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Comment by ericbarrett | Hacker News Reader

My napkin calculations are that the three channels are ~10ms apart, given the bomber is 69 feet nose-to-tail* and an assumption of 400 mph velocity.

* http://www.dept.aoe.vt.edu/~mason/Mason_f/B2Spr09.pdf

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The google maps scale shows the plane as being pretty close to that length as well. I think the satellite is high enough up that altitude scaling for aircraft is going to be minimal.

I see about a 10 foot separation between colors.

Velocity = distance / time

400 mph is 587 fps.

10 / 587 = 17 ms

Do they actually use satellites for their "satellite" view? I always assumed that it was done by aircraft.

Flying a plane around the planet sounds a lot more expensive financially and thermodynamically than just beaming down images from an object constantly falling into the views you want to see.

I looked it up. There are some satellite images, but a lot of it is done by aircraft.

You are only interested in urban areas which is less than 1% of total earth surface

It's also the case that often there are more legal regulations around aerial imagery compared to satellite imagery.

Or you could just strap a camera to a few commercial airliners and have full coverage.

You'd only have full coverage of the jet routes. They don't just fly willy nilly across the globe.

Depends on the needed resolution. You can cover a lot more earth, far more cheaply with a satellite with several meter wide pixels.

When I used to work in remote sensing I remember that Bing Maps had a very distinct resolution transition from spacecraft to aircraft. Aircraft coverage maps were a small subset of the total earth land area, but most US towns had some coverage at that time (~2012).

As you zoomed in, you could tell when Bing switched capture methods because the aircraft cameras were clearly off-nadir, and you'd see off-angle perception differences.

They only use aircraft around larger urban areas. Satellite imagery is used for rural areas. You can see the resolution difference by seeing how far you can zoom in.

If you zoom in on a road near the pinned location for this post, and then go to, say, downtown kansas city, and zoom in you'll see a pretty significant difference.

It's definitely a mix, depending on the area and the resolution. For urban places where they have 3D views and higher resolution, it's definitely planes. For a random place like this at low resolution, it's satellites.

One might be able to calibrate this by using cars in the area close to the shot [1]. Assuming most cars are driving at the speed limit

[1] https://www.google.com/maps/place/38%C2%B059'37.1%22N+93%C2%...

The cars aren't flying at altitude though. They're farther from the satellite.

How low are these satellites? I wouldn't expect the altitude of a plane to be any significant proportion of the altitude of a satellite.

The satellites are probably greater than 400 km altitude, or so, while the plane is probably 10 km. The plane is in the same rough order of magnitude as really tall mountains, which the satellite is presumably designed to compensate for, so I agree with your general assessment.

The Google Maps image copyright is for Maxar, which means the collector is likely Worldview-3, which is at nominally 614 km above the Earth.

https://earth.esa.int/eogateway/missions/worldview-3

The shadow is 750 m away and the sun is about 15 degrees off noon, so the B2 altitude is roughly 3 km. Cousin comment says the satellite altitude is 614 km, so I wouldn't expect parallax effects to be dominant. This is also evidenced by the fact that both the plane and ground are in focus and the plane is measured to be 70 feet long in google maps, which is what it is specified to be in reality.

That bomber is also likely quite low, since it's based out of an air force base in that area.

Sure, but when the word "calibrate" comes up, you want to account for even the not-very-significant factors.

I would assume that the plane or satellite moving affects this too, it's presumably being corrected but there might be some parallax.

You're absolutely correct, but as the aircraft is much closer to the ground than the imaging satellite I disregarded it. Spherical cows and all that :) I'm sure a military analyst would do the calculations!

It actually works out to be pretty close. A satellite flying ~17k mph at 250 miles altitude would largely have the same angular displacement from the ground as a plane moving 400 mph at 31,000 feet. Most satellites fly in an easterly direction, so the question would ultimately be what the inclination of the orbit is relative to the direction of flight.

That's super interesting! I love this discussion. Would the satellite be that low though? 250 miles (400km) is still going to experience significant atmospheric drag. Fine for a cheap mass deployment like Starlink, but I'd expect an imaging satellite to be set up for a longer mission duration. Then again maybe the cost of additional mass for station-keeping is worth it for the imaging quality.

https://globalcomsatphone.com/satellites-and-their-altitudes...

You can find a list of satellites here: https://www.ucsusa.org/resources/satellite-database

According to the image on Google maps it was taken by Maxar. Maxar appears to have a few satellites, looks like 5 in geostationary orbit (~35,700km), 3 at about 400km and 1 at about 500km.

Looks like you nailed it! ctrl+f pushbroom in the thread if you haven’t seen it.

Most imaging satellites are in sun synchronous orbits so likely in this case moving north or south and slightly retrograde e.g. west.

I'd bet orbital motion is negligible here because the distortion seems to the eye to be entirely in the direction of the plane's apparent motion vector and I don't see any significant skew to that.

I think this plane is flying much lower and slower. The B2s are based at Whiteman AFB, which is about 25 miles south of this photograph. The prevailing winds in this part of the country are from the south, so I suspect that it is about to turn onto the final leg of its approach for a landing to the south. I’d guess that it’s no more than 8-10K’ and traveling at a 2-300 knots max.

Definitely. Rule of thumb in a jet is to slow down if you're closer than 3x the altitude. So at 8k feet you want to be at least 24 nautical miles (27 normal miles) away or if closer you want to be slowing down (or descending). If you don't slow down before you're going to have to do so in the descend and typical jets can't really slow down much in that.

I asked google maps to get directions from there to Whiteman AFB. Unfortunately, flight directions were not available.

And you are now on a list...

I've been on a list for so so long now that this one is just another. No big deal.

Hey welcome to the list, I've been on it since Primary School believe it or not, but only found out a few years ago!

How did you find out?

Well no the speed of the satellite is irrelevant there, the background is not getting distorded at all

Interesting, I would have made the opposite assumption ("the satellite is moving so much faster than everything else, we can estimate the interval from the satellite's speed alone). It seems both speeds might have a similar impact, as per sibling comment.

Isn't the speed of the sat irrelevant as it should correct for this anyways for all images it takes?

yeah, exactly

Can you calculate the time of day from the shadow's length as well?

https://www.google.com/maps/place/39%C2%B001'18.5%22N+93%C2%...

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