Module: gravityEffector

Executive Summary

Abstract class that is used to implement an effector impacting a GRAVITY body that does not itself maintain a state or represent a changing component of the body (for example: gravity, thrusters, solar radiation pressure, etc.)

The module PDF Description contains further information on this module’s function, how to run it, as well as testing.

Message Connection Descriptions

The following table lists all the module input and output messages. The module msg connection is set by the user from python. The msg type contains a link to the message structure definition, while the description provides information on what this message is used for.

Module I/O Messages
Msg Variable Name	Msg Type	Description
centralBodyOutMsg	SpicePlanetStateMsgPayload	central planet body state output message

The gravity effector contains a list of GravBodyData objects which contain the planet mass and size properties etc. The following table lists the Spice planet ephemeris input message that can be connected to a GravBodyData object. If no message is connected, then the planet has zero position and orientation information by default.

Module I/O Messages
Msg Variable Name	Msg Type	Description
planetBodyInMsg	SpicePlanetStateMsgPayload	planet spice ephemerisis input message

User Guide

The user must provide a list of GravBodyData objects to the spacecraft using:

scObject.gravField.gravBodies = spacecraft.GravBodyVector(gravBodyList)

Each gravity body data object can be created using:

earth = gravityEffector.GravBodyData()
earth.planetName = 'earth_planet_data'
earth.mu = 0.3986004415E+15  # meters^3/s^2
earth.radEquator = 6378136.6  # meters
earth.isCentralBody = False

Note that the simIncludeGradBody.py helper file contains a gravity body factor class to facilitate setting up gravity bodies.

class GravBodyData

#include <gravityEffector.h>

Container for gravitational body data

This class is designed to hold all of the information for a gravity body. The nominal use-case has it initialized at the python level and attached to dynamics using the AddGravityBody method.

Public Functions

GravBodyData(): Initializes localPlanet to zero except for localPlanet.J20002Pfix, which is initialized to the identity matrix. Additionally, initializes gravityModel to a PointMassGravityModel.

void initBody(int64_t moduleID): Initializes the gravityModel, logging and raising an error if the initialization fails.

Eigen::Vector3d computeGravityInertial(Eigen::Vector3d r_I, uint64_t simTimeNanos)

Computes the gravitational acceleration

Parameters:

r_I – inertial position vector
simTimeNanos – simulation time (ns)

void loadEphemeris(): Read the ephemeris data from planetBodyInMsg if it’s linked. Otherwise, zeros this->localPlanet.

void registerProperties(DynParamManager &statesIn)

Creates the following properies in the given statesIn object.

- [planetName].r_PN_N
- [planetName].v_PN_N
- [planetName].mu
- [planetName].J20002Pfix
- [planetName].J20002Pfix_dot

vr_PN_N,v_PN_N, andmuare initialized to zero, whileJ20002PfixandJ20002Pfix_dot are initialized to the values stored inthis->localPlanet. This usually means that J20002Pfixis initialized to the identity matrix andJ20002Pfix_dot` to zero.

Public Members

bool isCentralBody = false: Flag indicating that object is center

std::shared_ptr<GravityModel> gravityModel: Model used to compute the gravity of the object

double mu = 0: [m3/s^2] central body gravitational param

double radEquator = 0: [m] Equatorial radius for the body

double radiusRatio = 1: [] ratio of polar over equatorial radius

std::string planetName = "": Gravitational body name, this is used as the Spice name if spiceInterface is used

std::string displayName = "": This is the name that is displayed in Vizard. If not set, Vizard shows planetName

std::string modelDictionaryKey = "": “” will result in using the current default for the celestial body’s given name, otherwise key will be matched if possible to available model in internal model dictionary

ReadFunctor<SpicePlanetStateMsgPayload> planetBodyInMsg: planet spice ephemeris input message

SpicePlanetStateMsgPayload localPlanet: [-] Class storage of ephemeris info from scheduled portion

BSKLogger bskLogger: — BSK Logging

Private Members

Eigen::MatrixXd *r_PN_N: [m] (state engine property) planet inertial position vector

Eigen::MatrixXd *v_PN_N: [m/s] (state engine property) planet inertial velocity vector

Eigen::MatrixXd *muPlanet: [m/s] (state engine property) planet inertial velocity vector

Eigen::MatrixXd *J20002Pfix: [m/s] (state engine property) planet attitude [PN]

Eigen::MatrixXd *J20002Pfix_dot: [m/s] (state engine property) planet attitude rate [PN_dot]

uint64_t timeWritten = 0: [ns] time the input planet state message was written

Friends

friend class GravityEffector

class GravityEffector : public SysModel 

#include <gravityEffector.h>

gravity effector class

Public Functions

void Reset(uint64_t currentSimNanos): Initializes every GravBodyData associated with this GravityEffector

void UpdateState(uint64_t currentSimNanos): Updates the central body, loads the ephemeris data for every GravBodyData, and writes the output messages.

void linkInStates(DynParamManager &statesIn): Links the correlation between times property

void registerProperties(DynParamManager &statesIn): Registers the gravity, inertial position, and inertial velocity properties. It also calls registerProperties for every associated GravBodyData.

void computeGravityField(Eigen::Vector3d r_cF_N, Eigen::Vector3d rDot_cF_N)

Calculate gravitational acceleration of s/c wrt inertial (no central body) or wrt central body

Parameters:

r_cF_N – is position of center of mass of s/c wrt frame
rDot_cF_N – is the derivative of above

void updateInertialPosAndVel(Eigen::Vector3d r_BF_N, Eigen::Vector3d rDot_BF_N): Updates the inertial position and velocity properties

void updateEnergyContributions(Eigen::Vector3d r_CN_N, double &orbPotEnergyContr): Computes the Potential Energy Contributions from every associated GravBodyData

void setGravBodies(std::vector<std::shared_ptr<GravBodyData>> gravBodies): Sets the GravBodyData associated with this effector

void addGravBody(std::shared_ptr<GravBodyData> gravBody): Adds a GravBodyData associated with this effector

void prependSpacecraftNameToStates()

Called to modify property names to prepend them by the string stored in nameOfSpacecraftAttachedTo

This can be used to make property names unique between different GravityEffector in a simulation with multiple dynamic objects.

Public Members

std::vector<std::shared_ptr<GravBodyData>> gravBodies: [-] Vector of bodies we feel gravity from

std::shared_ptr<GravBodyData> centralBody: Central body

std::string vehicleGravityPropName = "g_N": [-] Name of the vehicle gravity acceleration property

std::string systemTimeCorrPropName = "systemTime": [-] Name of the correlation between times

std::string inertialPositionPropName = "r_BN_N": [-] Name of the inertial position property

std::string inertialVelocityPropName = "v_BN_N": [-] Name of the inertial velocity property

std::string nameOfSpacecraftAttachedTo = "": [-] Name of the s/c this gravity model is attached to

Message<SpicePlanetStateMsgPayload> centralBodyOutMsg: central planet body state output message

BSKLogger bskLogger: — BSK Logging

Private Functions

Eigen::Vector3d getEulerSteppedGravBodyPosition(std::shared_ptr<GravBodyData> bodyData)

Compute planet position with Euler integration

Parameters:: bodyData – planet data

void writeOutputMessages(uint64_t currentSimNanos): Writes to centralBodyOutMsg if it is linked and there is a central body

Private Members

Eigen::MatrixXd *gravProperty: [-] g_N property for output

Eigen::MatrixXd *timeCorr: [-] Time correlation property

Eigen::MatrixXd *inertialPositionProperty: [m] r_N inertial position relative to system spice zeroBase/refBase coordinate frame, property for output.

Eigen::MatrixXd *inertialVelocityProperty: [m/s] v_N inertial velocity relative to system spice zeroBase/refBase coordinate frame, property for output.