[Blueflag]

4 Watts can jam signals to 200KM?? Hmmmm. I'm no RF electrical engineer, but I seem to recall a broadcast omnidirectional signal loses power proportional to the cube of the distance from the antenna. 4 Watts can jam to 200KM?? FReeper expert help requested.

[relee]

...4 Watts can jam to 200KM?? ...

Sure, if you use really big antennas!

[c-five]

...broadcast omnidirectional signal loses power proportional to the cube of the distance from the antenna. 4 Watts can jam to 200KM??

Square, I think. But anyway, the same law applies to the orbiting transmitters, which are much further away from the receiver than the jammer.

[expatpat]

The satelite signals being received are quite weak, unless you use a large antenna. There has to be noise-rejection processing in the receiver anyway.

[Myrddin]

The GPS signal from the satellite is below the noise floor. You recover it by generating the expect PN sequence and summing it with the received signal. The generated sequence is retarded on intervals until the generated sequence is in phase with the satellite signal. The sum is then above the noise floor. A 4 watt transmitter at 200 kilometers is competing with a satellite transmitting from 12,000 miles up (minimum distance to the surface). The terrestrial signal can easily swamp the GPS front end and cover the satellite signals. The PN sequences for the C/A channel are "advertised" by the satellites in the almanac. The jammer just needs to generate all the possible PN sequences with slightly more delivered energy than the actual satellites.

The current "phase" of the encrypted military channel is broadcast on the C/A channel. It only repeats every 7 days, so you need to get a fix on roughly where you are in the sequence AND possess the crypto key with which it is encrypted. Swamping access to the C/A channel signal will prevent picking up the "phase" reference for the military encrypted channel.

[El Gato]

4 Watts can jam signals to 200KM?? Hmmmm. I'm no RF electrical engineer, but I seem to recall a broadcast omnidirectional signal loses power proportional to the cube of the distance from the antenna. 4 Watts can jam to 200KM?? FReeper expert help requested.

It's the square of the distance, not the the cube. Remember though that the GPS satellites, whose signal is what is actually being jammed, are much farther away than 200km, so the signal to be overcome is pretty weak, even though it's intial stregth may have been stronger than the jammer.

[lepton]

I'm no RF electrical engineer, but I seem to recall a broadcast omnidirectional signal loses power proportional to the cube of the distance from the antenna. 4 Watts can jam to 200KM??

Geosync satellites are how far away?

[SENTINEL]

I also am very skeptical about 4 watts jamming out to 120 miles. Not to mention that the jdams most likely have a semi-directional antenna pointed generally up as the bomb falls.

My gut feel is that it would take much closer to 25,000 watts minimum to jam to that distance.

USMC HAWK Air Defense System Technician, top echelon.

Any RF EE's out there ?