Creating Stand-Alone Messages

Source Code

The python code shown below can be downloaded here.

The prior example showed how to connect messages that are embedded within the Basilisk modules. However, there are times where you might need to create a stand-alone copy of such a message. Some flight algorithm modules require the input of a stand-alone message that provides information about the spacecraft mass and inertia properties, or thruster or reaction wheel configuration information. For example, the module unit test ideally just runs the module being tested. Any input messages that this module needs should be created as stand-alone messages. This avoids the unit test script depending on other modules output messages, but makes the module test function on its own.

This tutorial shows you how to create a stand-alone message and connect the Module: cppModuleTemplate input message to it. The syntax is identical to connect a stand-alone message to a C module. Further, this example illustrates how the simulation can be started and stopped multiple times while the message or module variables are changed between runs.

To create a stand-alone message, the message payload (i.e. data) container must be created first. Let us assume the message is of type someMsg. The corresponding payload is called someMsgPayload. Thus, the payload container is created using:

msgData = messaging.someMsgPayload()

Essentially this is a python instance of the message structure definition found in architecture/msgPayloadDefC/SomeMsg.h. The content of the message payload is zero’d on creating it. If there is a variable in the structure that we want to change, this is done simply with:

msgData.variable = .....

Next, a message object is created and the message data is written to it. The message object is created using:

msg = messaging.someMsg()

The payload is written to the message using:

msg.write(msgData)

These steps can also be combined into a single line using:

msg = messaging.someMsg().write(msgData)

The simulation code below creates a stand-alone message that is then connected to the module input message.

import sys

import matplotlib.pyplot as plt
from Basilisk.architecture import messaging
from Basilisk.moduleTemplates import cppModuleTemplate
from Basilisk.utilities import SimulationBaseClass
from Basilisk.utilities import macros
from Basilisk.utilities import unitTestSupport


def run():
    """
    Illustration of creating stand-alone messages
    """

    #  Create a sim module as an empty container
    scSim = SimulationBaseClass.SimBaseClass()

    #  create the simulation process
    dynProcess = scSim.CreateNewProcess("dynamicsProcess")

    # create the dynamics task and specify the integration update time
    dynProcess.addTask(scSim.CreateNewTask("dynamicsTask", macros.sec2nano(1.)))

    # create modules
    mod1 = cppModuleTemplate.CppModuleTemplate()
    mod1.ModelTag = "cppModule1"
    scSim.AddModelToTask("dynamicsTask", mod1)

    # create stand-alone input message
    msgData = messaging.CModuleTemplateMsgPayload()
    msgData.dataVector = [1., 2., 3.]
    msg = messaging.CModuleTemplateMsg().write(msgData)

    # connect to stand-alone msg
    mod1.dataInMsg.subscribeTo(msg)

    # setup message recording
    msgRec = mod1.dataOutMsg.recorder()
    scSim.AddModelToTask("dynamicsTask", msgRec)

    #  initialize Simulation:
    scSim.InitializeSimulation()

    #   configure a simulation stop time and execute the simulation run
    scSim.ConfigureStopTime(macros.sec2nano(10.0))
    scSim.ExecuteSimulation()

    # change input message and continue simulation
    msgData.dataVector = [-1., -2., -3.]
    msg.write(msgData)
    scSim.ConfigureStopTime(macros.sec2nano(20.0))
    scSim.ExecuteSimulation()

    # plot recorded data
    plt.close("all")
    figureList = {}
    plt.figure(1)
    for idx in range(3):
        plt.plot(msgRec.times() * macros.NANO2SEC, msgRec.dataVector[:, idx],
                 color=unitTestSupport.getLineColor(idx, 3),
                 label='$r_{BN,' + str(idx) + '}$')
    plt.legend(loc='lower right')
    plt.xlabel('Time [sec]')
    plt.ylabel('Module Data [units]')
    figureList["bsk-5"] = plt.figure(1)
    if "pytest" not in sys.modules:
        plt.show()
    plt.close("all")

    return figureList


if __name__ == "__main__":
    run()

After the simulation runs for 10s, the stand-alone message data is changed and written into the message object. Note that the stand-alone message object itself doesn’t have to be re-created as this is still working and connected to the desired modules. Rather, we only have to update the content of the message.

Next, the simulation stop time is extended for an additional 10s to 20s total and the simulation is executed again. The resulting plot of the module output message is shown below.