actually works in the field, preferably using off the shelf hardware
modules of some sort, is a bit of a chore.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
