3 ms·

The distance between the transmitter and receiver is quite important. Signal strength is inversely proportional to the square of the distance. At some distance

The distance between the transmitter and receiver is quite important. Signal strength is inversely proportional to the square of the distance. At some distance you are going to have problems no matter how good the filters are. The problem here is that the receiver is mobile. Aircraft flying over C band satellite dishes (what used to use the current 5G band) used to cause interference to those dishes. In that case the problem was that the transmitter was mobile. Now the C band satellite people will have to fight interference from 5G transmitters in what remains of their band.

labcomputer 5y ago
With radar, it's actually worse! Received signal strength is proportional to the fourth power of distance (1/r^2 on the way to the target, then 1/r^2 on the way back to the radar). That's part of the reason 5G is such an issue, despite the ostensibly-large guard band. You need really good filters on the cellular 5G equipment to make sure it doesn't leak into the radar band.
- anfilt 5y ago
  Let's not also forget about drift on these filters. Those components will experience drift from age. That drift can also be accelerated from quiet a few factors such as heat ect...
- sandworm101 5y ago
  And many hundreds of emitters. All those towers and phones pumping out 1s and 0s will occasionally line up in a way thay looks like a higher/lower frquency. The aircraft looking down sees across multiple cells. All sorts of strange combinations might occur. Thats what guard bands are for.
- nwallin 5y ago
  > With radar, it's actually worse! Received signal strength is proportional to the fourth power of distance (1/r^2 on the way to the target, then 1/r^2 on the way back to the radar). I think that's specifically true when you're pointing a radar at an aircraft, but is not true for radar altimeters. When you're pointing a radar at an aircraft, the angular size of the aircraft is 1/r^2, so 1/r^2 of the emitted radiation hits the aircraft. Once emitted from the aircraft, the angular size of radar receiving dish is 1/r^2, so 1/r^2 of that radiation is detected. With a radar altimeter, you're pointing the radar at the ground. The angular size of the ground is 50% of the sphere, whether you're one foot above the ground or 100,000 feet. (...ok, maybe it's like 49.5% of the sphere...) As long as you're pointing it down-ish, 100% of the emitted radiation hits the ground. There is only one 1/r^2 relationship, on the return leg.
  - lutorm 5y ago
    Well, 100% of the emitted radiation hits the ground in terms of total power, but the radar is looking for the _closest_ reflection, which is some relatively small solid angle straight down. It doesn't change your conclusion though, that solid angle is independent of the distance to the ground.
    - nwallin 5y ago
      > Well, 100% of the emitted radiation hits the ground in terms of total power, but the radar is looking for the _closest_ reflection, which is some relatively small solid angle straight down. This fixed angle paints a larger and larger circle as altitude increases. If you double your altitude, you quadruple the size of the surface that's reflecting radar back at you. A bunch of stuff cancels out and you end up with 1/r^2 at the receiver, which is what we started with.
      - steerablesafe 5y ago
        An alternative explanation is that you treat the ground as a flat mirror, and the radar detects the distance of your mirror image. The distance between you and your mirror image is (2r), so the signal strength you receive from your mirror image is ~1/(2r)^2, still follows 1/r^2 .
  - fennecfoxen 5y ago
    I’m not arguing with the idea most of the signal gets to the ground, but, I’m worried that you get specifics quite wrong. Planes don’t want a reflection from 50% of the sphere. They want it from directly below. Other land is further from the plane and obscures the measurement. But they’re not broadcasting to the whole sphere either, because the radar instrument is not an isotropic radiator. That antenna has gain. It’s louder / more sensitive in a specific direction. Probably by a lot. You can’t make perfect antennas, but you don’t need any signal at all from most of the sphere, so you can gain many decibels. The angle of the beam you do have is probably fixed, and being fixed would imply that the area swept by the core of the beam is indeed proportional to the square of the altitude. But I don’t think two trips through an n^2 distance is n^4 unless I’m missing some of the physics. Shouldn’t it be (2n)^2 and this still proportional to the square of the altitude not to the hypercube? (And affected by scattering and bad reflectivity from the terrain, which seems likely to be independent of altitude?) This is different for detecting random entities in free space where most of the first n^2 trip really is lost. So it’s like you say but for different reasons??!?!
- hilbert42 5y ago
  "That's part of the reason 5G is such an issue, despite the ostensibly-large guard band." As I keep repeating here, if excessive or debilitating interference is being experienced between services under normal operating conditions then there has been a failure of Spectrum Management planning - absolutely no question about it. OK, we know such interference has happened, so who was responsible for the planning fuck-up? I've no argument about filters, RFI etc., what most are saying about the engineering issues makes sense. My argument is that the spectrum planning has failed and that much of the reason can be put down to deregulation in the 1980s when many governments essentially washed their hands of Spectrum Management (succumbed to commercial pressures and overly loosened engineering standards and now we're paying the penalty). It seems to me (given the international nature of the problem with non-US registered aircraft flying into US airspace, etc.) that we need to go back to the core of the problem and acknowledge these past 'regulations' errors before the problem can be fixed properly. Perhaps all radar altimeters will have to be changed (respecified with much tighter filters, better intermod figures, etc.) but given the fuck-up who should pay? After all, it's not the FAA's fault nor that of the aircraft industry (see my comments in other posts about the ITU and WRC). As I see it, this is a political issue, not a technical one.
zw123456 5y ago
Right, so, the issue is NOT the BTS that is being raised, it is the phone on the plane, which are a lot closer to the antenna on the plane, which is on the ass end of the plane which is the low point during take off and landing. this is the white paper that the FAA bases their concerns on https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In... that is the issue they are raising I have run the numbers and there is no way even at phone full power that it can interfere unless it was less than 2m away even then it would have to be free space, etc. the body losses, airframe, antenna back scatter all taken into account, very very unlikely unless the radar bandpass filters are from the 1960's
- upofadown 5y ago
  >...unless the radar bandpass filters are from the 1960's ... Were filters particularly bad in the '60s? If you claim to have run the numbers you will have to reveal your assumptions.
  - zw123456 5y ago
    yes, DSP's did not exist then :) The numbers are in the doc i posted.
- amirhirsch 5y ago
  Thanks for sharing. I’m going to read this report now. Are you in Radar? Potentially looking for a job at a well-funded startup making very high powered phased arrays in c and x bands?
- amirhirsch 5y ago
  The conclusions in the report are more severe than your comments are presenting. I don't think this is a good forum to discuss it further though. There is another comment suggesting a clash of two vastly different safety cultures that really hits the nail on the head, and I think that even the hypothesized/speculative risk to aviation needs addressing, though you are also probably right that the aircraft industry wants cash from the telcos for a retrofit that they should pay for themselves and were willing to drag their feet assuming FAA would trump the FCC in this matter (they will). Frankly if they want digital pulse-compression radar altimeters that are going to operate even with adversarial interference, I built/am-building the most advanced matched filter in the world though it will cost a bit more than a sub-$1K High Pass Filter plugged in line with the antennae which is probably sufficient (though avionics safety requirements will probably find a way to make it $100K anyway)

labcomputer 5y ago

With radar, it's actually worse! Received signal strength is proportional to the fourth power of distance (1/r^2 on the way to the target, then 1/r^2 on the way back to the radar). That's part of the reason 5G is such an issue, despite the ostensibly-large guard band. You need really good filters on the cellular 5G equipment to make sure it doesn't leak into the radar band.

anfilt 5y ago
Let's not also forget about drift on these filters. Those components will experience drift from age. That drift can also be accelerated from quiet a few factors such as heat ect...
sandworm101 5y ago
And many hundreds of emitters. All those towers and phones pumping out 1s and 0s will occasionally line up in a way thay looks like a higher/lower frquency. The aircraft looking down sees across multiple cells. All sorts of strange combinations might occur. Thats what guard bands are for.
nwallin 5y ago
> With radar, it's actually worse! Received signal strength is proportional to the fourth power of distance (1/r^2 on the way to the target, then 1/r^2 on the way back to the radar). I think that's specifically true when you're pointing a radar at an aircraft, but is not true for radar altimeters. When you're pointing a radar at an aircraft, the angular size of the aircraft is 1/r^2, so 1/r^2 of the emitted radiation hits the aircraft. Once emitted from the aircraft, the angular size of radar receiving dish is 1/r^2, so 1/r^2 of that radiation is detected. With a radar altimeter, you're pointing the radar at the ground. The angular size of the ground is 50% of the sphere, whether you're one foot above the ground or 100,000 feet. (...ok, maybe it's like 49.5% of the sphere...) As long as you're pointing it down-ish, 100% of the emitted radiation hits the ground. There is only one 1/r^2 relationship, on the return leg.
- lutorm 5y ago
  Well, 100% of the emitted radiation hits the ground in terms of total power, but the radar is looking for the _closest_ reflection, which is some relatively small solid angle straight down. It doesn't change your conclusion though, that solid angle is independent of the distance to the ground.
  - nwallin 5y ago
    > Well, 100% of the emitted radiation hits the ground in terms of total power, but the radar is looking for the _closest_ reflection, which is some relatively small solid angle straight down. This fixed angle paints a larger and larger circle as altitude increases. If you double your altitude, you quadruple the size of the surface that's reflecting radar back at you. A bunch of stuff cancels out and you end up with 1/r^2 at the receiver, which is what we started with.
    - steerablesafe 5y ago
      An alternative explanation is that you treat the ground as a flat mirror, and the radar detects the distance of your mirror image. The distance between you and your mirror image is (2r), so the signal strength you receive from your mirror image is ~1/(2r)^2, still follows 1/r^2 .
- fennecfoxen 5y ago
  I’m not arguing with the idea most of the signal gets to the ground, but, I’m worried that you get specifics quite wrong. Planes don’t want a reflection from 50% of the sphere. They want it from directly below. Other land is further from the plane and obscures the measurement. But they’re not broadcasting to the whole sphere either, because the radar instrument is not an isotropic radiator. That antenna has gain. It’s louder / more sensitive in a specific direction. Probably by a lot. You can’t make perfect antennas, but you don’t need any signal at all from most of the sphere, so you can gain many decibels. The angle of the beam you do have is probably fixed, and being fixed would imply that the area swept by the core of the beam is indeed proportional to the square of the altitude. But I don’t think two trips through an n^2 distance is n^4 unless I’m missing some of the physics. Shouldn’t it be (2n)^2 and this still proportional to the square of the altitude not to the hypercube? (And affected by scattering and bad reflectivity from the terrain, which seems likely to be independent of altitude?) This is different for detecting random entities in free space where most of the first n^2 trip really is lost. So it’s like you say but for different reasons??!?!
hilbert42 5y ago
"That's part of the reason 5G is such an issue, despite the ostensibly-large guard band." As I keep repeating here, if excessive or debilitating interference is being experienced between services under normal operating conditions then there has been a failure of Spectrum Management planning - absolutely no question about it. OK, we know such interference has happened, so who was responsible for the planning fuck-up? I've no argument about filters, RFI etc., what most are saying about the engineering issues makes sense. My argument is that the spectrum planning has failed and that much of the reason can be put down to deregulation in the 1980s when many governments essentially washed their hands of Spectrum Management (succumbed to commercial pressures and overly loosened engineering standards and now we're paying the penalty). It seems to me (given the international nature of the problem with non-US registered aircraft flying into US airspace, etc.) that we need to go back to the core of the problem and acknowledge these past 'regulations' errors before the problem can be fixed properly. Perhaps all radar altimeters will have to be changed (respecified with much tighter filters, better intermod figures, etc.) but given the fuck-up who should pay? After all, it's not the FAA's fault nor that of the aircraft industry (see my comments in other posts about the ITU and WRC). As I see it, this is a political issue, not a technical one.

zw123456 5y ago

Right, so, the issue is NOT the BTS that is being raised, it is the phone on the plane, which are a lot closer to the antenna on the plane, which is on the ass end of the plane which is the low point during take off and landing. this is the white paper that the FAA bases their concerns on https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-Interference-Assessment-Report_274-20-PMC-2073_accepted_changes.pdf https://www.rtca.org/wp-content/uploads/2020/10/SC-239-5G-In... that is the issue they are raising I have run the numbers and there is no way even at phone full power that it can interfere unless it was less than 2m away even then it would have to be free space, etc. the body losses, airframe, antenna back scatter all taken into account, very very unlikely unless the radar bandpass filters are from the 1960's

upofadown 5y ago
>...unless the radar bandpass filters are from the 1960's ... Were filters particularly bad in the '60s? If you claim to have run the numbers you will have to reveal your assumptions.
- zw123456 5y ago
  yes, DSP's did not exist then :) The numbers are in the doc i posted.
amirhirsch 5y ago
Thanks for sharing. I’m going to read this report now. Are you in Radar? Potentially looking for a job at a well-funded startup making very high powered phased arrays in c and x bands?
amirhirsch 5y ago
The conclusions in the report are more severe than your comments are presenting. I don't think this is a good forum to discuss it further though. There is another comment suggesting a clash of two vastly different safety cultures that really hits the nail on the head, and I think that even the hypothesized/speculative risk to aviation needs addressing, though you are also probably right that the aircraft industry wants cash from the telcos for a retrofit that they should pay for themselves and were willing to drag their feet assuming FAA would trump the FCC in this matter (they will). Frankly if they want digital pulse-compression radar altimeters that are going to operate even with adversarial interference, I built/am-building the most advanced matched filter in the world though it will cost a bit more than a sub-$1K High Pass Filter plugged in line with the antennae which is probably sufficient (though avionics safety requirements will probably find a way to make it $100K anyway)