9 ms·

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?
  - WJW 1y ago
    Ships like that are typically optimized to look small from "low" angles, ie from the perspective of other surface combatants and sea skimming anti ship missiles. The large flat surfaces are not so much used to reduce RCS by themselves, but mostly to reduce instances of "corner reflectors" like hatches and exposed cranes the like, which can have a RCS many times larger than their physical size due to their shape. See also the "Reduction" section on Wikipedia in the article about Radar Cross Section: (https://en.wikipedia.org/wiki/Radar_cross_section#Purpose_shaping https://en.wikipedia.org/wiki/Radar_cross_section#Purpose_sh...).
    - throw0101b 1y ago
      Would airborne radar be better able to find ships with these designs (at least relatively speaking)?
  - saltcured 1y ago
    As I recall, the faceted look of the early stealth aircraft was said to be a practical matter. It reduced the complexity of modeling the reflections during the design process. So with additional computational complexity, they could go back to smooth surfaces in later designs. I imagine there are similar issues with ship design. Since these things are wavelength specific, you probably have a bigger computational problem for a bigger vessel. You can't just solve for the design on a miniature and scale it up to build it.

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?
- WJW 1y ago
  Ships like that are typically optimized to look small from "low" angles, ie from the perspective of other surface combatants and sea skimming anti ship missiles. The large flat surfaces are not so much used to reduce RCS by themselves, but mostly to reduce instances of "corner reflectors" like hatches and exposed cranes the like, which can have a RCS many times larger than their physical size due to their shape. See also the "Reduction" section on Wikipedia in the article about Radar Cross Section: (https://en.wikipedia.org/wiki/Radar_cross_section#Purpose_shaping https://en.wikipedia.org/wiki/Radar_cross_section#Purpose_sh...).
  - throw0101b 1y ago
    Would airborne radar be better able to find ships with these designs (at least relatively speaking)?
- saltcured 1y ago
  As I recall, the faceted look of the early stealth aircraft was said to be a practical matter. It reduced the complexity of modeling the reflections during the design process. So with additional computational complexity, they could go back to smooth surfaces in later designs. I imagine there are similar issues with ship design. Since these things are wavelength specific, you probably have a bigger computational problem for a bigger vessel. You can't just solve for the design on a miniature and scale it up to build it.