agi.foundation.celestial

(agi.foundation.orbitpropagation-2025r1.jar)

Class TwoBodyGravity

java.lang.Object

agi.foundation.infrastructure.DefinitionalObject

agi.foundation.celestial.ForceModel

agi.foundation.celestial.TwoBodyGravity

All Implemented Interfaces:

ICloneWithContext, IEnumerateDependencies, IEquatableDefinition, IFreezable, IPartialDifferentiable

public class TwoBodyGravity
extends ForceModel
implements IPartialDifferentiable

Represents the acceleration from a simple two body gravity model.

Constructor Summary

Constructors

Modifier	Constructor and Description
	TwoBodyGravity() Initializes a new instance.
	TwoBodyGravity(Point targetPoint) Initializes a new instance based on the given target point.
	TwoBodyGravity(Point targetPoint, CentralBody centralBody, double gravitationalParameter) Initializes a new instance based on the given target point and central body.
protected	TwoBodyGravity(TwoBodyGravity existingInstance, CopyContext context) Initializes a new instance as a copy of an existing instance.

Method Summary

Modifier and Type	Method and Description
void	buildForceEvaluator(ResultantForceBuilder builder, EvaluatorGroup group) Build lists of principal and perturbation forces based on this model in order to create an overall resultant force.
static Cartesian	calculateAcceleration(Cartesian position, double gravitationalParameter) Calculates the acceleration at a position caused by two-body gravity.
protected boolean	checkForSameDefinition(ForceModel other) Checks to determine if another instance has the same definition as this instance and returns true if it does.
protected boolean	checkForSameDefinition(TwoBodyGravity other) Checks to determine if another instance has the same definition as this instance and returns true if it does.
Object	clone(CopyContext context) Clones this object using the specified context.
protected int	computeCurrentDefinitionHashCode() Computes a hash code based on the current properties of this object.
void	enumerateDependencies(DependencyEnumerator enumerator) Enumerates the dependencies of this object by calling DependencyEnumerator#enumerate(T) for each object that this object directly depends upon.
CentralBody	getCentralBody() Gets the body which creates the gravity and defines the inertial frame of propagation.
int	getDimension() Gets the dimension of the values produced by the object.
ForceEvaluator	getForceEvaluator(EvaluatorGroup group) Gets an instance of an evaluator that can compute the force.
double	getGravitationalParameter() Gets the gravitational parameter to use when computing gravity, in meters cubed per second squared.
PartialDerivativesEvaluator	getPartialDerivativesEvaluator(List<IPartialDifferentiable> independentVariables, EvaluatorGroup group) Gets an evaluator that calculates the partial derivatives of the two body gravity acceleration with respect to any of the independentVariables parameters that apply.
Point	getTargetPoint() Gets the target point at which to measure the gravitational acceleration.
void	setCentralBody(CentralBody value) Sets the body which creates the gravity and defines the inertial frame of propagation.
void	setGravitationalParameter(double value) Sets the gravitational parameter to use when computing gravity, in meters cubed per second squared.
void	setTargetPoint(Point value) Sets the target point at which to measure the gravitational acceleration.

Methods inherited from class agi.foundation.celestial.ForceModel

checkForSameDefinition, freezeAggregatedObjects, getForceEvaluator, getForceVector, getKind, getRole

Methods inherited from class agi.foundation.infrastructure.DefinitionalObject

areSameDefinition, areSameDefinition, areSameDefinition, areSameDefinition, areSameDefinition, collectionItemsAreSameDefinition, collectionItemsAreSameDefinition, collectionItemsAreSameDefinition, dictionaryItemsAreSameDefinition, freeze, getCollectionHashCode, getCollectionHashCode, getCollectionHashCode, getDefinitionHashCode, getDefinitionHashCode, getDefinitionHashCode, getDefinitionHashCode, getDefinitionHashCode, getDefinitionHashCode, getDictionaryHashCode, getIsFrozen, isSameDefinition, throwIfFrozen

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface agi.foundation.infrastructure.IEquatableDefinition

getDefinitionHashCode, isSameDefinition

Constructor Detail

TwoBodyGravity

public TwoBodyGravity()

Initializes a new instance. By default the CentralBody (get / set) is set to Earth from the CentralBodiesFacet and the GravitationalParameter (get / set) is set to WorldGeodeticSystem1984.GravitationalParameter.

TwoBodyGravity

public TwoBodyGravity(Point targetPoint)

Initializes a new instance based on the given target point. By default the CentralBody (get / set) is set to Earth from the CentralBodiesFacet and the GravitationalParameter (get / set) is set to WorldGeodeticSystem1984.GravitationalParameter.

Parameters:

targetPoint - The point at which to measure gravity.

TwoBodyGravity

public TwoBodyGravity(Point targetPoint,
                      CentralBody centralBody,
                      double gravitationalParameter)

Initializes a new instance based on the given target point and central body.

Parameters:

targetPoint - The point at which to measure gravity.

centralBody - The central body around which the point is defined.

gravitationalParameter - The gravitational parameter of the central body.

TwoBodyGravity

protected TwoBodyGravity(@Nonnull
                         TwoBodyGravity existingInstance,
                         @Nonnull
                         CopyContext context)

Initializes a new instance as a copy of an existing instance.

See ICloneWithContext.clone(CopyContext) for more information about how to implement this constructor in a derived class.

Parameters:

existingInstance - The existing instance to copy.

context - A CopyContext that controls the depth of the copy.

Throws:

ArgumentNullException - Thrown when existingInstance or context is null.

Method Detail

clone

public Object clone(CopyContext context)

Clones this object using the specified context.

This method should be implemented to call a copy constructor on the class of the object being cloned. The copy constructor should take the CopyContext as a parameter in addition to the existing instance to copy. The copy constructor should first call CopyContext.addObjectMapping(T, T) to identify the newly constructed instance as a copy of the existing instance. It should then copy all fields, using CopyContext.updateReference(T) to copy any reference fields.

A typical implementation of ICloneWithContext:

public static class MyClass implements ICloneWithContext {
    public MyClass(MyClass existingInstance, CopyContext context) {
        context.addObjectMapping(existingInstance, this);
        someReference = context.updateReference(existingInstance.someReference);
    }

    @Override
    public final Object clone(CopyContext context) {
        return new MyClass(this, context);
    }

    private Object someReference;
}

In general, all fields that are reference types should be copied with a call to CopyContext.updateReference(T). There are a couple of exceptions:

1. Fields that are aggregate parts of the object should always be copied. A referenced object is an aggregate if properties or methods on the object being copied can modify the externally-visible value of the referenced object.
2. If the semantics of the referenced object require that it have a single parent, owner, context, etc., and the object being copied is that parent, owner, or context, then the referenced object should always be copied.

If one of these exceptions applies, the CopyContext should be given an opportunity to update the reference before the reference is copied explicitly. Use CopyContext.updateReference(T) to update the reference. If CopyContext.updateReference(T) returns the original object, indicating that the context does not have a replacement registered, then copy the object manually by invoking a Clone method, a copy constructor, or by manually constructing a new instance and copying the values.

alwaysCopy = context.updateReference(existingInstance.alwaysCopy);
if (existingInstance.alwaysCopy != null && alwaysCopy == existingInstance.alwaysCopy) {
    alwaysCopy = (AlwaysCopy) existingInstance.alwaysCopy.clone(context);
}

If you are implementing an evaluator (a class that implements IEvaluator), the IEvaluator.updateEvaluatorReferences(agi.foundation.infrastructure.CopyContext) method shares some responsibilities with the copy context constructor. Code duplication can be avoided by doing the following:

1. For references to other evaluators ONLY, simply assign the reference in the constructor instead of calling CopyContext.updateReference(T). You should still call CopyContext.updateReference(T) on any references to non-evaluators.
2. Call IEvaluator.updateEvaluatorReferences(agi.foundation.infrastructure.CopyContext) as the last line in the constructor and pass it the same CopyContext passed to the constructor.
3. Implement IEvaluator.updateEvaluatorReferences(agi.foundation.infrastructure.CopyContext) as normal. See the reference documentation for IEvaluator.updateEvaluatorReferences(agi.foundation.infrastructure.CopyContext) for more information on implementing that method.

public MyClass(MyClass existingInstance, CopyContext context) {
    super(existingInstance, context);
    someReference = context.updateReference(existingInstance.someReference);
    evaluatorReference = existingInstance.evaluatorReference;
    updateEvaluatorReferences(context);
}

@Override
public void updateEvaluatorReferences(CopyContext context) {
    evaluatorReference = context.updateReference(evaluatorReference);
}

@Override
public Object clone(CopyContext context) {
    return new MyClass(this, context);
}

private Object someReference;
private IEvaluator evaluatorReference;

Specified by:

clone in interface ICloneWithContext

Specified by:

clone in class DefinitionalObject

Parameters:

context - The context to use to perform the copy.

Returns:

A new instance which is a copy of this object.

checkForSameDefinition

protected final boolean checkForSameDefinition(ForceModel other)

Checks to determine if another instance has the same definition as this instance and returns true if it does. Derived classes MUST override this method and check all new fields introduced by the derived class for definitional equivalence. It is NOT necessary to check base class fields because the base class will already have done that. When overriding this method, you should NOT call the base implementation because it will return false for all derived-class instances. Derived classes should check the type of other to preserve the symmetric nature of IEquatableDefinition.isSameDefinition(java.lang.Object).

Specified by:

checkForSameDefinition in class ForceModel

Parameters:

other - The other instance to compare to this one.

Returns:

true if the two objects are defined equivalently; otherwise false.

checkForSameDefinition

protected boolean checkForSameDefinition(TwoBodyGravity other)

Checks to determine if another instance has the same definition as this instance and returns true if it does. Derived classes MUST override this method and check all new fields introduced by the derived class for definitional equivalence. It is NOT necessary to check base class fields because the base class will already have done that. When overriding this method, you should NOT call the base implementation because it will return false for all derived-class instances. Derived classes should check the type of other to preserve the symmetric nature of IEquatableDefinition.isSameDefinition(java.lang.Object).

Parameters:

other - The other instance to compare to this one.

Returns:

true if the two objects are defined equivalently; otherwise false.

computeCurrentDefinitionHashCode

protected int computeCurrentDefinitionHashCode()

Computes a hash code based on the current properties of this object. Derived classes MUST override this method and compute a hash code that combines: a unique hash code seed, the base implementation result, and the hash codes of all new fields introduced by the derived class which are used in the TwoBodyGravity.checkForSameDefinition(agi.foundation.celestial.ForceModel) method.

Overrides:

computeCurrentDefinitionHashCode in class ForceModel

Returns:

The computed hash code.

enumerateDependencies

public void enumerateDependencies(DependencyEnumerator enumerator)

Enumerates the dependencies of this object by calling DependencyEnumerator#enumerate(T) for each object that this object directly depends upon. Derived classes which contain additional dependencies MUST override this method, call the base implementation, and enumerate dependencies introduced by the derived class.

Specified by:

enumerateDependencies in interface IEnumerateDependencies

Overrides:

enumerateDependencies in class ForceModel

Parameters:

enumerator - The enumerator that is informed of the dependencies of this object.

getTargetPoint

public final Point getTargetPoint()

Gets the target point at which to measure the gravitational acceleration.

setTargetPoint

public final void setTargetPoint(Point value)

Sets the target point at which to measure the gravitational acceleration.

getCentralBody

public final CentralBody getCentralBody()

Gets the body which creates the gravity and defines the inertial frame of propagation.

setCentralBody

public final void setCentralBody(CentralBody value)

Sets the body which creates the gravity and defines the inertial frame of propagation.

getGravitationalParameter

public final double getGravitationalParameter()

Gets the gravitational parameter to use when computing gravity, in meters cubed per second squared.

setGravitationalParameter

public final void setGravitationalParameter(double value)

Sets the gravitational parameter to use when computing gravity, in meters cubed per second squared.

getDimension

public final int getDimension()

Gets the dimension of the values produced by the object. For example, Vectors would have a dimension of three, and Scalars of one. A PartialDerivativesEvaluator created by this type will have a RowDimension (get) equal to this property, and a ColumnDimension (get) equal to the summation of the dimensions of the independent variables that this object is dependent on.

Specified by:

getDimension in interface IPartialDifferentiable

calculateAcceleration

@Nonnull
public static Cartesian calculateAcceleration(@Nonnull
                                                        Cartesian position,
                                                        double gravitationalParameter)

Calculates the acceleration at a position caused by two-body gravity. This low-level call is provided for convenience in simple use cases, in general you should produce a ForceEvaluator from this class that will model gravitational acceleration on an object over time.

Parameters:

position - The position of the target object.

gravitationalParameter - The gravitational parameter of the body being orbited.

Returns:

The gravitational acceleration.

getForceEvaluator

public ForceEvaluator getForceEvaluator(EvaluatorGroup group)

Gets an instance of an evaluator that can compute the force.

Overrides:

getForceEvaluator in class ForceModel

Parameters:

group - The group in which to create the evaluator and its dependents.

Returns:

The evaluator that computes the force.

Throws:

ArgumentNullException - Thrown when group is null.

buildForceEvaluator

public void buildForceEvaluator(ResultantForceBuilder builder,
                                EvaluatorGroup group)

Build lists of principal and perturbation forces based on this model in order to create an overall resultant force.

Specified by:

buildForceEvaluator in class ForceModel

Parameters:

builder - The builder object containing the lists and additional information on the Axes in which the overall resultant is defined.

group - The evaluator group in which to create evaluators.

getPartialDerivativesEvaluator

public final PartialDerivativesEvaluator getPartialDerivativesEvaluator(List<IPartialDifferentiable> independentVariables,
                                                                        EvaluatorGroup group)

Gets an evaluator that calculates the partial derivatives of the two body gravity acceleration with respect to any of the independentVariables parameters that apply.

Specified by:

getPartialDerivativesEvaluator in interface IPartialDifferentiable

Parameters:

independentVariables - The list of IPartialDifferentiable representing the independent variables that the created partial derivative evaluator calculates partials with respect to.

group - The evaluator group to contain the evaluator.

Returns:

The PartialDerivativesEvaluator.