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Sgp4Propagator Class

Propagates an orbit using the NORAD SGP4/SDP4 model as defined by the Center for Space Standards and Innovation (CSSI). More information and the algorithm's code can be found at www.centerforspace.com.

Note that the SGP4 propagator performs its propagation using the TAI time standard. If the user asks for a propagation over an interval and timestep specified in UTC, the system interprets this to mean that the reported values should be in UTC. However, in order to compute the correct timespans (potentially propagating through leap seconds), the system will convert to TAI in order to compare times. Since the length of a second is different in various time standards, the stepsize is measured in the time standard the user specifies even though the comparisons and propagation occurs in TAI. If the length of a given time step is important, this may be a necessary calculation. However, the performance of the SGP4 propagation can be improved by specifying input times in TAI.

The acceleration returned from the SGP4Propagator is identical to the J4Propagator, which is an approximation used to prevent code using this propagator from being unable to provide analysis in the absence of acceleration. In STK Components, there are a number of places that expect or require acceleration information from propagators. In order to support this, the Sgp4Propagator will return an acceleration when asked. However, because the SGP4 theory does not explicitly contain a representation of the acceleration and the velocity of the SGP4 theory does not correspond exactly with the position, the acceleration returned for SGP4 is an approximation. Compared with numerical differentiation of the position, the velocity returned from the SGP4 theory can have a discrepancy anywhere from a few millimeters to several meters. Since the acceleration perturbations from drag and third body effects are several orders of magnitude less than gravitational effects, the J4 perturbations are used here to approximate the acceleration for SGP4.

Inheritance Hierarchy

Namespace:  AGI.Foundation.Propagators
Assembly:  AGI.Foundation.Models (in AGI.Foundation.Models.dll) Version: 19.5.402.0 (19.5.402.0)
Syntax
public class Sgp4Propagator : CartesianOnePointPropagator

The Sgp4Propagator type exposes the following members.

Constructors
  NameDescription
Public methodSgp4Propagator
Initializes a new instance with default values.
Public methodSgp4Propagator(Sgp4Elements)
Constructs a new instance of an Sgp4Propagator using the specified initial conditions.
Public methodSgp4Propagator(IEnumerableSgp4Elements)
Constructs a new instance of an Sgp4Propagator using the specified initial conditions.
Protected methodSgp4Propagator(Sgp4Propagator, CopyContext)
Initializes a new instance as a copy of an existing instance.
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Properties
  NameDescription
Public propertyElementSets
Gets the set of elements from which to propagate.
Public propertyInitialConditions
Gets or sets the first set of elements in the ElementSets.
Public propertyIsFrozen
Gets a value indicating whether this object is frozen. A frozen object cannot be modified and an ObjectFrozenException will be thrown if an attempt is made to do so.
(Inherited from DefinitionalObject.)
Public propertyReferenceFrame
Gets True Equator, Mean Equinox (TEME) reference frame, the coordinate system in which SGP4 produces ephemeris.
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Methods
  NameDescription
Protected methodCheckForSameDefinition(CartesianOnePointPropagator)
Checks to determine if another instance has the same definition as this instance and returns 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 for all derived-class instances. Derived classes should check the type of other to preserve the symmetric nature of IsSameDefinition(Object).
(Overrides CartesianOnePointPropagatorCheckForSameDefinition(CartesianOnePointPropagator).)
Protected methodCheckForSameDefinition(DefinitionalObject)
Checks to determine if another instance has the same definition as this instance and returns 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 for all derived-class instances. Derived classes should check the type of other to preserve the symmetric nature of IsSameDefinition(Object).
(Inherited from CartesianOnePointPropagator.)
Protected methodCheckForSameDefinition(Sgp4Propagator)
Checks to determine if another instance has the same definition as this instance and returns 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 for all derived-class instances. Derived classes should check the type of other to preserve the symmetric nature of IsSameDefinition(Object).
Public methodClone
Clones this object using the specified context.
(Overrides DefinitionalObjectClone(CopyContext).)
Protected methodComputeCurrentDefinitionHashCode
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 CheckForSameDefinition(DefinitionalObject) method.
(Overrides CartesianOnePointPropagatorComputeCurrentDefinitionHashCode.)
Public methodCreatePoint
Creates a Point that represents the motion produced by this propagator.
(Overrides CartesianOnePointPropagatorCreatePoint.)
Public methodEnumerateDependencies
Enumerates the dependencies of this object by calling EnumerateT(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.
(Overrides DefinitionalObjectEnumerateDependencies(DependencyEnumerator).)
Public methodEquals
Determines whether the specified object is equal to the current object.
(Inherited from Object.)
Public methodStatic memberEstimateElements(DateMotionCollectionCartesian, Sgp4PropagatorSgp4EstimationInput)
Produces a batch least squares fit for a given set of trajectory observations (states). This overload will take the first position and velocity after the RequestedEstimationEpoch in the ephemerisInTEME and use the MeanElementsAtEpoch(JulianDate, Cartesian, Cartesian) to produce an initial guess.
Public methodStatic memberEstimateElements(DateMotionCollectionCartesian, Cartesian, Cartesian, Sgp4PropagatorSgp4EstimationInput)
Produces a batch least squares fit for a given set of trajectory observations (states). The specified position and velocity provide the values to use as the state expected at the RequestedEstimationEpoch in the configuration. The estimation takes this state and uses MeanElementsAtEpoch(JulianDate, Cartesian, Cartesian) to produce an initial guess for the least squares.
Protected methodFinalize
Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.
(Inherited from Object.)
Public methodFreeze
Freezes this object. Further attempts to modify it will result in an ObjectFrozenException.
(Inherited from DefinitionalObject.)
Protected methodFreezeAggregatedObjects
Called by Freeze to also freeze any objects that are considered to be a part of this object. Derived classes which contain additional aggregated objects MUST override this method, call the base implementation, and freeze aggregated objects introduced by the derived class. The objects that need to be frozen in this method are frequently created in this object's constructor and are not settable via properties.
(Overrides DefinitionalObjectFreezeAggregatedObjects.)
Public methodGetDefinitionHashCode
Gets a hash code representing the definition of this object.
(Inherited from DefinitionalObject.)
Public methodGetEvaluator
Gets an evaluator that can propagate at individual dates.

This evaluator propagates the orbit state. The result of evaluating will be a MotionT corresponding to the orbital state at the given JulianDate expressed in the propagator's ReferenceFrame.

Note: when evaluating with this evaluator, it may be more efficient to specify times using an arithmetically safe TimeStandard to avoid the need to convert in order to perform the propagation. The length of a time step may be different in different TimeStandards. So be careful when specifying times.

(Inherited from CartesianOnePointPropagator.)
Public methodGetEvaluator(EvaluatorGroup)
Gets an evaluator that can propagate at individual dates.

This evaluator propagates the orbit state from InitialConditions around the Earth. The result of evaluating will be the Motion<Cartesian> corresponding to the orbital position and velocity at the given JulianDate expressed in the EarthCentralBody's TrueEquatorMeanEquinoxFrame.

*Note* When evaluating with this evaluator, it will be more efficient to specify times using the TAI time standard to avoid the need to convert in order to perform the propagation. The length of a time step may be different in different TimeStandards. So be careful when specifying times.

(Overrides CartesianOnePointPropagatorGetEvaluator(EvaluatorGroup).)
Public methodGetHashCode
Serves as the default hash function.
(Inherited from Object.)
Protected methodGetMotionReferenceFrame
Gets the reference frame in which the motion is defined.
(Overrides CartesianOnePointPropagatorGetMotionReferenceFrame.)
Public methodGetType
Gets the Type of the current instance.
(Inherited from Object.)
Public methodIsSameDefinition
Determines if this object has the same definition as another object.
(Inherited from DefinitionalObject.)
Public methodStatic memberMeanElementsAtEpoch(JulianDate, Cartesian, Cartesian)
Produces an element set from a given Cartesian state at a given time. Since SGP4 uses perturbations, the classical orbital elements produced by ModifiedKeplerianElements may not correspond to the specified position and velocity when propagated as SGP4 elements. This method performs a simple iteration to correct the classical elements so that the resulting SGP4 elements will represent the specified position and velocity at the given time as accurately as possible. This will assume that the BStar value is zero.
Public methodStatic memberMeanElementsAtEpoch(JulianDate, Cartesian, Cartesian, Int32)
Produces an element set from a given Cartesian state at a given time. Since SGP4 uses perturbations, the classical orbital elements produced by ModifiedKeplerianElements may not correspond to the specified position and velocity when propagated as SGP4 elements. This method performs a simple iteration to correct the classical elements so that the resulting SGP4 elements will represent the specified position and velocity at the given time as accurately as possible. This will assume that the BStar value is zero.
Public methodStatic memberMeanElementsAtEpoch(JulianDate, Cartesian, Cartesian, Int32, Double)
Produces an element set from a given Cartesian state at a given time. Since SGP4 uses perturbations, the classical orbital elements produced by ModifiedKeplerianElements may not correspond to the specified position and velocity when propagated as SGP4 elements. This method performs a simple iteration to correct the classical elements so that the resulting SGP4 elements will represent the specified position and velocity at the given time as accurately as possible. This will use the specified value for BStar in the resulting element set. Also, note that this method may have difficulty converging for perfectly circular orbits.
Protected methodMemberwiseClone
Creates a shallow copy of the current Object.
(Inherited from Object.)
Public methodPropagate(JulianDate, JulianDate, Duration, Int32, ReferenceFrame)
Calculates position (and zero or more derivatives) for regular time steps over an interval.

Note: when propagating, it may be more efficient to specify times using an arithmetically safe TimeStandard to avoid the need to convert in order to perform the propagation. The length of a time step may be different in different TimeStandards. So be careful when specifying times.

(Inherited from CartesianOnePointPropagator.)
Public methodPropagate(JulianDate, JulianDate, Duration, Int32, ReferenceFrame, ITrackCalculationProgress)
Calculates position (and zero or more derivatives) for regular time steps over an interval.

Note: when evaluating with this evaluator, it may be more efficient to specify times using an arithmetically safe TimeStandard to avoid the need to convert in order to perform the propagation. The length of a time step may be different in different TimeStandards. So be careful when specifying times.

(Inherited from CartesianOnePointPropagator.)
Protected methodThrowIfFrozen
Throws ObjectFrozenException if this object IsFrozen. This method should be called from any method or property that modifies this object.
(Inherited from DefinitionalObject.)
Public methodToString
Returns a string that represents the current object.
(Inherited from Object.)
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Examples
An example of creating an Sgp4Propagator from a Two Line Element Set:
C#
string tleString =
    "1 20724U 90068A   02173.73395695 -.00000086  00000-0  00000-0 0  1771\n" +
    "2 20724  56.1487  21.0845 0183651 226.6216 131.8780  2.00562381 85500\n";
TwoLineElementSet tle = new TwoLineElementSet(tleString);
Sgp4Propagator propagator = new Sgp4Propagator(tle);
See Also