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Scenario file manual

The scenario file lists out the initial conditions for the simulation, the orrery model and the flight plans for any spacecraft in the simulation.

The easiest way to learn about scenario files is to look at the annotated example we just ran.

To get a list of spacecraft programs and how to use them do

build/groho programs

Syntax highlighting

Very elementary syntax highlighting for .gro files can be found under syntax-highlighting. For VS Code copy (or link) the syntax-highlighting/groho folder to ~/.vscode/extensions/groho, something like

ln -s ~/Code/groho/syntax-highlighting/groho/ ~/.vscode/extensions/groho

A Textmate grammar is found at syntax-highlighting/groho/syntaxes/groho.tmLanguage.json

Orrery model

You pass a list of kernel files to the simulator. Optionally, you can indicate a subset of NAIF codes that the simulator should load from the kernel file.

spk de432s.bsp ; load everything from this file

pick 809 899
spk nep086.bsp ; only load 809 and 899 from this file

A SPK file contains barycenters as well as physical bodies. Often ephemeris are stored relative a barycenter. For example:

301 -> 3 -> 0
299 -> 2 -> 0
809 -> 8 -> 0
899 -> 8 -> 0

When loading objects the simulator follows the following rules

  1. If a barycenter and the main body of the barycenter are both loaded, the barycenter is used for coordinate transforms but is not used for gravity computations: the main body is used. The trajectory of the barycenter is also, in this case, not saved.
  2. If a body encountered more than once, the ephemeris is loaded from the first file it is encountered in.

In the given example, the barycenter 8 is not used for gravity computations since 899 is loaded.

Flight plans

Flight plans start with a line indicating the name of the spacecraft

plan Durga

This is followed by a list of events. Each event specifies a start time, duration and, a spacecraft program and how it should run. For example the line:

2050.01.01:0.5 3600 orbit 301 200x200

will turn on a program that thrusts the spaceship till it achieves a 200x200 km orbit around the moon or till the duration is up, whichever is earlier. (A duration is required because this makes restarts easier. It allows the simulator to more easily decide the time point upto which computations can be reused)

Each program has access to the state of the solarsystem (modeling a perfect IMU and perfect knowledge of the solar system) and produces only one output: a thrust vector for the spaceship.

Multiple programs can not run at the same time. If a program's event time occurs before the previous program has terminated, it will be put into a queue, so that they will run as soon as the earlier program has finished.

To describe another spacecraft's flight plan, simply use another plan statement. All events coming after this, will be associated with this new spacecraft.

The insert directive

insert followed by a file path inserts the text of that file into the original file at that point. This can be done recursively. In this manner, multiple files can be combined to form the complete scenario file. For example

start 2050.01.01:0.5
end 2055.01.01:0.5

spk de432s.bsp
pick 809 899
spk nep086.bsp 

insert flightplan1.txt
insert flightplan2.txt

is a neat way of splitting out the flightplans of the two spacecraft in the simulation into two additional files. This can make the writing of the simulation more manageable.

Plot description file manual

fig:
  figsize: [6, 7]

panels:
  main:
    subplot: 211
  moon:
    subplot: 212
    ref: 399
    targets: [-1000, 301]
    dt: 60

Description files are YAML files. They contain two top level sections: fig and panels. The fig section indicates overall figure properties.

The panels section is a dictionary of suplots that we want drawn. The subplot command indicates where the plot should be placed on an imaginary grid. So 211 means, "on an imaginary grid 2 rows x 1 column, make this the first plot"

If nothing else is supplied this plots all the data in the simulation referenced to the solar system barycenter.

If ref is passed the plot is translated to have the body indicated by ref in the center. If targets is passed (this has to be a list), only that list of bodies are plotted. If dt is passed, the trajectories are spline interpolated with an interval of dt seconds.