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

The present repo contains the `tle2ssc` Python script which converts
NORAD Two-Line Element sets (TLE) from CelesTrak into into
Solar System Catalog (SSC) files that can be read by Celestia.

The `tle2ssc` script is set by default to retrieve the latest orbits
for various GNSS ("GPS") satellite systems.
It could be easily updated for any other source of TLEs.

*Celestia Animated Time Lapse GNSS Orbits*
![celestia-gnss-orbits-anim](pics/celestia-gnss-orbits-anim.gif)


# Quick & Dirty Install
If you just want to import the pre-generated file into Celestia for
a look, just copy these files into your `extras` folder or similar.

```
# crufty below, private dir better
sudo mkdir -p /usr/share/celestia/extras-standard/gnss/models/
# Whatever user...
sudo chown -R debian:debian /usr/share/celestia/extras-standard/gnss

# Get SSC file
wget -O /usr/share/celestia/extras-standard/gnss/gnss.ssc \
  https://spacecruft.org/spacecruft/celestia-gnss/raw/branch/main/extras/gnss/gnss.ssc

# Get 3D model
wget -O /usr/share/celestia/extras-standard/gnss/models/galileo-gnss.cmod \
  https://spacecruft.org/spacecruft/celestia-gnss/raw/branch/main/extras/gnss/models/galileo-gnss.cmod

# Then (re)start Celestia.
```

# Install celestia-gnss
Install thusly to generate your own `.ssc` files.

```
git clone https://spacecruft.org/spacecruft/celestia-gnss
cd celestia-gnss
sudo apt update
sudo apt install python3-pip python3-virtualenv
# Many ways to do python setup, here is one:
virtualenv -p python3 env
source env/bin/activate
pip install --upgrade pip
pip install -r requirements.txt
```

# Run celestia-gnss
Example (crufty example, if you have perms):

```
./tle2ssc > /usr/share/celestia/extras-standard/gnss/gnss.ssc
```


# Use in Celestia
Use thusly, hooman.

*Celestia Showing GNSS Satellites with Labels*
![celestia-gnss-screenshot](pics/celestia-gnss-screenshot.png)

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

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

Each system, GPS, Galileo, Glonass, Beidou, can be used,
or the GNSS files which include all four, plus more regional systems.


# Screenshots
Screenshots of running Celestia with GNSS satellites.

*GNSS Satellites Orbiting Earth*
![celestia-gnss-orbits](pics/celestia-gnss-orbits.png)

*Celestia with Moon orbit, Earth, and GNSS satellites.*
![celestia-moon-gnss](pics/celestia-moon-gnss.png)

*Celestia MultiView.*
![celestia-multiview-gnss](pics/celestia-multiview-gnss.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

Celestia forum thread about celestia-gnss:

* https://celestia.space/forum/viewtopic.php?f=6&t=22559

## TLE Conversion

TLE Conversion Spreadsheet by Grant Hutchison from July 2003,
the basis for the formulas in the `tle2ssc` script.

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

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

Useful formula from here too:

* https://kaitlyn.guru/projects/two-line-elements-tle/

I had this exact printout on my desk (trimmed to size!) when I came across
this gem from @fisadev via Poliastro docs:

***How are satellite orbits disseminated?***

<img src="pics/wtf-tle.jpg" width="600">


* https://docs.poliastro.space/en/latest/examples/Loading%20OMM%20and%20TLE%20satellite%20data.html

Perhaps from page 7 of the PDF,
*Strategic Center for Networking, Integration, and Communications
Orbit Propagation Front-End Software Development*.

* https://ntrs.nasa.gov/citations/20180008672

Not used, but perhaps could be useful from Libre Space Foundation:

* https://gitlab.com/librespacefoundation/python-satellitetle

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

Example `.ssc` file contents:

```
"GSAT0101 (PRN E11)" "Sol/Earth" {
    Class "spacecraft"
    Mesh "galileo-gnss.cmod"
    Radius 0.005

  EllipticalOrbit {
    Epoch 2459722.35154914
    Period 0.58659637
    SemiMajorAxis 29600.176
    Eccentricity 0.00041090
    Inclination 56.9865
    AscendingNode 22.0582
    ArgOfPericenter 29.0248
    MeanAnomaly 331.0391
  }
  Obliquity 56.9865
  EquatorAscendingNode 22.0582
  RotationOffset 296.9294
  # Orientation  [  ]
}
```

## TLE Sources

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

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

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

## SGP4
SPG4

*  https://pypi.org/project/sgp4/

## Skyfield
Skyfield

* https://rhodesmill.org/skyfield/earth-satellites.html


# 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`.

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

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

![celestia-galileo-screenshot](pics/celestia-galileo-screenshot.png)

## 3D Model Conversion
The `galileo.obj` file from the ESA can be read by `cmodview` in
the `celestia-tools` package. The file can be saved in `cmodview`
as `galileo-gnss.cmod` and then read by Celestia.

The ESA supplies textures, but the aren't applied in the Celestia model (yet).

### Blender
`blender-2.79b` didn't work to convert a `.obj` file to `.3ds`
that rendered in Celestia.


# TODO
Do.

* All GNSS satellites are using the Galileo 3D model.

* Textures need to be added to 3D models.

* Orientation of satellite 3D models is incorrect.

* Confirm orbits are reasonably correct.


# Status
Alpha, under development.

Output works in Celestia. Orbits are not confirmed correct.


# License / Copyright
Upstream sources under their respective copyrights.

Copyright (C) 2022, 2023 Jeff Moe.

License: GPLv2 or any later version.