.. _scenarioSmallBodyNav:

scenarioSmallBodyNav
====================

Overview
--------

This scenario demonstrates how to use the ``smallBodyNavEKF()`` for state estimation about a small body. In this example,
Bennu is used. However, any small body could be selected as long as the appropriate gravitational parameter is set.

In this scenario, :ref:`simpleNav` and :ref:`planetEphemeris` provide measurements to the EKF in the form of :ref:`navTransMsgPayload`,
:ref:`navAttMsgPayload`, and :ref:`ephemerisMsgPayload` input messages. The EKF takes in these measurements at each timestep
and updates the state estimate, outputting this state estimate in its own standalone message, a :ref:`smallBodyNavMsgPayload`,
as well as navigation output messages - :ref:`navTransMsgPayload` and :ref:`ephemerisMsgPayload`.

.. note:: This module is only meant to provide a somewhat representative autonomous small body proximity operations navigation solution for POMDP solvers. Therefore, realistic measurement modules do not exist to support this module, and not every source of uncertainty in the problem is an estimated parameter.

.. attention::

    To see the asteroid Bennu in Vizard the asteroid asset bundle must be installed.  See
    the Vizard `Download <https://github.com/lasp/xmera/Vizard/VizardDownload.html>`__ web page.

The relative position estimate and the estimation error and covariance may be found in the plots below.

.. image:: /_images/Scenarios/scenarioSmallBodyNav1.svg
   :align: center

.. image:: /_images/Scenarios/scenarioSmallBodyNav3.svg
   :align: center

Likewise, the relative velocity estimate and the estimation error and covariance may be found in the plots below.

.. image:: /_images/Scenarios/scenarioSmallBodyNav2.svg
   :align: center

.. image:: /_images/Scenarios/scenarioSmallBodyNav4.svg
   :align: center

In the next four plots, the attitude and rate estimates and error plots of the small body frame with respect to the
 inertial frame are displayed.

.. image:: /_images/Scenarios/scenarioSmallBodyNav5.svg
   :align: center

.. image:: /_images/Scenarios/scenarioSmallBodyNav6.svg
   :align: center

.. image:: /_images/Scenarios/scenarioSmallBodyNav7.svg
   :align: center

.. image:: /_images/Scenarios/scenarioSmallBodyNav8.svg
   :align: center

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

      python3 scenarioSmallBodyNav.py