You can't use "efficient antenna" and "100 kHz" in the same sentence. Oh,
wait...
----- Original Message -----
From: "J. Forster" <j...@quikus.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Sunday, July 28, 2013 3:06 PM
Subject: Re: [time-nuts] GPS Spoofing
The point about the duty cycle being low is correct. And, there are
commercial linear power amps, like the used ones made by ENI and others,
that can easily put out 1 kW plus narrow pulses.
Furthermore, the pulse generator is trivial to make with a Rb, 3 or more
Tektronix DD501s, a simple OR gate and a gated oscillator at about 100
kHz. I've cobbled up that setup several times as a LORAN-A simulator.
The main difficulty is getting a reasonable match to an efficient antenna
at 100 kHz.
-John
=================
Hi
Since it's a pulse system, and you get to position your pulse for maximum
effect, I don't see any reason to generate CW power. Simply mimic the
lowest power slave in the chain. There's very little redundancy with
Loran, so spoofing one station will mess it up. No need to mask the entire
chain. At most you would need to hit two low power slaves.
Math wise:
Wavelength is 10,000 ft / 3,000M. Throw things off by ~10% of that and you
have problems in a harbor. You would need to play a bit to see weather a
pulse every so often does the trick or not. Is that 20 db below the slave
or not ? You'd have to play with it. It's in that range. A spoof that says
they are on the other side of the world isn't going to work. One that says
you are on the north side of the channel (when you are on the south side)
is what would work.
Power within a pulse set at a 5:1 duty cycle. For a 50,000 us GRI you
have another 50:1. For longer GRI's you might add another 2:1. Net is a
peak to average ratio of 250-1000 to 1. Put another way, a 500W pulse is ~
1 average.
Power at 100 KHz = what's in a fairly cheap switching power supply. Plug
it into the wall. A couple hundred watts (or even KW) pulse is cheap. Say
you have 120W out of the wall (or a car battery). If the math above is
correct and you can run 80% efficiency, that's a pretty powerful pulse.
It's probably cheaper to generate something at 50:1 rather than the whole
> 200:1. A 5KW is a *lot* of RF, even into a simple antenna.
Antenna - there's a couple ways to do that. All of them are tradeoffs
(size / cost / power). The cheap way is to use a wire that's already
there…. Since you don't need to propagate (near field), the antenna
efficiency could be higher than you would think for some antennas.
Is it easier than that with some smarts involved in the pulse - probably
yes. Do the smarts raise the hardware cost significantly? - you'd have to
build a few and find out. What really drives this or that Loran receiver
nuts? I'm quite sure you could work that out with one to play with.
Am I gong into the Loran-C jammer business? No, so don't contact me off
list to buy one. The point is not *have* I built one, but could one be
built easily.
Bob
On Jul 28, 2013, at 1:29 PM, "Poul-Henning Kamp" <p...@phk.freebsd.dk>
wrote:
In message <dab33aef-98ef-4503-89a7-657f0d25a...@rtty.us>, Bob Camp
writes:
I'm not talking about taking out Loran-C over the entire North
Atlantic.
The target is a harbor sized area. For that, you certainly do not need
a
600' antenna or megawatts of power.
No, you need about 600W (continuous) and a loop-antenna about 5m in
diameter.
Do the math, It's not as easy as you think.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by
incompetence.
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