# Spacecruft Celestia GNSS
Celestia is a "real-time space simulation that lets you experience our
universe in three dimensions".

It contains many celestial objects, but few that are man made, such
as Hubble. It has a TLE for the ISS, but it doesn't update it.

There is a spreadsheet that you can enter a TLE into it and it
generates an `.ssc` file that Celestia can read. Using a spreadsheet
is a bit awkward, and hard to automate.

It would be real swell to be able to add TLEs to Celestia more easily.
Then all the GNSS satellites could be added. Or SatNOGS...

It will take the data that generates this:

![spacecruft-galileo](pics/galileo.png)

And render it in this application:
![celestia-screenshot](pics/celestia-screenshot.png)


# See Also

## Spacecruft
Spacecruft:

* https://spacecruft.org/spacecruft/gnss-earth

* https://spacecruft.org/spacecruft/celestia

## Celestia
Upstream Celestia:

* https://celestia.space/

* https://github.com/CelestiaProject/Celestia

## TLE Conversion

TLE Conversion Spreadsheet:

* https://www.classe.cornell.edu/~seb/celestia/hutchison/spreadsheets.html

* https://www.classe.cornell.edu/~seb/celestia/hutchison/tle-xls.zip

## Solar System Catalog
*"Solar System Catalog" files are primarily used to define the objects which
can be found in a stellar system, such as planets, moons, asteroids, comets,
and spacecraft. They also have four other uses: they can define alternative
surface textures, locations on or near an object, and orbital reference points.
An SSC file can also edit objects which have been defined in other SSC catalog files.*


Info on `.ssc` files:

* https://www.classe.cornell.edu/~seb/celestia/creating-ssc-files.html

* https://en.wikibooks.org/wiki/Celestia/SSC_File


In Celestia name "Galileo" is used for the *other* spacecraft, not
for the EU GNSS. So the name used in development here adds `-gnss`,
ala, `galileo-gnss.ssc`, whereas the other GNSS don't.

Example `.ssc` file contents:

```
"1997-035A" "Sol/Earth" {
  Class "spacecraft"
  # Mesh
  Radius  0.005

  EllipticalOrbit {
    Epoch  2459718.94467728
    Period  0.49859567
    SemiMajorAxis  26560.345
    Eccentricity  0.0057297
    Inclination  55.5062
    AscendingNode  156.8049
    ArgOfPericenter  51.2220
   MeanAnomaly  309.3262
  }
  Obliquity  55.5062
  EquatorAscendingNode  156.8049
  RotationOffset  80.2289
  # Orientation  [  ]
}
```

## Galileo
Galileo 3D models are from the ESA:

* https://gssc.esa.int/education/galileo3d/


## TLE Sources

Beidou (China)
* https://celestrak.com/NORAD/elements/beidou.txt

Galileo (Europe)
* https://celestrak.com/NORAD/elements/galileo.txt

Glonass (Russia)
* https://celestrak.com/NORAD/elements/glo-ops.txt

GNSS (All)
* https://celestrak.com/NORAD/elements/gnss.txt

GPS (USA)
* https://celestrak.com/NORAD/elements/gps-ops.txt


## Spice
Spice TLE:
"Evaluate NORAD two-line element data for earth orbiting spacecraft."

* https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/evsgp4.html

## pyorbital
Orbital and astronomy computations in python.

* https://github.com/pytroll/pyorbital/

* https://pyorbital.readthedocs.io/


# 3D Models
Celestia uses an old 3D proprietary format from an old 3D application in
`.3ds` format. It also has it's own format `.cmod`.

## 3D Model Conversion
Recent versions of Blender don't export `.3ds` format. But old versions
of Blender, such as 2.79b do work. Get an old version here:

* https://download.blender.org/release/Blender2.79/

Launch `blender`, import, export.

```
# Go to https://gssc.esa.int/education/galileo3d/
# and download the Galileo_3D.zip file.
# It generates a unique URL, afaict.
unzip Galileo_3D.zip

# Get old Blender
wget https://download.blender.org/release/Blender2.79/blender-2.79b-linux-glibc219-x86_64.tar.bz2
tar xf blender-2.79b-linux-glibc219-x86_64.tar.bz2
cd blender-2.79b-linux-glibc219-x86_64
./blender
```

Blender creates a new file on launch.

Go to `File --> Import --> Wavefront (.obj)`.

Import the file, such as `galileo.obj`.

Go to `File --> Export --> 3D Studio (.3ds)`.

Save the file, such as `galileo-gnss.3ds`.

The `galileo-gnss.3ds` file created this way didn't render in Celestia. `:(`


# Install celestia-gnss
Install thusly.

```
git clone https://spacecruft.org/spacecruft/celestia-gnss
cd celestia-gnss
sudo apt update
sudo apt install python3-pip
# Many ways to do python setup, here is one:
pip3 install --upgrade --user pip
# Make sure your path includes `~/.local/bin`, if not, add it.
which pip
# Should be ~/.local/bin/pip
pip install --user -r requirements.txt
```

# Run celestia-gnss
Run thusly.

Convert a TLE, example:
```
./tle2ssc extras/galileo-gnss/galileo.txt
```


# Use in Celestia
Then copy the satellites you want into your Celestia extras dir.
Crufty bad way to do it, example:

```
sudo cp -a extras/galileo-gnss/ /usr/share/celestia/extras-standard/
```


# Issues
`pyorbital` doesn't support geostationary orbits.
It gives the error:
`Deep space calculations not supported`

I thought that was what `pyorbital` was for. It is mentioned in a
bug report, but not in the docs. Hmm.

See issue:

* https://github.com/pytroll/pyorbital/issues/71

```
I think the Deep space error comes when trying to use TLEs for computing
geostationary or deep space orbits, with pyorbital doesn't support yet.
```

# License / Copyright
Upstream sources under their respective copyrights.

Copyright (C) 2022 Jeff Moe.

License: GPLv2 or any later version.