progression of technologies

jim bell jdb10987 at yahoo.com
Sun Jul 12 14:53:05 PDT 2015


  From: "wirelesswarrior at Safe-mail.net" <wirelesswarrior at Safe-mail.net>
-------- Original Message --------


From: jim bell <jdb10987 at yahoo.com>
Apparently from: cypherpunks-bounces at cpunks.org
To: Tom Ritter <tom at ritter.vg>, "dan at geer.org" <dan at geer.org>
Cc: cpunks <cypherpunks at cpunks.org>
>It would seem that a GPS jammer keyfob, that could be turned off when desired, would do the trick. The GPS >signal at ground level is very weak and only works due to a huge process gain. Narrowband jamming will >desensitize the closeby tracker's front end easily countering that process gain without jamming more distanrt >GPS receivers. Building such a jammer from cheap and available components should be easy.
Yes, jamming ordinary (non-military) GPS would be easy.  The two frequencies involved are: 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal).  (The former is the civilian signal; the latter is the military signal.)  The system uses spread-spectrum techniques, but even these can be defeated readily with only very tiny amounts of power.  The ERP (effective radiated power) of the L1 signal is about 250 watts (meaning that the signal is equivalent to an isotropic, or non-directional, radiator emitting 250 watts.)  Due to the inverse-square law, the emissions at 20,200 km altitude are equivalent a signal a trillion times (120 decibels) weaker at a distance of 20,200,000/1,000,000 or 20.2 meters away:  A signal 250 watts/1 trillion is 0.25 nanowatts.  I don't know offhand what the spread-spectrum advantage of the system is, but a nearby signal of 1 microwatt would probably obliterate any received signal.  You will notice that the ratio of the two frequencies, L1 and L2, is very close (about 0.19% difference) to 9:7, being two odd numbers.  This is quite convenient.  What would be needed is a square wave oscillator at a frequency of 1.57542 GHz/9, or  175.04666 Mhz.  This signal, filtered to remove signal much below 1.2 GHz, will emit harmonics at both the L1 and L2 signals.  It's not going to be an especially 'clean' jammer, having spurs also at the 3rd harmonic of 175.04666 MHz as well as the fifth harmonic, as well as the odd harmonics above the 9th.  Making the 175 MHz squarewave would be a simple matter, using a PLL (Phase Lock Loop) frequency multiplier and a lower-frequency crystal.  I think there are probably programmable-frequency oscillators on the market too, although I haven't looked into that in many years.
          Jim Bell, N7IJS.   "The LAST 'Tech-plus' ham in the world"





  
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