Hacker News Reader

Comment by _kst_ | Hacker News Reader

The article says it affects "GPS signals down to 50 feet AGL". (I believe that's Above Ground Level.) So it shouldn't affect surface navigation.

View on HN

Does anyone have a good explanation for how that's possible? Aren't the same GPS signals used regardless of elevation? Coming from a satellite, wouldn't they have to traverse the area where they're supposedly "jammed" before they can make it to ground level?

Sure, but signal interference is about mixing two signals together. Once the GPS signal has passed the interference signal (below 50 ft) it shouldn't matter anymore

I would guess that this is due to the jamming antenna's visibility over the horizon.

You can't jam a signal in the air, all you can do is transmit a jamming signal towards where the receiver should be. And if the receiver is on the ground, out of your line of sight, then you can't do anything about it.

Jamming doesn't block the signal, it shouts over the signal. So, if your receiver can't hear the jammer, it will be unaffected, even if receivers between you and the satellite can hear the jammer.

IIRC, there exist things for GPS called "null steering antennas" that allow a receiver point a "null" at a jamming source to block it. I believe they're considered controlled munitions by the US government.

Turn on a GPS jammer, you'll jam everything up to a certain distance away that has line of sight to the antenna. Anything on the ground that is over the horizon won't be affected, but anything above ground has a much longer horizon distance.

Reply on news.ycombinator.com