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The Spaceflight Now article DigitalGlobe books two launches with SpaceX for Earth-imaging fleet goes beyond describing the two launches to talk about DigitalGlobe's future plans.

Fitted with imaging telescopes and detectors built by Raytheon, the WorldView Legion satellites will capture panchromatic, or black-and-white, images with spatial resolution of around 1 foot, or 30 centimeters. That is comparable to the imaging resolution offered by DigitalGlobe’s newest satellites, WorldView 3 and WorldView 4.

DigitalGlobe says the WorldView Legion fleet will double the company’s ability to capture the highest-resolution 30-centimeter-class imagery, and triple capacity over the parts of the planet with the highest imaging demands. The development, construction and launch of the new Earth observation fleet is expected to cost around half of the WorldView 1, WorldView 2 and GeoEye 1 missions it will replace.

[...]

The combination of DigitalGlobe’s existing satellites, the Scout network and WorldView Legion will be able to observe “the most rapidly changing areas on Earth as frequently as every 20 to 30 minutes, from sunrise to sundown,” the company said in a statement.

Am I misunderstanding the article, or is there a suggestion that ~30cm resolution would be available "as frequently as every 20 to 30 minutes" (daytime) for the world's most rapidly changing areas?

Question: Assuming agile, state-of-the-art ADCS, how many satellites would be necessary to to re-image a specific equatorial or mid-latitude area at even medium to high resolution (if not ~30 cm) "as frequently as every 20 to 30 minutes"?

Are we talking hundreds, or thousands? The article mentions Planet Labs and about 200 Doves for once-daily coverage per site as reference, but if I understand correctly those are generally nadir-fixed and use the constellation's pattern to rescan, whereas DigitalGlobe's satellites might be targetting a specific site for higher cadence.

I'm interested in understanding DigitalGlobe's plans and how the "as frequently as every 20 to 30 minutes" statement stacks up to their plans; maybe I'm just reading it wrong?

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First, let's see how many satellites we need to cover any place on Earth once every 30 minutes: Let's take a convenient, almost polar orbit with a period of 100 minutes (corresponding to about 650 km height). This gives 3 satellites needed on each orbit, with an equal spacing. Now let's assume we are fine with images taken at up to 30° off-nadir angle. This gives us a ground track width of 660 km. The equator is about 40,000 km long, so we need 60 different orbits to be able to cover any equatorial area once every 30 minutes for a total of 180 satellites.

Now, this is a vast overestimate of the actual needed number: Any area further away from the equator is overflown much more frequently. Polar regions will be in view every 8 minutes. If we restrict ourselves to one image every 30 minutes for a latitude of 40°, we are down to 130 satellites. And if we take the maximum achievable off-nadir angles of 65° into account we get down to 20 orbits / 60 satellites.

Additionally, the article you quoted states

WorldView Legion satellites will fly in a mix of polar sun-synchronous orbits, [...] and mid-latitude orbits that do not pass over the poles

If we interpret

the most rapidly changing areas on Earth

as "the most densely populated areas in the US and Europe" (which might be a fair assumption for a commercial company), we can restrict the imaging area further to latitudes between 30° and 50°. A quick, back-of-the-envelope estimation results in maybe 8 different orbits needed for a total of 24 satellites, while the the total fleet for this feat will be about 20 satellites.

Please note that this last number is purely based on geometry, I didn't check whether such orbits are stable without a lot of station keeping.

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  • $\begingroup$ Thanks, I'm having trouble understanding what this collection of orbits really looks like. Can you describe the hypothetical constellation a bit more? Is this a single ring of 20 satellites in a single, near-equatorial ring, or six to eight polar orbits with three each? It's hard to know if the number 20 is right without a clear enough description of the constellation to test it. This is why I tried to indicate that an answer which referred to their actual plans is what I'm hoping for. $\endgroup$ – uhoh Mar 30 '18 at 11:34
  • $\begingroup$ The ground track of ISS is maybe a good reference - it spends a substantial time over interesting regions and crosses Europe 3-4 times a day. Half of these are during night. With some optimizations to inclination and orbital period, it should be able to take one image a day of any place in Europe - times 24 to cover 12 hours of daylight each 30 minutes. $\endgroup$ – asdfex Mar 30 '18 at 13:23
  • $\begingroup$ So DigitalGlobe's plan is to use something like six or seven polar orbits with three satellites in each orbit? Or is this just a proof of concept? $\endgroup$ – uhoh Mar 30 '18 at 14:32
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    $\begingroup$ It's speculation, I don't have more information than you have. As they state, it will be a mix of polar and "medium" inclination orbits. $\endgroup$ – asdfex Mar 30 '18 at 14:34

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