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The article said the signal is stronger. That is the new anti-jamming countermeasure.

by cwt137 8y ago

tialaramex 8y ago
Just broadcasting a similarly strong signal won't jam GPS family transmissions. This is because the transmission is based on a pseudorandom bit sequence. It looks like noise unless you also know what the pseudorandom sequence should be. If you do you can pick this out from any not related signal trying to jam it, until the jammers pour in so much more power that it overwhelms your receiver. The new birds know a pseudorandom sequence that was state of the art ten years ago rather than forty years ago (times approximate). This is a shared key system. If you either break the system or know the key, you get position data or can effectively jam it. You can put several transmissions on the same bird, and give away some of the keys, voila you're a public service. Keep one or more for your military and they've got position information that can't be jammed.
- cameldrv 8y ago
  Anything can be jammed, you just need enough power. Having a secret spread spectrum code helps. It's the same principle but easier to understand in the frequency hopping case. In that case, suppose you want as much jamming power as signal power, you have to spread it out across the whole band because you don't know where the next frequency hop is going to be. So if say the system uses 1/100 of the available bandwidth at any one time, now the jammer needs to be 100x as powerful to jam the whole band with the same power as the jammed system. Still, the satellite puts out a low power signal from thousands of miles away, so the jammer starts with an advantage. Where the new block iii satellites will improve this is with a spot beam that can be directed at a particular area on earth where the jamming is expected, and locally get about 100x the power. I suspect that you'll see more directional antennas and phased arrays applied to GPS receivers as well, since the angular direction to the satellite is known, and it's difficult to put the jammer right in the path of between the receiver and the satellite.

The article said the signal is stronger. That is the new anti-jamming countermeasure.

by cwt137 8y ago

The article said the signal is stronger. That is the new anti-jamming countermeasure.

Just broadcasting a similarly strong signal won't jam GPS family transmissions. This is because the transmission is based on a pseudorandom bit sequence. It looks like noise unless you also know what the pseudorandom sequence should be. If you do you can pick this out from any not related signal trying to jam it, until the jammers pour in so much more power that it overwhelms your receiver. The new birds know a pseudorandom sequence that was state of the art ten years ago rather than forty years ago (times approximate). This is a shared key system. If you either break the system or know the key, you get position data or can effectively jam it. You can put several transmissions on the same bird, and give away some of the keys, voila you're a public service. Keep one or more for your military and they've got position information that can't be jammed.

cameldrv 8y ago
Anything can be jammed, you just need enough power. Having a secret spread spectrum code helps. It's the same principle but easier to understand in the frequency hopping case. In that case, suppose you want as much jamming power as signal power, you have to spread it out across the whole band because you don't know where the next frequency hop is going to be. So if say the system uses 1/100 of the available bandwidth at any one time, now the jammer needs to be 100x as powerful to jam the whole band with the same power as the jammed system. Still, the satellite puts out a low power signal from thousands of miles away, so the jammer starts with an advantage. Where the new block iii satellites will improve this is with a spot beam that can be directed at a particular area on earth where the jamming is expected, and locally get about 100x the power. I suspect that you'll see more directional antennas and phased arrays applied to GPS receivers as well, since the angular direction to the satellite is known, and it's difficult to put the jammer right in the path of between the receiver and the satellite.