[time-nuts] The Demise of LORAN (was Re: Reference oscillator accuracy)

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Nov 16 08:10:17 UTC 2009


Various not so random notes:

The power needed to jam GPS depends a lot on receiver state, during
TTFF it takes virtually nothing.

Therefore most "real" jammers will periodically blast at high power,
to "dislodge" any locked receivers and then continue at low power
to keep them off the signal.  They are also built spectrum efficient,
by emitting a signal designed specifically to interfere with GPS'
spread-spectrum encoding, either by trying to lure the receivers
to aim for the jammer (lowest  power) or just by mimicking the worst
kind of power for acquisition & tracking (higher power).

Most receivers have hard-limiting inputs, so overload is a slightly
more involved concept than for analog inputs, but it is still
possible.

The jammers which were quoted earlier are not "real jammers": they
are just simple noise-sources, and long range is a negative sales
parameter, because they are intended for "personal protection": a
long range would increase the risk that they get detected.

Their main customer base is drug-runners, fraudulent businessmen,
infidel husbands and criminals sentenced to home-confinement with
a GPS a ancle-bracelet.   Many of those jammers does not work as
well as advertised.  Some of them are even "trojaned" and emit a
signature signal for the benefit of law-enforcement.

The infamous tv-preamp case was so efficient because it trippled
the frequency of a local TV signal, due to instability, went into
saturation/clipping and had a circular antenna with convenient
dimensions to radiation of the resulting blanket of noise around
the GPS frequency.

Unfortunately, nobody tought about measuring its power-consumption
or if they did, they didn't publish it.  Given the kind of UHF
transistors usually used in antenna-preamps, we are very likely
talking no more than 1W.

I an urban/hi-rise environment, havoc can be played with jammers
that use glass facades as reflectors for the signal.  The story
about the "Mexican LORAN-C jammer" is instructive in how that
complicates finding the trouble.

GPS antennas on planes in the air do receive some help from being
above it all, and pilots can still fly without GPS.  The trouble
starts once CATIII landings on GPS become routine.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at 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.




More information about the Time-nuts_lists.febo.com mailing list