.. _BskSim_scenarios_scenario_BasicOrbitFormation:

scenario_BasicOrbitFormation
============================

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 :ref:`BSK_FormationDynamics.py <BSK_FormationDynamics>` and :ref:`BSK_FormationFSW.py <BSK_FormationFSW>` files.

The script is found in the folder ``xmera/examples/BskSim/scenarios`` and executed by using::

      python3 scenario_BasicOrbitFormation.py

The simulation mimics the basic simulation in the earlier tutorial in :ref:`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 :ref:`BSK_FormationFSW`.
However the flight software mode can also be set to "standby".

The simulation layout is shown in the following illustrations.

.. image:: /_images/static/test_scenario_BasicOrbitFormation.svg
   :align: center

.. image:: /_images/static/Simulation_container_layout_BasicOrbitFormation.svg
   :align: center


Configuring the scenario file
-----------------------------
The simulation layout is very similar to the one used for the :ref:`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 :ref:`BSK_FormationDynamics` and :ref:`BSK_FormationFSW` files.
These two files have been created for this specific formation flying implementation into Xmera.

After initializing the interfaces and making sure that the :ref:`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 :ref:`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.



BSK_FormationDynamics file description
--------------------------------------
Looking at the :ref:`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 Xmera 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 :ref:`BSK_FormationDynamics.py <BSK_FormationDynamics>` file.

BSK_FormationFsw file description
---------------------------------
The setup of the FSW file (:ref:`BSK_FormationFSW.py <BSK_FormationFSW>`) 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 :ref:`BSK_FormationFsw` to -1. Otherwise
the orientation and rates of both spacecraft will not converge!


Illustration of Simulation Results
----------------------------------

::

    showPlots = True

.. image:: /_images/Scenarios/scenario_BasicOrbitFormation_attitude_error_chief.svg
   :align: center

.. image:: /_images/Scenarios/scenario_BasicOrbitFormation_rate_error_chief.svg
   :align: center

.. image:: /_images/Scenarios/scenario_BasicOrbitFormation_attitude_error_deputy.svg
   :align: center

.. image:: /_images/Scenarios/scenario_BasicOrbitFormation_rate_error_deputy.svg
   :align: center

.. image:: /_images/Scenarios/scenario_BasicOrbitFormation_orbits.svg
   :align: center