6 ms·

The DoD (or the NRO) had this tech in the 80s? Wasn’t this SAR originally in the form of ground penetrating radar invention for finding nukes in ground siloes?

geomark 6y ago
Also used to track submerged submarines by detecting ocean surface disturbances caused by the underwater wake of the submarine. Same benefits: works at night and through cloud cover.
- lemonspat 6y ago
  That is astounding! something I was unaware about before. At what depth could it be measured?
  - grendelt 6y ago
    Almost any depth. A big steel tube is going to displace water. Once you have some baseline measurements (of known underwater landforms), then small imperceptible variations would stick out.
    - SirLuxuryYacht 6y ago
      Not possible. The uncertainty of bathymetry in areas only mapped using gravity is larger than the dimensions of a sub. Even in very well mapped areas, think of the effort in the Indian ocean to find MH370, this would require the assumption that the sub is moving, which is not always the case, e.g. if the sub halts propulsion and drifts with a current.
      - geomark 6y ago
        Well yes, it's the surface disturbance of the underwater wake that they are detecting. So the sub has to be moving under its own power. But there is no need for high resolution and the wake of the sub stands out starkly even with a large field of view. So I'm not sure it can evade detection by halting propulsion and drifting. The point at which it halted would be known and it would be detected as soon as it restarted, unless it was carried by an unknown current far enough to be out of view of the satellite.
  - baybal2 6y ago
    The infamed laser on Tu 142 was said to get to only get to 50m, but that was a "direct imager" which relied on direct reflection, and seventies-eighties era technology. Newer ones are directly sensing density, and temperature changes in the seawater by sensing Raman shift, and are supposedly much better. No known data in public of course.
- aerostable_slug 6y ago
  Tracking submerged boats from space is the holy grail of ASW, and as best as I can tell it doesn't work outside littoral zones (where sometimes you can optically detect the sub) and contrived tests. It has been tried with SAR, blue/green lasers, IR, all kinds of stuff, and luckily it doesn't work in the real world. When it does, it will be massively destabilizing as it places the "invulnerable" second strike capability that guarantees deterrence against peer states at risk.
  - baybal2 6y ago
    > When it does, it will be massively destabilizing as it places the "invulnerable" second strike capability that guarantees deterrence against peer states at risk. It never been such to begin width, at least by the late cold war, some portion of missile subs were reliably tailed by adversaries without any high tech trickery. North Korea, Russia, and China each already have more submarines than USA, and would have no problem to put 10 subs to tail each Ohio.
    - unchocked 6y ago
      Tailing some portion of missile subs won't do - a single missile sub is an existential threat so deterrence isn't destabilized until the adversary can credibly eliminate all of them within ~10 minutes. It takes a nuclear sub to trail a nuclear sub. NK and China have exclusively, and primarily (respectively) diesel subs. These have to surface and run their engines to charge batteries. They cannot make 20+ knots submerged for long, and they cannot track deeply submerged submarines from the surface. Surfacing and running diesels would make their presence known. They cannot range far from port, and certainly cannot track missile boats under the arctic ice. Russia has a sufficiently large nuclear fleet, but it is outdated and probably outclassed by US technology. There is probably greater disparity between the powers than existed during the cold war, at which time deterrence was credible.

geomark 6y ago

Also used to track submerged submarines by detecting ocean surface disturbances caused by the underwater wake of the submarine. Same benefits: works at night and through cloud cover.

lemonspat 6y ago
That is astounding! something I was unaware about before. At what depth could it be measured?
- grendelt 6y ago
  Almost any depth. A big steel tube is going to displace water. Once you have some baseline measurements (of known underwater landforms), then small imperceptible variations would stick out.
  - SirLuxuryYacht 6y ago
    Not possible. The uncertainty of bathymetry in areas only mapped using gravity is larger than the dimensions of a sub. Even in very well mapped areas, think of the effort in the Indian ocean to find MH370, this would require the assumption that the sub is moving, which is not always the case, e.g. if the sub halts propulsion and drifts with a current.
    - geomark 6y ago
      Well yes, it's the surface disturbance of the underwater wake that they are detecting. So the sub has to be moving under its own power. But there is no need for high resolution and the wake of the sub stands out starkly even with a large field of view. So I'm not sure it can evade detection by halting propulsion and drifting. The point at which it halted would be known and it would be detected as soon as it restarted, unless it was carried by an unknown current far enough to be out of view of the satellite.
- baybal2 6y ago
  The infamed laser on Tu 142 was said to get to only get to 50m, but that was a "direct imager" which relied on direct reflection, and seventies-eighties era technology. Newer ones are directly sensing density, and temperature changes in the seawater by sensing Raman shift, and are supposedly much better. No known data in public of course.
aerostable_slug 6y ago
Tracking submerged boats from space is the holy grail of ASW, and as best as I can tell it doesn't work outside littoral zones (where sometimes you can optically detect the sub) and contrived tests. It has been tried with SAR, blue/green lasers, IR, all kinds of stuff, and luckily it doesn't work in the real world. When it does, it will be massively destabilizing as it places the "invulnerable" second strike capability that guarantees deterrence against peer states at risk.
- baybal2 6y ago
  > When it does, it will be massively destabilizing as it places the "invulnerable" second strike capability that guarantees deterrence against peer states at risk. It never been such to begin width, at least by the late cold war, some portion of missile subs were reliably tailed by adversaries without any high tech trickery. North Korea, Russia, and China each already have more submarines than USA, and would have no problem to put 10 subs to tail each Ohio.
  - unchocked 6y ago
    Tailing some portion of missile subs won't do - a single missile sub is an existential threat so deterrence isn't destabilized until the adversary can credibly eliminate all of them within ~10 minutes. It takes a nuclear sub to trail a nuclear sub. NK and China have exclusively, and primarily (respectively) diesel subs. These have to surface and run their engines to charge batteries. They cannot make 20+ knots submerged for long, and they cannot track deeply submerged submarines from the surface. Surfacing and running diesels would make their presence known. They cannot range far from port, and certainly cannot track missile boats under the arctic ice. Russia has a sufficiently large nuclear fleet, but it is outdated and probably outclassed by US technology. There is probably greater disparity between the powers than existed during the cold war, at which time deterrence was credible.