4 ms·

More coverage of RF sensing, including laptops/phones with radios+NPU to sense their human: 2025, "Espargos: ESP32-based WiFi sensing array", 30 comments, http

More coverage of RF sensing, including laptops/phones with radios+NPU to sense their human: 2025, "Espargos: ESP32-based WiFi sensing array", 30 comments, https://news.ycombinator.com/item?id=43079023 https://news.ycombinator.com/item?id=43079023 2024, "How Wi-Fi sensing became usable to track people's movements", https://www.technologyreview.com/2024/02/27/1088154/wifi-sensing-tracking-movements/ https://www.technologyreview.com/2024/02/27/1088154/wifi-sen... 2023, "What Is mmWave Radar?: Everything You Need to Know About FMCW", 30 comments, https://news.ycombinator.com/item?id=35312351 https://news.ycombinator.com/item?id=35312351 2022, "mmWave radar, you won't see it coming", 180 comments, https://news.ycombinator.com/item?id=30172647 https://news.ycombinator.com/item?id=30172647 2021, "The next big Wi-Fi standard is for sensing, not communication", 200 comments, https://news.ycombinator.com/item?id=29901587 https://news.ycombinator.com/item?id=29901587

Animats 1y ago
Right. The longer range versions of multistatic radar are used to detect stealth aircraft.[1][2] All that careful stealth geometry to minimize direct reflections doesn't help much when the emitters and receivers are in different locations. [1] https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of-iranian-sky-ihomegrown-ilong-range-iradars https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of... [2] https://www.yiminzhang.com/pdf/radar13_passive.pdf https://www.yiminzhang.com/pdf/radar13_passive.pdf
- 4gotunameagain 1y ago
  No but the highly classified radar absorbing compounds that stealth aircraft are wrapped in definitely help :)
  - kuschku 1y ago
    How does it help if you're passing between transmitter and receiver? (either directly, or by bouncing a radar signal off the ionosphere and receiving it again) You should still show up as a shadow?
    - ruined 1y ago
      direct is practically useless, because that's point-to-point. lighting up the ionosphere that way seems like the hardest case scenario, requiring a very powerful transmitter, somehow ignored by sensitive high-resolution scanning over a large area of sky. and you'd be disrupted by other occluding objects like water vapor there is precedent https://en.m.wikipedia.org/wiki/Over-the-horizon_radar https://en.m.wikipedia.org/wiki/Over-the-horizon_radar but it seems like a limiting factor is suitable frequencies and resolution
  - WJW 1y ago
    (context: I used to be involved in the design of military radar systems for the Dutch navy) The radar absorbing compounds of stealth aircraft are highly optimized for specific wavelengths (usually X-band) and fall off heavily outside that frequency band. Similarly, the radar cross section of stealthy aircraft is highly optimized for specific purposes (usually evading GBAD in the forward direction) and rapidly falls off in other scenarios. Most "stealth" aircraft are actually fairly visible from other directions. That said, multistatic radar with transmitters-of-opportunity like cell towers and civil radio stations has always been in strong competition with fusion power as "the tech that is forever 10 years in the future". The transmitters are often not very powerful compared to dedicated radar systems and worse, they transmit energy in the horizontal plane rather than upwards where the planes are. The frequencies involved are much lower, which inherently leads to less radial accuracy unless you use VERY large antennas. Unlike a dedicated radar system the signals they send out are typically not shaped optimally for radar purposes, so signal processing like pulse compression becomes much harder. Because the signals are inherently not as predictable as normal radar signals you need MUCH more computing power. Finally, atmospheric conditions become fiendishly tricky for long range, because signal delays between each transmitter-target-receiver triple will be different. This means resolution goes way down if there's too many clouds or ionospheric interference, often to the point of uselessness. Many of those problems are mostly terrible when trying to detect aircraft at long range though, and largely go away for short range surface use like in port. I'm still not entirely sure why for a port, which is stationary and requires tons of infrastructure investment anyway, this system would be preferable to a normal civilian type radar system. You can get a conventional one for at most a few tens of thousands, while this system apparently requires a trailer full of RF signal processing equipment. That is likely to cost at least in the order of magnitude more, while probably being less accurate.
    - throw0101b 1y ago
      > (context: I used to be involved in the design of military radar systems for the Dutch navy) […] Most "stealth" aircraft are actually fairly visible from other directions. Is that different than ships, which in recent years/decades have tended to look a certain way (a 'finite' number of fixed angles): * https://en.wikipedia.org/wiki/Knud_Rasmussen-class_patrol_vessel https://en.wikipedia.org/wiki/Knud_Rasmussen-class_patrol_ve... * https://en.wikipedia.org/wiki/Absalon-class_frigate https://en.wikipedia.org/wiki/Absalon-class_frigate Do ships have to have a low return (?) at more angles?
- Grayskull 1y ago
  Well, you don't even need a radar. Tamara sensor could detect B-2, when it had it's onboard radar on. https://en.wikipedia.org/wiki/Tamara_passive_sensor https://en.wikipedia.org/wiki/Tamara_passive_sensor
  - rcxdude 1y ago
    stealth does in general go out the window when you turn on your radar. It's much like dressing in black and then running around with a flashlight at night. (and yes, there are equivalents to the various forms of night vision here, with associated tradeoffs)
    - engineer_22 1y ago
      Nah, there is much secret about radar
  - ta20240528 1y ago
    It turns out for the Iran drama, that radar's like the Tamara have to survive the F-35s first, then the F-15s… Which they don't. Then the B-2s fly in in unopposed. The key to the B-2s is dropping the F-35s. Which seems to be hard.
  - chipsa 1y ago
    Doubt: the APQ-181 radar on the B-2 is a Ku band radar, about 15 GHz. Tamara is about 1 GHz. This is entirely incompatible frequency ranges. Also, the APQ-181 is a LPI radar, which means it’s specifically designed to avoid correlation of signals such that you can track by the signals emitted. There are presumably some downsides to working in LPI, but the upside is that the signal is designed to be indistinguishable from an increased noise floor.
  - wat10000 1y ago
    I'm pretty sure that "don't operate your radar in enemy airspace" is right below "don't email your flight plan to the enemy" on the list of tips for stealth pilots who want to survive a mission.
- psunavy03 1y ago
  . . . because we all saw how effective Iranian air defenses were at countering stealth aircraft recently.
  - prasadjoglekar 1y ago
    To be fair, Iranian air defences were awful at countering non stealthy Israeli aircraft as well. The stealth bombers were just the most convenient vehicle for carrying the massive bomb.
    - psunavy03 1y ago
      Open-source reporting has Israeli F-35s kicking the door down on the Iranian IADS. Strong data point as to their effectiveness when properly employed as compared to some of the woo-woo airshow fanboying over things like bistatic radar. As always, the side who can best maximize the capabilities of their platforms while hiding/compensating for their limitations is the one who will win.
knetl 1y ago
More on Wi-Fi RF sensing: 2014, "We Can Hear You with Wi-Fi!", https://dl.acm.org/doi/abs/10.1145/2639108.2639112 https://dl.acm.org/doi/abs/10.1145/2639108.2639112 2015, "Keystroke Recognition Using WiFi Signals", https://dl.acm.org/doi/abs/10.1145/2789168.2790109 https://dl.acm.org/doi/abs/10.1145/2789168.2790109 2022, "Human Biometric Signals Monitoring based on WiFi Channel State Information using Deep Learning", https://arxiv.org/abs/2203.03980 https://arxiv.org/abs/2203.03980
throw0101b 1y ago
> 2021, "The next big Wi-Fi standard is for sensing, not communication", 200 comments, https://news.ycombinator.com/item?id=29901587 https://news.ycombinator.com/item?id=29901587 See 802.11bf: * https://en.wikipedia.org/wiki/WiFi_Sensing https://en.wikipedia.org/wiki/WiFi_Sensing

Animats 1y ago

Right. The longer range versions of multistatic radar are used to detect stealth aircraft.[1][2] All that careful stealth geometry to minimize direct reflections doesn't help much when the emitters and receivers are in different locations. [1] https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of-iranian-sky-ihomegrown-ilong-range-iradars https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of... [2] https://www.yiminzhang.com/pdf/radar13_passive.pdf https://www.yiminzhang.com/pdf/radar13_passive.pdf

4gotunameagain 1y ago
No but the highly classified radar absorbing compounds that stealth aircraft are wrapped in definitely help :)
- kuschku 1y ago
  How does it help if you're passing between transmitter and receiver? (either directly, or by bouncing a radar signal off the ionosphere and receiving it again) You should still show up as a shadow?
  - ruined 1y ago
    direct is practically useless, because that's point-to-point. lighting up the ionosphere that way seems like the hardest case scenario, requiring a very powerful transmitter, somehow ignored by sensitive high-resolution scanning over a large area of sky. and you'd be disrupted by other occluding objects like water vapor there is precedent https://en.m.wikipedia.org/wiki/Over-the-horizon_radar https://en.m.wikipedia.org/wiki/Over-the-horizon_radar but it seems like a limiting factor is suitable frequencies and resolution
- WJW 1y ago
  (context: I used to be involved in the design of military radar systems for the Dutch navy) The radar absorbing compounds of stealth aircraft are highly optimized for specific wavelengths (usually X-band) and fall off heavily outside that frequency band. Similarly, the radar cross section of stealthy aircraft is highly optimized for specific purposes (usually evading GBAD in the forward direction) and rapidly falls off in other scenarios. Most "stealth" aircraft are actually fairly visible from other directions. That said, multistatic radar with transmitters-of-opportunity like cell towers and civil radio stations has always been in strong competition with fusion power as "the tech that is forever 10 years in the future". The transmitters are often not very powerful compared to dedicated radar systems and worse, they transmit energy in the horizontal plane rather than upwards where the planes are. The frequencies involved are much lower, which inherently leads to less radial accuracy unless you use VERY large antennas. Unlike a dedicated radar system the signals they send out are typically not shaped optimally for radar purposes, so signal processing like pulse compression becomes much harder. Because the signals are inherently not as predictable as normal radar signals you need MUCH more computing power. Finally, atmospheric conditions become fiendishly tricky for long range, because signal delays between each transmitter-target-receiver triple will be different. This means resolution goes way down if there's too many clouds or ionospheric interference, often to the point of uselessness. Many of those problems are mostly terrible when trying to detect aircraft at long range though, and largely go away for short range surface use like in port. I'm still not entirely sure why for a port, which is stationary and requires tons of infrastructure investment anyway, this system would be preferable to a normal civilian type radar system. You can get a conventional one for at most a few tens of thousands, while this system apparently requires a trailer full of RF signal processing equipment. That is likely to cost at least in the order of magnitude more, while probably being less accurate.
  - throw0101b 1y ago
    > (context: I used to be involved in the design of military radar systems for the Dutch navy) […] Most "stealth" aircraft are actually fairly visible from other directions. Is that different than ships, which in recent years/decades have tended to look a certain way (a 'finite' number of fixed angles): * https://en.wikipedia.org/wiki/Knud_Rasmussen-class_patrol_vessel https://en.wikipedia.org/wiki/Knud_Rasmussen-class_patrol_ve... * https://en.wikipedia.org/wiki/Absalon-class_frigate https://en.wikipedia.org/wiki/Absalon-class_frigate Do ships have to have a low return (?) at more angles?
Grayskull 1y ago
Well, you don't even need a radar. Tamara sensor could detect B-2, when it had it's onboard radar on. https://en.wikipedia.org/wiki/Tamara_passive_sensor https://en.wikipedia.org/wiki/Tamara_passive_sensor
- rcxdude 1y ago
  stealth does in general go out the window when you turn on your radar. It's much like dressing in black and then running around with a flashlight at night. (and yes, there are equivalents to the various forms of night vision here, with associated tradeoffs)
  - engineer_22 1y ago
    Nah, there is much secret about radar
- ta20240528 1y ago
  It turns out for the Iran drama, that radar's like the Tamara have to survive the F-35s first, then the F-15s… Which they don't. Then the B-2s fly in in unopposed. The key to the B-2s is dropping the F-35s. Which seems to be hard.
- chipsa 1y ago
  Doubt: the APQ-181 radar on the B-2 is a Ku band radar, about 15 GHz. Tamara is about 1 GHz. This is entirely incompatible frequency ranges. Also, the APQ-181 is a LPI radar, which means it’s specifically designed to avoid correlation of signals such that you can track by the signals emitted. There are presumably some downsides to working in LPI, but the upside is that the signal is designed to be indistinguishable from an increased noise floor.
- wat10000 1y ago
  I'm pretty sure that "don't operate your radar in enemy airspace" is right below "don't email your flight plan to the enemy" on the list of tips for stealth pilots who want to survive a mission.
psunavy03 1y ago
. . . because we all saw how effective Iranian air defenses were at countering stealth aircraft recently.
- prasadjoglekar 1y ago
  To be fair, Iranian air defences were awful at countering non stealthy Israeli aircraft as well. The stealth bombers were just the most convenient vehicle for carrying the massive bomb.
  - psunavy03 1y ago
    Open-source reporting has Israeli F-35s kicking the door down on the Iranian IADS. Strong data point as to their effectiveness when properly employed as compared to some of the woo-woo airshow fanboying over things like bistatic radar. As always, the side who can best maximize the capabilities of their platforms while hiding/compensating for their limitations is the one who will win.

knetl 1y ago

More on Wi-Fi RF sensing: 2014, "We Can Hear You with Wi-Fi!", https://dl.acm.org/doi/abs/10.1145/2639108.2639112 https://dl.acm.org/doi/abs/10.1145/2639108.2639112 2015, "Keystroke Recognition Using WiFi Signals", https://dl.acm.org/doi/abs/10.1145/2789168.2790109 https://dl.acm.org/doi/abs/10.1145/2789168.2790109 2022, "Human Biometric Signals Monitoring based on WiFi Channel State Information using Deep Learning", https://arxiv.org/abs/2203.03980 https://arxiv.org/abs/2203.03980

throw0101b 1y ago

> 2021, "The next big Wi-Fi standard is for sensing, not communication", 200 comments, https://news.ycombinator.com/item?id=29901587 https://news.ycombinator.com/item?id=29901587 See 802.11bf: * https://en.wikipedia.org/wiki/WiFi_Sensing https://en.wikipedia.org/wiki/WiFi_Sensing