-------------- Original message --------------
From: Fred Jensen <[EMAIL PROTECTED]>
> [EMAIL PROTECTED] wrote:
> > Modern DF arrays can fix a position from a single location. Modern
> > DF arrays measure the phase differences very accurately and can use
> > the phase difference to obtain directivity and azimuth.
>
> How does one obtain a complete set of coordinates for the origin of a
> received signal using only one passive receiving site (i.e. no
> transponder for time delay measurement)? I understand bearing (azimuth)
> from the rx site ... how does one obtain distance?
>
Throughout the late '60s and up until June of this year Stanford University and
SRI International were heavely involved in research and development of OTH-B
(Over The Horizon Backscatter) radar systems. Actually Stanford got out of the
business in 1970 when it was forced to drop its classified research, but I
digress. During this period I developed signal processing and control software
for SRI's OTH radar test bed. This was a bistatic system with the receive site
located near Los Banos, California and the transmit site near Lost Hills, a
separation of about 85 km. The receive antenna consisted of 256 vertical
monopoles separated by 10 meters for a total length of 2.5 km, using analog
beamforming to produce 32 independent sub-arrays. The full array provided
azimuthal resolution of 0.25 deg at 10 MHz.
Which is the long way around of addressing the distance measuring question...no
you cannot determine distance based on a single monostatic "look". We developed
a number of techniques for distance determination. The radar waveform was
swept-frequency CW ("Chirp").The waveform generators at the receive and
transmit sites were synchronized with HP cesium clocks with the receive site's
time delay offset to allow "range gating" of the received backscatter. By
looking at the time delay of the backscatter, distance to the echo could be
determined. But, wait, just knowing time delay doesn't solve the equation, you
need to know the propagation path and the height of the ionosphere to determine
the path length and with a little geometry the ground distance to the echo. A
separate backscatter sounding and vertical incidence sounding transmitter was
used to determine the ionospheric layer structure and hence ground range from a
given "slant range".
We also deployed "repeaters" at known time delays in the radar coverage area to
provide "ground truth" and verify the accuracy of our ionospheric path
determination.
So it was a sad day in June when the sites were "bulldozed", having spent a
great deal of my professional life at one site or the other. The threat that
OTH was designed to meet disappeared. The Air Force built a system in Maine
based on our design, also one in Idaho which never went operational. The Navy
still has an operational system on the East coast which looks into the
Caribbean but that's about it for US OTH radar.
73,Doug W6JD
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