[Sanzig]

Eh, not really. Synthetic Aperture Radar satellites used for marine ship detection have extremely wide sensor swath widths, and ships show up as very bright radar targets against the ocean. Detecting a large ship, even in a very large search area, is almost trivial.

Identifying a ship is harder, but not insurmountable. In particular, large ships like aircraft carriers tend to have very identifiable radar signatures if your resolution is high enough.

[throwaway894345]

How do these work? I would think radar would have a very difficult time seeing a ship against the backdrop of the ocean from so high above. Is the satellite bouncing radar waves off the side of the ship as the satellite is near the horizon? Even if you can detect a ship, I'm having a hard time imagining a sufficiently high radar resolution for such a wide sensor swath width at such an extreme range. Is the idea that you locate it with the wide sensor swath and then get a detailed radar signature from a more precise sensor?

[OneDeuxTriSeiGo]

Even with an extremely low resolution radar hit they are very identifiable.

Most naval vessels move in groups/squadrons. Carriers basically always travel with a "carrier strike group"/CSG of a dozen other ships and destroyers often travel in "destroyer squadrons"/DESRONs. So any time you see a cluster of hits, just by the relative responses of each hit you can narrow down and guess the entire CSG/DESRON in one go and then work out which responses map to which ship in the CSG/DESRON once you have a good idea of which group you are looking at.

This is especially true because ships even within the same class have varying ages, different block numbers, and differing retrofits. So each one has a unique signature to it.

But also if you aren't completely certain you can always come back with a second high resolution pass and then it's trivial to identify each ship just visually.

[throwaway894345]

Granted, but how does satellite radar actually see ships at all? How do the ships not blend into the ocean (the relative difference between the distances between ship<->satellite and ocean<->satellite is minescule)?

EDIT: the sibling comment already provided a high quality answer: https://news.ycombinator.com/item?id=47458766

[Sanzig]

SAR operates in side-looking slant geometry.

Consider shooting a ray at the ocean at an oblique angle from a satellite: it bounces off and scatters away from you. Hardly any of the energy scatters back towards you.

Now, put a ship there. The ray bounces off the surface of the ocean and scatters up into the side of the ship, and from geometry, it's going to bounce off the ship and come straight back towards its original source. You get tons of energy coming back at you.

A ship on the ocean is basically a dihedral corner reflector, which is a very good target for a radar.

> I'm having a hard time imagining a sufficiently high radar resolution for such a wide sensor swath width at such an extreme range. Is the idea that you locate it with the wide sensor swath and then get a detailed radar signature from a more precise sensor?

That's one approach, there are so-called "tip and cue" concepts that do exactly this: a lead satellite will operate in a wide swath mode to detect targets, and then feed them back to a chase satellite which is operating in a high resolution spotlight mode to collect detailed radar images of the target for classification and identification.

However, aircraft carriers are big, so I don't think you'd even need to do the followup spotlight mode for identification. As an example, RADARSAT-2 does 35 meter resolution at a 450 km swath for its ship detection mode. That's plenty to be able to detect and identify an aircraft carrier, and that's a 20 year old civilian mission with public documentation, not a cutting edge military surveillance system. There are concepts for multi-aperture systems that can hit resolutions of less than ten meters at 500 km swath width using digital beamforming, like Germany's HRWS concept.

tl;dr: Radar works very well for this.

[mrguyorama]

>A ship on the ocean is basically a dihedral corner reflector, which is a very good target for a radar.

This is why the Zumwalt and other low observable designs are going back to roughly tumblehome hulls:

https://en.wikipedia.org/wiki/Zumwalt-class_destroyer#/media...

If only it could actually do anything. I genuinely don't understand how we refused to retrofit any weapon system to the gun mounts. We have 5inch guns. They aren't the magic cannon it was designed for but do they really not fit? Apparently we are now putting hypersonic missiles in those mounts instead.

Can't exactly make a Carrier that shape though.

[jasonwatkinspdx]

A Zumwalt with 5 inch gun offers almost no mission capability above a simple coast guard cutter.

They're putting hypersonics on it because they've got 3 hulls and might as well get some value out of them, but not because it's what you'd design for from scratch.

The Zumwalt program was dumb from day 1. It was driven by elderly people on the congressional arms committees that have romantic notions of battleships blasting it out.

The reality is since the development of anti ship missiles, sitting off the coast and plinking at someone is suicidal, even if you have stealth shaping and uber guns of some sort.

It was a DoA mission concept.

[greedo]

The Zumwalt class are being refitted to carry CSP. And the boutique gun system is really a complex thing, it's not like packing in a bunch of VLS containers.

[throwaway894345]

This is cool. Thanks for the detailed follow up!

[jasonwatkinspdx]

> I would think

Just do a youtube search and you'll find plenty of talking head explainer videos. Ignore the talking head and just look at the imagery and data they share.