64 lines
2.3 KiB
Markdown
64 lines
2.3 KiB
Markdown
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# Where's the Sat?
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**Category**: Space and Things
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**Points (final)**: 43
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**Solves**: 107
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> Let's start with an easy one, I tell you where I'm looking at a satellite, you tell me where to look for it later.
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**Given files**: `stations.zip`
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## Write-up
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by [erin (`barzamin`)](https://imer.in).
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Like all 106 other teams probably did, we used Python and [Skyfield](https://rhodesmill.org/skyfield/), an astronomical computation library.
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The challenge gives us questions like
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```
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Please use the following time to find the correct satellite:(2020, 3, 18, 11, 43, 3.0)
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Please use the following Earth Centered Inertial reference frame coordinates to find the satellite:
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[-305.58833718148855, 5030.717506174544, 4485.770450701875]
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Current attempt:1
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What is the X coordinate at the time of:(2020, 3, 18, 4, 24, 46.0)?
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```
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We can easily grab the time and ECI coordinates (which turned out to be International Terrestrial Reference Frame coordinates) with some expect-style goodness:
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```{.python}
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r.readuntil('find the correct satellite:')
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t = ts.utc(*[float(x) for x in r.readuntil('\n').decode().strip()[1:-1].split(', ')])
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r.readuntil('coordinates to find the satellite:')
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eci_coords = np.array([float(x) for x in r.readuntil('\n').decode().strip()[1:-1].split(', ')])
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```
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We're given a list of candidate satellites (or space stations, rather) in two-line element form; find the closest one at the given time:
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```{.python}
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satellites = load.tle_file('./stations.txt')
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match = satellites[np.argmin([np.linalg.norm(s.at(t).position.km-eci_coords) for s in satellites])]
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```
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The challenge then wants us to project ITRF coordinates for three times it gives us. Read the challenge timestamps and tell it where the satellite is:
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```{.python}
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for _ in range(3):
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r.readuntil('X coordinate at the time of:')
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new_t = ts.utc(*[float(x)
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for x in r.readuntil('?\n', drop=True).decode().strip()[1:-1].split(', ')])
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print(new_t.utc_jpl())
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x,y,z = match.at(new_t).position.km
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r.send(f'{x}\n')
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r.send(f'{y}\n')
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r.send(f'{z}\n')
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r.interactive()
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```
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The key can easily be grabbed from the output.
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### Full code
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```{.python include=wheres.py}
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```
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## Resources and other writeups
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- <https://rhodesmill.org/skyfield/>
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- <https://en.wikipedia.org/wiki/Earth-centered_inertial>
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- <https://en.wikipedia.org/wiki/International_Terrestrial_Reference_System_and_Frame>
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