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The largest earth-imaging constellation in the world can image the planet every day. Getting the data to the ground is a challenge, so my understanding is that

The largest earth-imaging constellation in the world can image the planet every day. Getting the data to the ground is a challenge, so my understanding is that they don't bother transmitting images obscured by clouds, or images of empty ocean. https://www.planet.com/ https://www.planet.com/ Updating the cadence to an ~hourly image would require ~2000 satellites, and updating it to every minute would require another 1.5 orders of magnitude... approximately 50,000 satellites. The downlink capabilities to pull that data off the satellites would be colossally huge. A tremendous number of groundstations with a huge array of antennae constantly coordinating and pulling data down. I'm sure Planet is heading in that direction, but it's actually hard to imagine the commercial value of such a mind-blowingly massive infrastructure investment. In contrast to some of the other commenters, I do _not_ think that the NRO or any other collection of government organizations has a planetary image datastream anywhere close to this. The number of satellites and quantity of data required is really mind blowingly immense. I think that the downlink stations would be impossible to conceal, so if we had such a capability, we'd know about it. It's also not necessary for their work - they typically task a satellite with watching an area of interest if they need to. They don't need 24/7 planet-wide monitoring.

ajross 8y ago
> A tremendous number of groundstations with a huge array of antennae constantly coordinating and pulling data down. In the limit where you're talking about a satellite "instantaneously local" to every location on the planet, that becomes just one ground antenna per satellite to handle whichever one is nearest. Yeah, 50k stations are expensive, but not nearly as much as the satellites they are talking to.
- sitkack 8y ago
  Satellites could easily spoof cellular towers, there is literally billions of ground stations, willing or not.
  - jsjohnst 8y ago
    Actually, no they couldn’t, assuming you’re talking about cell phones being the ground station. One, your cell phone doesn’t transmit with enough power to reach even a LEO satellite, and the existing protocols dictate a two way handshake. Second, what do you anticipate the phone to do with the data once it’s received?
  - zaarn 8y ago
    Cellular towers aren't able to talk that far.
    - sitkack 8y ago
      http://lteuniversity.com/ask_the_expert/f/59/p/3370/11922.aspx#11922 http://lteuniversity.com/ask_the_expert/f/59/p/3370/11922.as... Or replace "cellular tower" with long-range wifi https://en.wikipedia.org/wiki/Long-range_Wi-Fi#Notable_links https://en.wikipedia.org/wiki/Long-range_Wi-Fi#Notable_links My point stands that there exists a possible path to get TB to PB of data down to earth through existing handsets. A collection of handsets could form a phased array receiver, everything is basically SDR anyways.
      - zaarn 8y ago
        The satellite will likely have less than 30 seconds to connect to such a tower, upload the data and get new task data. And it looks like the connection in your example used aimed antennas, not just broadcasting the signal onto the surface of the planet, which means you loose a lot of signal strength.
- jsjohnst 8y ago
  50k ground stations would be multiple orders of magnitude more than we’ve ever had. Also, 75% of the earth’s globe is water, if it’s a 1:1 pairing as you suggest, where do you expect to put 75% of those ground stations?
  - ajross 8y ago
    A floating radio station in the middle of the ocean is still much cheaper than a flying radio station in orbit is all I'm saying.
    - jsjohnst 8y ago
      You might assume that fairly, but depending on parameters involved that might not be true.
- natosaichek 8y ago
  If you need to downlink data every minute, stations must be spaced around the earth such that every satellite passes over a groundstation every minute. LEO orbital period is about 100 minutes, so with a circumference of 25000 miles, ground stations would have to be spaced about every 250 miles over the surface of the earth. The earth has a surface area of about 200 million square miles, so divided into 250x250 mile chunks, one would only need about 3200 groundstation sites, but each would have to be able to communicate with ~20-30 satellites at any given moment. Because the satellites are small and relatively low power, the ground stations used to communicate with a high speed link probably use a 3-5 meter dish. So each groundstation site needs either a large array of rapid-slew tracking antennas or a big ol' phased array with some seriously hardcore software on board. A bank of ~20-30 3-5 meter antennae capable of slew and pitch to track a LEO target is a pretty serious investment. Each tracking antenna probably costs more than 100k... they'll have to have chilled antennae for low noise, and some heavy duty mechanisms to move them around. so 3200 sites * 30 dishes per site * 100k per dish is about 10 billion dollars - not including the cost of land or humans. A phased array capable of directing a tight beam to 30 individual targets simultaneously all with different movement parameters is a many 10's of million dollar piece of equipment, and will only inflate the above number. If one builds 50,000 satellites similar to what Planet has already built, the cost comes down into the single digit thousands of dollars per unit. Call it 10k. At that point, launch costs dominate the economics. Each sat has a mass of (say) 5 KG. The falcon heavy can launch 50,000 kg, but for cubesats some significant fraction of the mass will be in the deployment mechanism, so call it 7500 satellites per launch. That's 7 launches, at ~100 million each, so call it a round billion dollars to build and launch the satellites.

ajross 8y ago

> A tremendous number of groundstations with a huge array of antennae constantly coordinating and pulling data down. In the limit where you're talking about a satellite "instantaneously local" to every location on the planet, that becomes just one ground antenna per satellite to handle whichever one is nearest. Yeah, 50k stations are expensive, but not nearly as much as the satellites they are talking to.

sitkack 8y ago
Satellites could easily spoof cellular towers, there is literally billions of ground stations, willing or not.
- jsjohnst 8y ago
  Actually, no they couldn’t, assuming you’re talking about cell phones being the ground station. One, your cell phone doesn’t transmit with enough power to reach even a LEO satellite, and the existing protocols dictate a two way handshake. Second, what do you anticipate the phone to do with the data once it’s received?
- zaarn 8y ago
  Cellular towers aren't able to talk that far.
  - sitkack 8y ago
    http://lteuniversity.com/ask_the_expert/f/59/p/3370/11922.aspx#11922 http://lteuniversity.com/ask_the_expert/f/59/p/3370/11922.as... Or replace "cellular tower" with long-range wifi https://en.wikipedia.org/wiki/Long-range_Wi-Fi#Notable_links https://en.wikipedia.org/wiki/Long-range_Wi-Fi#Notable_links My point stands that there exists a possible path to get TB to PB of data down to earth through existing handsets. A collection of handsets could form a phased array receiver, everything is basically SDR anyways.
    - zaarn 8y ago
      The satellite will likely have less than 30 seconds to connect to such a tower, upload the data and get new task data. And it looks like the connection in your example used aimed antennas, not just broadcasting the signal onto the surface of the planet, which means you loose a lot of signal strength.
jsjohnst 8y ago
50k ground stations would be multiple orders of magnitude more than we’ve ever had. Also, 75% of the earth’s globe is water, if it’s a 1:1 pairing as you suggest, where do you expect to put 75% of those ground stations?
- ajross 8y ago
  A floating radio station in the middle of the ocean is still much cheaper than a flying radio station in orbit is all I'm saying.
  - jsjohnst 8y ago
    You might assume that fairly, but depending on parameters involved that might not be true.
natosaichek 8y ago
If you need to downlink data every minute, stations must be spaced around the earth such that every satellite passes over a groundstation every minute. LEO orbital period is about 100 minutes, so with a circumference of 25000 miles, ground stations would have to be spaced about every 250 miles over the surface of the earth. The earth has a surface area of about 200 million square miles, so divided into 250x250 mile chunks, one would only need about 3200 groundstation sites, but each would have to be able to communicate with ~20-30 satellites at any given moment. Because the satellites are small and relatively low power, the ground stations used to communicate with a high speed link probably use a 3-5 meter dish. So each groundstation site needs either a large array of rapid-slew tracking antennas or a big ol' phased array with some seriously hardcore software on board. A bank of ~20-30 3-5 meter antennae capable of slew and pitch to track a LEO target is a pretty serious investment. Each tracking antenna probably costs more than 100k... they'll have to have chilled antennae for low noise, and some heavy duty mechanisms to move them around. so 3200 sites * 30 dishes per site * 100k per dish is about 10 billion dollars - not including the cost of land or humans. A phased array capable of directing a tight beam to 30 individual targets simultaneously all with different movement parameters is a many 10's of million dollar piece of equipment, and will only inflate the above number. If one builds 50,000 satellites similar to what Planet has already built, the cost comes down into the single digit thousands of dollars per unit. Call it 10k. At that point, launch costs dominate the economics. Each sat has a mass of (say) 5 KG. The falcon heavy can launch 50,000 kg, but for cubesats some significant fraction of the mass will be in the deployment mechanism, so call it 7500 satellites per launch. That's 7 launches, at ~100 million each, so call it a round billion dollars to build and launch the satellites.