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Comment by foota | Hacker News Reader

I don't believe parent is right, but satelites don't stay in one place unless they're on the equator, because otherwise they have to be moving. This means that you need many satelites to maintain coverage of a single spot.

I don't know how many military satelites China has, but I would have assumed it would be sufficient to cover the pacific sufficiently to find an aircraft carrier. (the obvious caveat here being clouds, which are fairly common over the ocean)

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The JWST has a 6.5 meter mirror. The largest (known) spy satellites have a mirror of ~3m diameter. At GEO (geostationary orbit) that would provide an imaging resolution of about 7 meters. An aircraft carrier is about 337x76 meters. So from geostationary altitudes, a satellite similar to a KH-11 would see an American aircraft carrier as a blob of about 48 "pixels". This is probably enough signal to track all aircraft carriers around the globe in real time. It would have a field of view roughly the size of Houston (50x50 miles) and would have enough electricity from solar panels to power reaction wheels to stay pointed at carrier groups indefinitely. (~15-year lifespan would be limited by xenon supply for ion thrusters that keeps the satellite in GEO orbit)

The Chinese Yaogan-41 satellite is in geostationary orbit and might have a mirror in the 4m range.

https://www.csis.org/analysis/no-place-hide-look-chinas-geos...

> It would have a field of view roughly the size of Houston (50x50 miles)

Wait, what?

Like, this is a whole bunch of extremely unreliable numbers being stacked on top of each other to reach an unsupported conclusion, but how is a 50 square mile field of view supposed to find something in the middle of the pacific?

The satellite moves, so every orbit it captures a globe spanning strip that is 50 miles wide (here uncritically accepting the 50 miles figure).

And the carrier isn't going to be in the middle of the pacific, its going to want to launch strikes, so its going to be within (say 500 miles) of Chinese military targets, which does narrow down the size of the haystack somewhat.

But yes, this is a significant challenge. On the modern battlefield it is usually significantly harder to find something than to kill something after you have found it.

You only need to find the aircraft carrier once (say, when it docks) and then the satellite can remain pointed at it forever.

Correct me if I'm wrong, but satellites... orbit. They don't just fly in arbitrary directions following a target.

Even something like a rain storm can lose visual track of a carrier group, or even just night time, at which point you've now got an increasingly large area to search again to find the carrier which can be anywhere inside a several hundred mile circle.

> but satelites don't stay in one place

What?

> unless they're on the equator

What?

> because otherwise they have to be moving

What?

It was admittedly a bit sloppy. The more accurate version (IANAKSPP, I am not a kerbal space program player) would be that the only way to maintain a satelite in one position (without expending an infeasible amount of energy) is to position it above the equator and sync its speed with the Earth's rotation, allowing it to stay in a single position above the Earth. Satelites always have to be moving fast enough such that the centripetal force is sufficient to counteract the Earth's gravitational pull, otherwise they would fall back onto the planet.

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