From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: More on HF DF (or ADF)
Date: 12 Nov 2002 10:52:29 -0800
NNTP-Posting-Date: 12 Nov 2002 18:52:29 GMT
There must, of course, be some modulation that covers a broad enough
spectrum that you have something to correlate.
For synchronization, who needs GPS? You can synchronize very
accurately with a lot of other available signals. One example is
broadcast television, which nicely transmits synchronization pulses
and tone bursts. An ordinary television receiver locks to the color
sync tone burst within a very few degrees...probably within a
nanosecond or so. I'm under the impression that you have to get
fairly fancy with GPS to get that good...we've been doing some
GPS-synchronized acquisition using internet-controlled "receivers,"
and my understanding is that you don't get much better than 50
nanosecond relative sync stability. That's not bad, of
course...that's only 50 feet of propagation distance. There's also a
patented technique for getting withing a couple hundred nanoseconds,
just using internet connections (and hardware that reads special
packets). In any such system, of course, you should expect to
calibrate out systematic errors.
Several years ago, I thought about how simply you might be able to
make such a system. You'd like to make receivers which have very
little drift of their delay times. I wondered how well you could do
with something like a "direct conversion" I and Q receiver, digitizing
the outputs and feeding them to a DSP. Put emphasis on a high TOI, so
you can leave the bandwidth wide, so you don't have to have filters
with narrow bandwidth and long delays that can drift around. As with
lots of other ideas, I never found time to develop that one. Sigh.
The "receivers" in the system I mentioned above have a 10MHz
bandwidth, and have the luxury of residing in temperature-controlled
environments, so phase (time) shifts aren't much of a problem. Of
course, you have to be careful with your synchronization signal also.
Joop van der Velden wrote in message news:...
> Jim Weir wrote:
> > There has been some discussion on this NG about how to detect the location of a
> > transmitter in the HF region (80 and 40 meters is the subject of some interest
> > here) using everything from a handheld radio's loopstick to a curtain array that
> > will cover several dozen acres.
> It seems to me that nowadays, when we all can have nanosecond-accurate
> gps-based synchonized clocks also synchronizing our receivers, it should
> be possible to find the direction and distance of a transmitter without
> any directional antenna.
> With 3 or more distant receivers, correlating their reception, it should
> be possible to pinpoint a transmitter within milliseconds. Is anybody
> aware of amateur projects that uses this principle?