8 ms·

I don't think there's enough detail here to decide whether or not to be mad. The L band is huge, 1000MHz-2000MHz. GPS uses a few tiny slices of this band. Ce

I don't think there's enough detail here to decide whether or not to be mad. The L band is huge, 1000MHz-2000MHz. GPS uses a few tiny slices of this band. Cell phones, amateur radio, weather radar (though not precipitation-detection radar), ADS-B, and a ton of other non-military things are in that band. Adding 5G may or may not have any negative effect on GPS. I doubt they are proposing to run their 5G network on top of the GPS frequencies, after all, because everyone knows that would break GPS (which already operate well below the noise floor). One time when we were working on testing Wifi interference, we tried to buy every possible 2.4GHz device to see what they did to our routers. We found these TV extenders (connect camera to one end, connect TV to the other) that claimed to be 2.4GHz but didn't interfere at all. We looked more closely with a spectrum analyzer and they were actually 1.3GHz and completely stepped all over the GPS frequencies. Did the FCC stop Amazon from importing these things? Nope. I guess my point is... the threat is real, but someone going out of their way to request permission from the government (as is legally required) is probably not going to cause many problems.

core-questions 7y ago
>everyone knows that would break GPS (which already operate well below the noise floor). Can you explain the noise floor bit?
- Junk_Collector 7y ago
  There exists a fundamental level of electrical noise at all frequencies typically controlled by the background temperature that sets the minimum level of signal you can resolve before it gets washed out in noise. GPS uses multiple coherent (noise being random instead of coherent does not combine at the same rate) copies of the signal that can be recombined to increase their effective signal, so even though they are transmitted below the universal noise floor, they are above it once recombined. *edited for spelling
  - core-questions 7y ago
    Thanks, that's very interesting. Appreciated.
- jrockway 7y ago
  The reply that you already have is pretty good. https://en.wikipedia.org/wiki/Gold_code https://en.wikipedia.org/wiki/Gold_code is a good jumping off point for further research (as is the wikipedia article on GPS).
josh2600 7y ago
When light squared was shut down for messing with gps frequencies in their satellite network, the military did not say that light^2 was actually broadcasting outside of the expected frequency band. The deal is that the military just doesn’t use filters on most of their broadcast and reception equipment so their operating frequency is actually way wider than they say publicly. This is because filters cost money and reduce the effectiveness of the broadcast system. If you could get away with operating over a much wider spectrum for any service, you probably would. This is why when the military says “this interferes with gps even though it’s outside of the gps band were supposed to be using” what they mean is that they are using a much wider band of spectrum than what’s in the specs. Ultimately, the fcc doesn’t have regulatory authority over the dod.
- michaelt 7y ago
  > The deal is that the military just doesn’t use filters Do you have a citation for that? I would have thought the issue is not that GPS receivers don't use filters, but rather than no filter has a perfectly sharp cut-off and GPS signals are exceptionally faint. For example, if the GPS signal is 0.1 femtowatts at 1575MHz, even if you've got a filter that can remove 99.99999% of a noise signal at 1600MHz you can still overwhelm the GPS signal with 1 nanowatt of power.
  - javajosh 7y ago
    Oh man, I bet there's a nice market of GPS jammers out there - especially useful for the home team in any action, since they don't need help with directions. I wonder if that counts as a weapon?
    - wl 7y ago
      Some truckers use them for manipulating the trackers on their vehicles. Some of them are stupid enough to have them active when they are anywhere near an airport, which makes it far more likely they'll get caught.
  - duskwuff 7y ago
whoopdedo 7y ago
> Did the FCC stop Amazon from importing these things? Did you report it to the FCC so they'd know?
beambot 7y ago
Perhaps it's not about the nominal operation as a 5G tower; rather, a bunch of powerful software-defined radios that could interfere upon command is a pretty reasonable attack vector.
Rebelgecko 7y ago
I'm not an RF person, but the way it was explained to me is that GNSS signals are in the neighborhood of 1560mhz-1610mhz. With the advent of GPSIII, even more of the important signals are getting pushed outwards towards the extremes of that range. Ligado/Lightsquared initially wanted permission to basically surround GPS (1526-1559mhz and 1610-1660mHz) After lots of complaints, they amended their request to leave a buffer of 20mhz or so between their signals and GPS. However, Ligado wants to transmit at ~10watts, and the GPS signal is in the neighborhood of a femtowatt by the time it hits a receiver. That can apparently still cause interference. For the GPS chip in your phone it may not make a huge difference. However testing has shown degraded performance in aviation GPS receivers more than a mile away from a 10watt transmitter. For even higher precision receivers (think surveyor equipment or maybe an autonomous tractor) the radius of the degradation stretches more than 2 miles from the transmitter. Ligado wants to put their transmitters ¼ of a mile apart. Source for some of those numbers here, and an entertaining talk if you're ever able to see it in person: https://rntfnd.org/wp-content/uploads/Brad-at-UAG-Users-Advisory-Group-Oct-2019.pdf https://rntfnd.org/wp-content/uploads/Brad-at-UAG-Users-Advi...
- andromeduck 7y ago
  Why can't we just greatly improve the signal strength at source by an order of magnitude? Like I know the new satellites can use beam forming to increase power for some large area for military use but it looks like even so total power is about ~500W right now. What's stopping us from deploying 10KW sats?
  - dralley 7y ago
    >Why can't we just greatly improve the signal strength at source by an order of magnitude? Like I know the new satellites can use beam forming to increase power for some large area for military use but it looks like even so total power is about ~500W right now. What's stopping us from deploying 10KW sats? Even assuming that the very smart and capable people who developed GPS haven't thought about that already, there is zero-point-zero percent chance that the government will (or should) turn 24 satellites that cost 500 million dollars each into very expensive flying paperweights, for the benefit of some tiny 5G startup.
iav 7y ago
Ligado’s biggest investors are JPMorgan, Centerbridge, and Fortress. David Redl, the former head of the NTIA, supported Ligado’s mission while the GPS (part of DoD but also co-run by the DOT) was against it for as long as this issue existed. The FCC pushed this through because Ajit Pai is retiring at the end of the year (regardless if Trump wins) and wants a legacy of approving more commercial uses of spectrum.
ChuckMcM 7y ago
Your GPS receiver is not going to be able to "see" the current satellites. All of the other people who share this spectrum do so "horizontally" if you will, this frequency band isn't reflected by atmospheric effects so omni-directional emitters are not visible to antenna that are looking "up" to the sky. The problem with this proposal is that the company wants to broadcast DOWN from satellites which these antennas will see and it will reduce their sensitivity to neighboring signals in the same band. Since you need 4 - 6 satellites for a good tracking lock on GPS, the likelyhood is that you won't be able to get that because of inter-band interference. Can you fix it? Sure, you can add a $10 - $50 front end selector on the GPS devices to increase their selectivity to GPS only bands. That will raise the cost of "having" GPS across the board and legacy devices won't see the benefit so they will become useless. It is hard to imagine that any technical voices at the FCC thought this was a good, or even reasonable, idea. And on a humorous note, "U.S. Space Force Gen. John Raymond." I chuckle that the Space Force already has generals but of course it does, but not ones that "came up through the ranks, from Spaceman 1st class."
- izend 7y ago
  Chuck your comments are always filled with great information and humour. Thank you for your years of contribution to HN so that lurkers like myself can be informed and entertained.

core-questions 7y ago

>everyone knows that would break GPS (which already operate well below the noise floor). Can you explain the noise floor bit?

Junk_Collector 7y ago
There exists a fundamental level of electrical noise at all frequencies typically controlled by the background temperature that sets the minimum level of signal you can resolve before it gets washed out in noise. GPS uses multiple coherent (noise being random instead of coherent does not combine at the same rate) copies of the signal that can be recombined to increase their effective signal, so even though they are transmitted below the universal noise floor, they are above it once recombined. *edited for spelling
- core-questions 7y ago
  Thanks, that's very interesting. Appreciated.
jrockway 7y ago
The reply that you already have is pretty good. https://en.wikipedia.org/wiki/Gold_code https://en.wikipedia.org/wiki/Gold_code is a good jumping off point for further research (as is the wikipedia article on GPS).

josh2600 7y ago

When light squared was shut down for messing with gps frequencies in their satellite network, the military did not say that light^2 was actually broadcasting outside of the expected frequency band. The deal is that the military just doesn’t use filters on most of their broadcast and reception equipment so their operating frequency is actually way wider than they say publicly. This is because filters cost money and reduce the effectiveness of the broadcast system. If you could get away with operating over a much wider spectrum for any service, you probably would. This is why when the military says “this interferes with gps even though it’s outside of the gps band were supposed to be using” what they mean is that they are using a much wider band of spectrum than what’s in the specs. Ultimately, the fcc doesn’t have regulatory authority over the dod.

michaelt 7y ago
> The deal is that the military just doesn’t use filters Do you have a citation for that? I would have thought the issue is not that GPS receivers don't use filters, but rather than no filter has a perfectly sharp cut-off and GPS signals are exceptionally faint. For example, if the GPS signal is 0.1 femtowatts at 1575MHz, even if you've got a filter that can remove 99.99999% of a noise signal at 1600MHz you can still overwhelm the GPS signal with 1 nanowatt of power.
- javajosh 7y ago
  Oh man, I bet there's a nice market of GPS jammers out there - especially useful for the home team in any action, since they don't need help with directions. I wonder if that counts as a weapon?
  - wl 7y ago
    Some truckers use them for manipulating the trackers on their vehicles. Some of them are stupid enough to have them active when they are anywhere near an airport, which makes it far more likely they'll get caught.
- duskwuff 7y ago

whoopdedo 7y ago

> Did the FCC stop Amazon from importing these things? Did you report it to the FCC so they'd know?

beambot 7y ago

Perhaps it's not about the nominal operation as a 5G tower; rather, a bunch of powerful software-defined radios that could interfere upon command is a pretty reasonable attack vector.

Rebelgecko 7y ago

I'm not an RF person, but the way it was explained to me is that GNSS signals are in the neighborhood of 1560mhz-1610mhz. With the advent of GPSIII, even more of the important signals are getting pushed outwards towards the extremes of that range. Ligado/Lightsquared initially wanted permission to basically surround GPS (1526-1559mhz and 1610-1660mHz) After lots of complaints, they amended their request to leave a buffer of 20mhz or so between their signals and GPS. However, Ligado wants to transmit at ~10watts, and the GPS signal is in the neighborhood of a femtowatt by the time it hits a receiver. That can apparently still cause interference. For the GPS chip in your phone it may not make a huge difference. However testing has shown degraded performance in aviation GPS receivers more than a mile away from a 10watt transmitter. For even higher precision receivers (think surveyor equipment or maybe an autonomous tractor) the radius of the degradation stretches more than 2 miles from the transmitter. Ligado wants to put their transmitters ¼ of a mile apart. Source for some of those numbers here, and an entertaining talk if you're ever able to see it in person: https://rntfnd.org/wp-content/uploads/Brad-at-UAG-Users-Advisory-Group-Oct-2019.pdf https://rntfnd.org/wp-content/uploads/Brad-at-UAG-Users-Advi...

andromeduck 7y ago
Why can't we just greatly improve the signal strength at source by an order of magnitude? Like I know the new satellites can use beam forming to increase power for some large area for military use but it looks like even so total power is about ~500W right now. What's stopping us from deploying 10KW sats?
- dralley 7y ago
  >Why can't we just greatly improve the signal strength at source by an order of magnitude? Like I know the new satellites can use beam forming to increase power for some large area for military use but it looks like even so total power is about ~500W right now. What's stopping us from deploying 10KW sats? Even assuming that the very smart and capable people who developed GPS haven't thought about that already, there is zero-point-zero percent chance that the government will (or should) turn 24 satellites that cost 500 million dollars each into very expensive flying paperweights, for the benefit of some tiny 5G startup.

iav 7y ago

Ligado’s biggest investors are JPMorgan, Centerbridge, and Fortress. David Redl, the former head of the NTIA, supported Ligado’s mission while the GPS (part of DoD but also co-run by the DOT) was against it for as long as this issue existed. The FCC pushed this through because Ajit Pai is retiring at the end of the year (regardless if Trump wins) and wants a legacy of approving more commercial uses of spectrum.

ChuckMcM 7y ago

Your GPS receiver is not going to be able to "see" the current satellites. All of the other people who share this spectrum do so "horizontally" if you will, this frequency band isn't reflected by atmospheric effects so omni-directional emitters are not visible to antenna that are looking "up" to the sky. The problem with this proposal is that the company wants to broadcast DOWN from satellites which these antennas will see and it will reduce their sensitivity to neighboring signals in the same band. Since you need 4 - 6 satellites for a good tracking lock on GPS, the likelyhood is that you won't be able to get that because of inter-band interference. Can you fix it? Sure, you can add a $10 - $50 front end selector on the GPS devices to increase their selectivity to GPS only bands. That will raise the cost of "having" GPS across the board and legacy devices won't see the benefit so they will become useless. It is hard to imagine that any technical voices at the FCC thought this was a good, or even reasonable, idea. And on a humorous note, "U.S. Space Force Gen. John Raymond." I chuckle that the Space Force already has generals but of course it does, but not ones that "came up through the ranks, from Spaceman 1st class."

izend 7y ago
Chuck your comments are always filled with great information and humour. Thank you for your years of contribution to HN so that lurkers like myself can be informed and entertained.