Data Providers: Satellite: Ephemeris Diff: ICRF

The relative ephemeris of the assigned object with respect to the primary object, 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. The relative ephemeris can be requested in a variety of coordinate systems. 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	Dimension	Type	Description
Time	DateFormat	Real Number	Time.
x	DistanceUnit	Real Number	The X component of the position vector.
y	DistanceUnit	Real Number	The Y component of the position vector.
z	DistanceUnit	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	DistanceUnit	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	AngleUnit	Real Number	The angle between the inertial velocity of the primary object and the inertial velocity of the secondary object at the given time
Miss Distance	DistanceUnit	Real Number	The component of the relative position vector perpendicular to the relative velocity vector.
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.
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.

Name

Dimension

Type

Description

Time

DateFormat

Real Number

Time.

DistanceUnit

Real Number

The X component of the position vector.

DistanceUnit

Real Number

The Y component of the position vector.

DistanceUnit

Real Number

The Z component of the position vector.

Rate

Real Number

X Cartesian component of velocity.

Rate

Real Number

Y Cartesian component of velocity.

Rate

Real Number

Z Cartesian component of velocity.

Range

DistanceUnit

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

AngleUnit

Real Number

The angle between the inertial velocity of the primary object and the inertial velocity of the secondary object at the given time

Miss Distance

DistanceUnit

Real Number

The component of the relative position vector perpendicular to the relative velocity vector.

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.

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.