Data Provider Groups | Data Provider Elements

Ephemeris Diff

The relative ephemeris of the assigned object with respect to the primary object, as observed from the requested coordinate system, expressed in Cartesian components as a function of time. The relative velocity vector is computed as observed in the requested coordinate system, and expressed in components of that frame.

Available for these objects: Aircraft, GroundVehicle, LaunchVehicle, Missile, Satellite, Ship

Type: Time-varying data.

Availability: Reports | Graphs | Dynamic Displays | Strip Charts

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

Data Provider Groups

Data can be requested in a variety of coordinate systems, where the origin of the coordinate system is the object's central body. The available coordinate systems depend on the object's central body. Nominally, the systems Fixed, Inertial, J2000, TrueOfDate, and MeanOfDate are supported, although some central bodies (notably the Earth and Sun) have more.

The following lists the systems available for Earth.

Name	Description
TrueOfDateRotating	A variation of the Fixed system where pole wander is ignored. The equator is the same as the TrueOfDate equator and the frame spins about the TrueOfDate z-axis.
Fixed	A coordinate system attached to the central body and rotating with it. The z-axis is nominally along the rotation axis.
ICRF	The relative ephemeris of the assigned object with respect to the primary object, as observed from the requested coordinate system, expressed in Cartesian components as a function of time. The relative velocity vector is computed as observed in the requested coordinate system, and expressed in components of that frame.
MeanOfDate	The mean equator mean equinox coordinate system evaluated at the requested time.
MeanOfEpoch	The mean equator mean equinox coordinate system evaluated at the epoch of the object.
TrueOfDate	The true equator true equinox coordinate system evaluated at the requested time.
TrueOfEpoch	The true equator true equinox coordinate system evaluated at the epoch of the object.
B1950	The mean equator mean equinox coordinate system evaluated at the beginning of the Besselian year 1950 (31 December 1949 22:09:46.866 = JD 2433282.4234591).
TEMEOfEpoch	The true equator mean equinox coordinate system evaluated at the epoch of the object.
TEMEOfDate	The true equator mean equinox coordinate system evaluated at the requested time.
AlignmentAtEpoch	The nonrotating coordinate system coincident with the Fixed system evaluated at the object's coordinate reference epoch.
J2000	The mean equator mean equinox coordinate system evaluated at the J2000.0 epoch (2000 January 1.5 TDB = JD 2451545.0 TDB).

Data Provider Elements

Name	Dimension	Type	Description
Time	Date	Real Number or Text	Time.
x	Distance	Real Number	The X component of the position vector.
y	Distance	Real Number	The Y component of the position vector.
z	Distance	Real Number	The Z component of the position vector.
vx	Rate	Real Number	X Cartesian component of velocity.
vy	Rate	Real Number	Y Cartesian component of velocity.
vz	Rate	Real Number	Z Cartesian component of velocity.
Range	Distance	Real Number	The range (i.e., distance between the primary and secondary object) at the given time.
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.
Speed	Rate	Real Number	Magnitude of the velocity vector of the projection of the selected point onto the reference plane.
Approach Angle	Angle	Real Number or Text	The angle between the inertial velocity of the primary object and the inertial velocity of the secondary object at the given time.
Miss Distance	Distance	Real Number	The component of the relative position vector perpendicular to the relative velocity vector.
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.