Thread

Astronomer watches Starlink satellites sinking to build a 'planetary barometer'

67 points · 19 comments · whh

sandworm101 · 1790366324 · focus · HN ↗
The author seems to treat the atmosphere as a, well, sphere. It isnt. It is not of equal depth at all areas. As starlink (and every other sat) only travels in part of the atmosphere each day they cannot measure the entire atmosphere. So you measure a thousand starlinks to get an average. Ok, but a significant bulge on one side of the planet will slow all sats, giving the false impression of a homegenous trend. So you have to track very specific sats in very specific survey orbits... but also update that list as the orbital shell rotates around the earth. It is a very complex problem.
1. schiffern · 1790367242 · focus · HN ↗
  Unfortunately the data doesn't have the required time or space resolution. The drag term is averaged over many orbits, and you can't even use perigee/apogee drift to localize it since the atmosphere rotates so it gets averaged over all longitudes.
  You can do some rudimentary localization by latitude, however, by comparing satellites in different inclinations. You also get data by altitude, of course.
  1. sandworm101 · 1790367714 · focus · HN ↗
    What you want is realtime accelerometer records from satellites in flight, exposing their drag factors over specific parts of the planet. The only tricky variable would be their orientation, but that is certainly recorded too.
    Something like this: <a href="https://en.wikipedia.org/wiki/GOCE" rel="nofollow">https://en.wikipedia.org/wiki/GOCE
    schiffern · 1790368730 · focus · HN ↗
    Based on losing "a few meters of altitude per day," if we assume 5 meter/day at 500 km that's 3 nano-gee. Sensitive piezoresistive accelerometers seem to run in the single digit micro-gee range.[0]
    Easier to look at the altitude loss and infer the acceleration indirectly. With access to the raw higher time-resolution altitude data, mapping should be possible.
    SpaceX has in the past gone above and beyond to expose TLE data for scientists and astronomers[1], so it's possible that people inside SpaceX would be open to collaborating on such a project.
    [0] <a href="https://www.pcb.com/sensors-for-test-measurement/accelerometers/high-sensitivity-icp" rel="nofollow">https://www.pcb.com/sensors-for-test-measurement/acceleromet...
    [1] <a href="https://youtu.be/MNc5yCYth5E?t=1719" rel="nofollow">https://youtu.be/MNc5yCYth5E?t=1719

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