# Where's the Sat?
**Category**: Space and Things 
**Points (final)**: 43 
**Solves**: 107

> 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.

**Given files**: `stations.zip`

## Write-up
by [erin (`barzamin`)](https://imer.in).

Like all 106 other teams probably did, we used Python and [Skyfield](https://rhodesmill.org/skyfield/), an astronomical computation library.
The challenge gives us questions like
```
Please use the following time to find the correct satellite:(2020, 3, 18, 11, 43, 3.0)
Please use the following Earth Centered Inertial reference frame coordinates to find the satellite:
[-305.58833718148855, 5030.717506174544, 4485.770450701875]
Current attempt:1
What is the X coordinate at the time of:(2020, 3, 18, 4, 24, 46.0)?
```

We can easily grab the time and ECI coordinates (which turned out to be International Terrestrial Reference Frame coordinates) with some expect-style goodness:
```{.python}
r.readuntil('find the correct satellite:')
t = ts.utc(*[float(x) for x in r.readuntil('\n').decode().strip()[1:-1].split(', ')])

r.readuntil('coordinates to find the satellite:')
eci_coords = np.array([float(x) for x in r.readuntil('\n').decode().strip()[1:-1].split(', ')])
```

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:
```{.python}
satellites = load.tle_file('./stations.txt')

match = satellites[np.argmin([np.linalg.norm(s.at(t).position.km-eci_coords) for s in satellites])]
```

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:
```{.python}
for _ in range(3):
	r.readuntil('X coordinate at the time of:')
	new_t = ts.utc(*[float(x)
		for x in r.readuntil('?\n', drop=True).decode().strip()[1:-1].split(', ')])
	print(new_t.utc_jpl())

	x,y,z = match.at(new_t).position.km
	r.send(f'{x}\n')
	r.send(f'{y}\n')
	r.send(f'{z}\n')

r.interactive()
```

The key can easily be grabbed from the output.

### Full code
```{.python include=wheres.py}
```

## Resources and other writeups
- <https://rhodesmill.org/skyfield/>
- <https://en.wikipedia.org/wiki/Earth-centered_inertial>
- <https://en.wikipedia.org/wiki/International_Terrestrial_Reference_System_and_Frame>