1. scenario_BasicOrbitFormation
1.1. Overview
This script sets up two 3-DOF spacecraft orbiting the Earth. The goal of the scenario is to
highlight how the BSK_Sim structure of a formation flying tutorial is different from the basic orbit scenario,
demonstrate how to create a formation flying scenario, and
how to customize the BSK_FormationDynamics.py and BSK_FormationFSW.py files.
The script is found in the folder basilisk/examples/BskSim/scenarios and executed by using:
python3 scenario_BasicOrbitFormation.py
The simulation mimics the basic simulation in the earlier tutorial in scenario_BasicOrbit.
The flight software mode is set to inertial3D. The goal of this mode is to align the body axes of the spacecraft with an inertial 3D point guidance coordinate system defined in BSK_FormationFsw. However the flight software mode can also be set to “standby”.
The simulation layout is shown in the following illustrations.
1.2. Configuring the scenario file
The simulation layout is very similar to the one used for the scenario_BasicOrbit file. Two simulation processes are created: one which contains dynamics modules, and one that contains the Flight Software (FSW) modules. First of all, it can be observed that the Dynamics- and FSW files used are the BSK_FormationDynamics and BSK_FormationFsw files. These two files have been created for this specific formation flying implementation into Basilisk.
After initializing the interfaces and making sure that the scenario_BasicOrbitFormation
class inherits from the BSKSim class,
it is time to configure the initial conditions using the configure_initial_conditions method.
It can be observed that two sets of
orbital elements are created. Each set corresponding to one spacecraft. After that the initial
conditions are set for each spacecraft.
After that the function that logs the outputs can be observed. Again this looks very similar to
the log_outputs method
in the scenario_BasicOrbit file, however one discrepancy can be noticed. Looking at
the code below it can be observed that
two instances of the simpleNavObject are logged (simpleNavObject and simpleNavObject2 respectively).
Each object corresponds
two one of the spacecraft. The same is true for the FSW objects. More on this will be discussed later.
The same is true for the pull_outputs method. Also in this function, it can be observed
that the outputs of two instances
of a specific object are pulled.
1.3. BSK_FormationDynamics file description
Looking at the BSK_FormationDynamics file, it can be observed that the dynamics
process consists of two tasks named DynamicsTask
and DynamicsTask2 respectively. These tasks are added to the dynamics process and to each task,
an instance of a specific object is added.
The gravity body (Earth in this case) is created using the gravBodyFactory and is attached
as a separate object to each spacecraft.
After that each object is added to the corresponding task. Something that is very important is the message names. In case multiple spacecraft are implemented in Basilisk it is necessary to manually connect an output message of one module to the input of a different module. This can be seen in the module-initialization methods in the BSK_FormationDynamics.py file.
1.4. BSK_FormationFsw file description
The setup of the FSW file (BSK_FormationFSW.py) in case of
formation flying is very similar to the setup of the dynamics file.
Also in this case, an instance of each task is initialized that corresponds to
one of the two spacecraft. Furthermore, it is
necessary to manually set the input- and output message names for the FSW
modules. In order to make this tutorial work properly its
is very important to set the self.mrpFeedbackRWs.Ki and self.mrpFeedbackRWs2.Ki
variables in BSK_FormationFsw to -1. Otherwise
the orientation and rates of both spacecraft will not converge!
1.5. Illustration of Simulation Results
showPlots = True
- scenario_BasicOrbitFormation.run(showPlots)[source]
The scenarios can be run with the followings setups parameters:
- Parameters:
showPlots (bool) – Determines if the script should display plots