In article ,
jc wrote:

Chad Irby wrote:

snip
You're forgetting something.

Power.

So what is the power of the satellites at what distance. Square dist law.

You're leaving out off-axis rejection and a lot of other factors
(vertically-oriented receive antennas, for one).

Then there's the whole problem that the US military has been working on
anti-jam GPS tech since *before* they launched the first satellite...

Not to mention, of course, that you're transmitting that 1 watt
omnidirectionally,

Last (only) time I have tried for an isotropic ( within ~ 3 dB, which
is what you need for GPS jamming from balloons, not omnidirectional)
antenna with 125 mW input we got ~ 120 dBm at 30 clicks which in flat
terrain, enough to get a reliable signal with an 11 dB omnidirctional
recieve antenna. Now what is the level from the satellites?

What was the bandwidth of that signal? Remember that you have to cover
a good swath of frequency (two freqs, actually). A continuous-wave 1/8
watt sine over a narrow band is not useful for jamming a GPS signal,
which is coming in over a much wider bandwidth (you're going to need
white noise over a couple of octaves, at *least*, before getting useful
power). Also remember that you have to cover enough of the sky to swamp
*all* of the angles that a GPS receiver can cover, since it's going to
reject anything except a "GPS-like" signal (part of the coolness of the
GPS system).

The normal "low" receive level is a few dB below your claimed level - no
more than ten or sixteen. And that's worst-case in open ground.

A vertically-hung omni antenna is a good approximation of an isotropic
antenna in this case, since it's most probably going to be somewhat
offset from the balloon to whatever you're trying to protect. You're
also going to need to keep launching jammers all of the time, since
you're going to need a lot of them, high enough up in the air to hit the
upward-pointing receive antennas on GPS-guided weapons (or with *much*
higher transmit power, by a factor of a hundred or so), all of the time.

Suddenly, the "$5 balloon-borne jammer" is a "$500 balloon-borne
jammer that only lasts a few hours."

How long does an RPG blast go for and how much does it cost

A lot more than one of those $5 jammers, but it does a *bunch* more
damage. One RPG can kill a million dollar tank and all of its crew,
while a couple of dozen GPS jammers will, if you're lucky, make someone
wait until you get tired of using them before they drop those $30,000
bombs (or fall back on more-expensive TV-guided bombs). It's also a lot
easier to sell than those "well, you have to buy a lot of them and keep
launching them all of the time, *forever*" jammers... to give you an
idea, to cover someplace like Iraq, you're going to need to launch a few
*thousand* of those gadgets all at once, and be prepared to launch that
same number every couple of hours (a day at most, since launching them
high enough to cover the wide areas you want will subject them to
high-speed winds that will push them all over the place).

Once again, you're up against battery life, along with a *lot* higher
actual power level than you seem to be allowing for. The Russian-made
jammers we found in Iraq had an output power of four watts, but needed
25 watts of power to run. They also, by the way, didn't work worth a
damn versus US GPS weapons...

--
cirby at cfl.rr.com

Remember: Objects in rearview mirror may be hallucinations.
Slam on brakes accordingly.