Re: garage remote jamming

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

"Dave Houston" <nobody@xxxxxxxxxxxx> wrote in message
news:45e9594b.914007187@xxxxxxxxxxxxxxxx
> "Robert Green" <ROBERT_GREEN1963@xxxxxxxxx> wrote:
>
> >There are a number of spread spectrum techniques.  The one I am familiar
> >with:
> >
> >http://en.wikipedia.org/wiki/Frequency_hopping
> >
> >clearly switches between frequencies.
>
> I believe Jeff was describing DSSS (Direct Sequence Spread Spectrum) which
> does use only one carrier frequency but is still subject to jamming.
See...
>
>      http://rfdesign.com/images/archive/0900McCune90.pdf

We're veering off into one of the semantic excursions that often obscure the
much simpler, original question:  Can a system like RadioRA be jammed by a
transmitter of much greater power at the "same frequency?"

The system that Jeff's talking about, DSSS, is described at Wikipedia:

"This noise signal is a pseudorandom sequence of 1 and -1 values, at a
frequency much higher than that of the original signal, thereby spreading
the energy of the original signal into a much wider band."

Band of what?  (-:

"As this description suggests, a plot of the transmitted waveform has a
roughly bell-shaped envelope centered on the carrier frequency, just like a
normal AM transmission, except that the added noise causes the distribution
to be much wider than that of an AM transmission."

Distribution of what? (-:

Certainly spread spectrum can performed in a variety of ways, and though
these technologies obviously provide greater *resistance* to jamming than
simple radio designs, nothing I've read so far suggests that any one of them
is *immune* to jamming.

They seem best suited for rejecting the low power interference created by
other, similar devices.  DSSS seems especially adept at recovering very weak
signals (like satellite transmissions) from a sea of *normal* radio noise.
It also seems quite useful in providing synchronization information between
multiple transmitters, a great asset to
GPSs.  But the military can jam civilian GPSs and they even admit to that
capacity openly, along with the ability to keep military GPSs in operation
while they do it.

******************************************************************
http://www.newscientist.com/article.ns?id=dn3474

The Pentagon has told New Scientist it will not implement a global GPS
blackout for civilian users if war starts in Iraq, as seems increasingly
likely.

"We would not create a global problem for transport out of spite for
Saddam," says a spokesman at the US Department of Defense. However, he
admitted that the US military does have the capability to jam civilian GPS
signals regionally, and did not rule this option out.

. . .

"A GPS expert at the University of New Brunswick in Canada, Richard Langley
says "the civilian GPS signal relies on a so-called "coarse/acquisition
code" (c/a), which enables a receiver to determine the distance to the
satellite. But the US military relies on the "precise code" (p).  The p code
is transmitted over a much wider bandwidth than the c/a code explains
Langley: "So you can jam the narrower c/a code without jamming the wider
signal."

******************************************************************

Simple brute force would probably knock out a lot of gear right near the
jamming source.  A much larger radius could be achieved if the pseudorandom
sequence of 1 and -1 values of DSSS or the frequency map of a frequency
hopping system were known.

There have been a number of situations I know of where radio traffic shot up
so much in such a short time that communications broke down nearly
completely.  9/11 and the recent tidal wave come to mind.  A lot of radio
gear that's never active simultaneously otherwise comes on line and it all
fails.  I'm not sure what can be done to address those issues but a lot of
problems resulted from the breakdown of communications at a time when
communications were critical.  So there's still room for progress in radio
comm.

Of course, this will be more of an issue when HA switches all have embedded
GPS receivers to detect sinkholes or landslides or other tectonic activities
. . . (-:

--
Bobby G.