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Comment by koudelka | Hacker News Reader

How often is the best earth-facing satellite imagery updated?

It'd be really cool to sic handwriting recognition on it to continuously look for "SOS".

Planet are looking to offer multiple times per day. Although I think that requires the camera to be slewed and a site targeted rather than being able to cover the whole earth every single orbit.

Remember that cloud cover is really common in a lot of places. The UK in January averages less than 2 hours per day of sunlight. You need enough satellites to image everywhere several times per day to allow for that. And even then you may have overcast weather for several days at a time.

Don't forget HELF (https://i.imgur.com/hQAT2Go.png).

The best ones have a turnaround of hours. I imagine it could get expensive quite quickly as the area to do that search would be absolutely massive.

Sentinel satellite data is open, I'm not sure if the resolution is good enough though.

At 10m resolution and ~5day revisit time, it might be possible to optimize for satellite detection. Smoke is relatively easy to spot in satellite imagery. This may be one's best bet. It's also important to note that fires (anything hot really) is also easily detectable. 30-40m high burning SOS letters would be an interesting strategy.

One of the things to keep in mind is that for Sentinel-2 the mean local solar time at the descending node is 10:30am. That may also help optimize at which time any specific attempts are made.

It would be difficult to keep text burning for days on end. You're probably better off writing SOS in high contrast, with a regular fire nearby.

Could work against you if the updated images get filtered out under the cloud cover/fog category.

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