2019/04/27

Ship detection by synthetic aperture radar imaging of wakes

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A radar could indirectly 'see' a moving ship by 'seeing' its wake even 36 years ago:
source (chapter 12)
I've seen even more impressive SAR imagery of a moving ship being visible by its wake (SEASAT imagery, 1978), published in the mid-80's already. The principle of detecting a moving ship by sensing its wake should nevertheless be obvious by this available lower quality image.

Radar's ability to find ships by their wakes has multiple important consequences in the naval domain:
  • radar stealth for surface ships is likely of little use against high altitude or orbital radars which can employ a SAR mode
  • as a consequence, radar satellites (even civilian ones meant for land imagery) may be of great importance to navies
  • as another consequence, the supersonic and high-flying anti-ship missiles (and Chinese quasiballistic anti-ship missiles) may have a vastly better capability to discriminate real moving ships from decoys than the typical Western approach of radar-guided seaskimmer missiles
  • and as yet another consequence, there's an additional realm for camouflage and deception; wakes* (this may also help a bit against wake-homing torpedoes)
I simply meant to point this out in a bit more detail than before because I find this hardly ever mentioned in the context of naval surface warfare and anti-shipping air warfare.

related:
Radar stealth is helpful at long distances, but  (...) In theory it's even possible that a missile could climb after being detected and look at the sea surface to spot ship's the wake pattern to tell real ship targets from decoys and boats (...).

S O
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20 comments:

  1. So, Radar stealth in ships has no tangible utility to impede long range detection by a "peer adversary", as such would have ocean reconaissance satellites or similar, and it has, most likely, no utility against "modern" Anti-Ship missiles (based on your assessment in the quoted article).

    We all know it's useless for the usual great power games of canon boat diplomacy and launching cruise missiles from out of range of any danger. So, why even spend money on it? I mean, even more than in aircraft, the tradeoffs of hull shaping and the issues of maintaining RAM coatings in the agressive sea environment make the utility extraordinary questionable...

    [And yes, I agree to much of your writings about the 'utility' of current (European) Navis... ;)]

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    1. A theoretical utility would be that if the ship's reflections are weak, you can disguise the ship by adding reflectors that mimic the returns from a normal cargo ship.
      Meanwhile, the rpm of the screws could be modified (especially with variable pitch screws) to match a cargo ship's at that speed.

      This matches the disguise approach used by some auxiliary merchantman commerce raiders and Q ships in the world wars.

      I do not see any indications (or outside preparations) on modern warships for any such disguises, though. Furthermore, the usefulness of a disguise depends on the plausibility that a low value target or neutral ship with that appearance would be in that area and show that navigation behaviour.
      Emissions control would need to be very restrictive as well.

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  2. Although SAR can certainly see wakes if it knows where to look, it's the knowing where to look part that I'm not seeing.

    Can the radar do so over tens of thousands of square miles at a time.

    @Mister
    Smaller radar return = harder to see.
    That's an absolute truth.

    Whether it's a warhead seeker or a giant fixed over the horizon deal.
    RCS reduction does not require exotic coatings, the goal is reduction not removal.

    @Sven
    Radar reduction and reflecting panels aren't rare, as far as I'm aware they're common carry

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    1. Radar reflectors are carried as decoys and to make civilian navigation radars useful in peacetime. I've never seen anything that hints at really changing the shape of a radar echo rather than just its intensity.

      Even civilian SAR satellites boast 100+ km width of scan (while moving at about 25.000 km/h). The issue with satellites is rather their position for search (not so much for tracking) than search area.

      Quasi-ballistic missiles do only need to find ships and discern them from decoys, they are supposed to be shot at the approximate position (maybe +/-20 nm).
      Same with supersonic anti-ship missiles.

      Combat aircraft would be forced to search from extreme distances or pop up and return to very low altitude by very long range SAMs such as SM-2ER or SM-6. I suppose their pilots would think of defending AEW and fighters as the main obstacle. Finding and shooting the ships would be rather simple afterwards.

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    2. I should probably elaborate on the ID thing. SAR radar can be used to identify ships. Even poor quality SAR resolution can be helpful in discerning a destroyer from a cargo ship by detecting the position and approximate length of the superstructures.
      See 2nd image here:
      https://defense-and-freedom.blogspot.com/2018/02/modern-warships-iii-aaw.html

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  3. South China sea isn't the Pacific. Sea floor sensors in chokepoints, 'fishing vessels', high altitude drones, 'disposable' drones.
    I don't think much could be expected to hide their approx position for long, even if the sats all go boom.
    I wonder if the yanks assume they will be able to hide.

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    1. Presumably the Americans don't plan to land in Beijing on day 1...

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    2. I know the yanks are preparing for a fight. I just don't have a clue what they think that will entail.

      No way theyre invading the mainland, but what about land defence of Taiwan? What about logistical support of Taiwan?

      If the yanks don't move quickly, Taiwan will fire off a few days worth of fireworks then sue for peace.

      They can't though, can they? No way they could escort a convoy into Taiwan during the opening phase of a war. That would be a fleet battle. (Any attempt at a 'covert' buildup is impossible these days, we live in the modern world now)

      The rat hole goes on from there. Sea control. Stand off exchanges. Chinese economic warfare. Global trade balances. Subsea fun, etc...

      And on. And on...

      The tyranny of distance and geography has it so the US can not engage China militarily. Maybe they know that, maybe they are just managing the decline (more correctly managing China's rise). No reason to 'lose' prematurely.

      If that is the case, they are spending an awful lot of money on kit that will never be used.

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    3. https://defense-and-freedom.blogspot.com/2009/08/taiwans-defence-army.html
      That was ten years ago, and little changed.

      Taiwan doesn't seem to bet on its military for deterrence or defence much. It embraced the PRC economically and is symbiotic by now.
      I doubt there will ever be a fight about Taiwan.

      Likewise, there's no point in invading the Koreas. South Korea should abstain from naval arms racing with its shipyards to not provoke the PRC, though.

      Japan is distant, I don't see much conflict potential there, either.

      The least unlikely scenario for an American-Sino War is thus IMO that the Americans once again vote some moron into the presidency and said moron escalates, maybe even establishes a far blockade. That could prompt the PRC to take hostages in order to break the blockade.

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    4. The point I was trying to make was, what do the yanks think they are going to do. I agree with you that Taiwan isn't going to fight, but do the yanks think that? That leads to the rat hole. What type of war do the yanks think they are going to fight?

      "The pivot to the Pacific" happened was under Obama. Rhetoric was basically the same. Economics aside, the military component was the same. Whinge about the Spratly Islands, and go on "Freedom of navigation" jaunts with their carrier battlegroups.

      The diplomatic alliance side is more complicated. Though cash rules everything and even the yanks don't disagree that China is going to have a larger (non PPP) economy, nothing they can do to stop that. So China can just 'buy' its allies. (The pushback against that is hilarious, "We share common values with our allies" I laugh and I laugh...)

      It pisses me off that humanity is seemingly going to waste decades and trillions on this discordant, self contradictory 'strategy'. Zombie empire. If they are going to fall what difference does it make (scorched earth), the US nation is contingent on it empire. Without it, it can not exist in its current form. So what difference does the amount of debt that the yankish state has when it fails make?

      Look back at history, empires are all barrels of morons. They all rot from the brain. "We're number one. We're number one. AMERICA FIRST!!!! AMERICA FIRST!!!" My lot is no different, just look at brexit.

      Its a rat hole. There doesnt have to be any logic behind any of it. Its a failure on my part to think that there should be.

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    5. I think the Americans who write articles and forum posts simply see a superpower challenged and conclude that it should confront the challenge with military power, no more reasoning than that. Their idea of military power appears to be a distant blockade coupled with naval land attack (more concern about range of naval aircraft than attention to cruise missiles).
      I've yet to see any remarks about sending the army into South Korea in force or any remarks about deploying the air force to South Korea or Taiwan in force.
      Air superiority far from friendly airbases or carriers appears to be impossible with the available tech.

      I think the Americans are insane in their budgeting and too traditional, too path-dependent in their structuring of forces. They waste a colossal amount of resources every year - the waste exceeds their public infrastructure spending shortfall and what they would need to convert their electrical power supply to near-carbon neutrality.

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    6. Completely off topic, of everything really, so apologies.

      Any predictions on the EU elections for your lot? Over here everything seems subdued, everyone is confused and exhausted. Even the reactionaries arent getting much buzz, though they try. Spanish elections weren't that bad when you consider the constraints.

      I would have thought the culture war side of election propoganda would have been at a higher pitch by now. Maybe it will hit big next week.

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    7. There are no common news about European policies or politics, so the elections for the European parliament will be a different story in every member country.
      https://defense-and-freedom.blogspot.com/2012/10/european-unification.html

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    8. "for your lot", the Germans.

      The internet is over rated as a form of communication. I say someone asks the Chinese to turn it off.

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    10. Actually, German companies are big in the business of producing the materials and parts for batteries and such. That may matter more than where the battery assembly is at.
      The German industry's strength is more in the medium-sized supplier companies than in the famous brand OEMs large companies.

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  4. It's nice to be able to see the enemy ships like that, you can avoid Pearl Harbor type disaster, but is it enough to illuminate a target for a missile? The biggest advantage of stealth is that it's difficult to fire at it - you might know it's roughly there, but you have to send something up close to be able to fire at it. This is a big advantage, and I'm not sure this technology annuls it - unless the anti-ship missile carries a nuclear warhead (as was the case during the Cold War).

    Am I missing something? Or is there a way of using this information for fire-control with modern technology? I'm not an expert nor very up-to-date in military technology, so it's quite possible I'm wrong.

    (Sorry if it's a duplicate, I'm not sure my previous comment went through, I think not.)

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    1. To find opposing ships on an ocean is one important step. It's particularly important in regard to the question whether the USN could 'bomb' the PRC with carrier aviation. Their Super Hornets, Growlers and also Lightning II in LO configuration are not terribly long-ranged.

      The missiles for ship attack have different means to detect and identify their targets. See the warships series, chapters AAW and ASuW.

      https://defense-and-freedom.blogspot.com/2018/01/modern-warships-i-introduction.html

      A Chinese quasiballistic anti-ship missile that cannot sense wakes might fall for a towed radar decoy. One that can sense wakes might go for actual ships only. I'm no physicist or radar-specialised engineer, so I leave it at these hints about what I think may be possible.

      Also keep in mind that a fleet that's found might be discouraged from sending as many aircraft on offensive missions as one whose commander deems the fleet to be undetected.

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    2. Thanks! I'll read the blog posts you proposed and then I might come back if I have further questions.

      Yes, what you write seems to make sense.

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