Relative Motion

The relative ephemeris of the assigned object with respect to the primary object, expressed in Cartesian components, as a function of time.

The relative ephemeris is computed with respect to two rotating frames, the RIC (Radial, In-Track, Cross-Track) frame and the NTC (Normal, Tangential, and Cross-Track) frames, that are defined using the primary's ephemeris.

Cross-track refers to the direction perpendicular to the position and inertial velocity; in-track refers to the direction perpendicular to both the radial and cross-track (positive in the direction of motion); tangential refers to the direction along the velocity vector; and normal refers to the direction perpendicular to the velocity and cross-track directions (positive outward along radial).

If the ephemeris times of the assigned object do not match those of the primary object, then the ephemeris of the assigned object is interpolated to the ephemeris times of the primary object. This can lead to nonzero values reported between primary and assigned objects that should be at the same position.

Available for these objects: LaunchVehicle, Missile, Satellite

Type: Time-varying data.

Availability: Reports | Graphs | Dynamic Displays | Strip Charts

Pre-data required: "<TruncObjectPath>" - e.g. "Satellite/Sat1"

Data Provider Elements

Name	Dimension	Type	Description
Time	Date	Real Number or Text	Time.
Radial	Distance	Real Number	The radial component of the relative position vector.
In-Track	Distance	Real Number	The in-track component of the relative position vector.
Cross-Track	Distance	Real Number	The cross-track component of the relative position vector.
Range	Distance	Real Number	The range (i.e., distance between the primary and secondary object) at the given time.
Radial Rate	Rate	Real Number	The radial component of the relative velocity vector as observed in the RIC rotating frame.
In-Track Rate	Rate	Real Number	The in-track component of the relative velocity vector as observed in the RIC rotating frame.
Cross-Track Rate	Rate	Real Number	The cross-track component of the relative velocity vector as observed in the RIC rotating frame.
Range Rate	Rate	Real Number	The rate of change of the magnitude of the relative position vector. This is computed using the analytical formula dot_product(relative_position_vector, relative_velocity_vector) / magnitude(relative_position_vector). This is an exact formula when the velocity is the derivative of position. Some propagators (e.g., SGP4, J2Perturbation, J4Perturbation), however, produce ephemerides where the velocity is not exactly the derivative of position so that this formula does not compute the derivative of range exactly in such cases.
Range Rate Numerical	Rate	Real Number	The rate of change of the magnitude of the relative position vector. This is computed using numerical differentiation of the range. The central difference will be used for interior points; forward or backward differences will be used at ephemeris start and stop times.
Range Rate Dot	Acceleration	Real Number	The time derivative of the range rate, as evaluated using the analytical formula.
Range Rate Dot Numerical	Acceleration	Real Number	The time derivative of the range rate, as evaluated using numerical differentiation.
Tangential	Distance	Real Number	The tangential component of the relative position vector.
Tangential Rate	Rate	Real Number	The tangential component of the relative velocity vector as observed in the NTC rotating frame.
Normal	Distance	Real Number	The normal component of the relative position vector.
Normal Rate	Rate	Real Number	The normal component of the relative velocity vector as observed in the NTC rotating frame.
Time Difference	Time	Real Number	Alternative measure for in-track position difference, computed as duration. The duration is computed by dividing the in-track component of the relative position vector by the inertial velocity of the primary object.
SpeedRelToRICFrame	Rate	Real Number	The magnitude of the relative velocity vector, as observed in the RIC rotating frame.
Radial Vel Diff	Rate	Real Number	The radial component of the vector difference between the assigned object's and the primary's inertial velocities.
In-Track Vel Diff	Rate	Real Number	The in-track component of the vector difference between the assigned object's and the primary's inertial velocities.
Cross-Track Vel Diff	Rate	Real Number	The cross-track component of the vector difference between the assigned object's and the primary's inertial velocities.
From Object	Distance	Text	The class name and name of the From-object.
To Object	Distance	Text	The class name and name of the To-object.
Position Cov Sigma	Unitless	Real Number	Position difference measured in terms of the position covariance. Reports N where the current relative position vector lies on a N sigma ellipsoid, where the ellipsoid if obtained from the position covariance of the primary satellite. This can only be computed if position covariance is available.
Position Combined Cov Sigma	Unitless	Real Number	Position difference measured in terms of the combined position covariance of primary and target satellites. Reports N where the current relative position vector lies on an N sigma ellipsoid, where the ellipsoid is obtained from the sum of the covariances matrices of the two objects. This can only be computed if position covariance is available.