Orbit/Location/Trajectory Uncertainty

Orbiter motion model option

When you select the Orbiter Motion Model, ODTK provides the initial uncertainty in the orbit state of the satellite in terms of a covariance matrix for errors in radial (R), in-track (I), and cross-track (C) position and velocity.

R_sigma	RI_correlation	RC_correlation	RRdot_correlation	RIdot_correlation	RCdot_correlation
...	I_sigma	IC_correlation	IRdot_correlation	IIdot_correlation	ICdot_correlation
...	...	C_sigma	CRdot_correlation	CIdot_correlation	CCdot_correlation
...	...	...	Rdot_sigma	RdotIdot_correlation	RdotCdot_correlation
...	...	...	...	Idot_sigma	IdotCdot_correlation
...	...	...	...	...	Cdot_sigma

In the diagonal from radial position (R_sigma) to cross-track velocity (Cdot_sigma), enter the estimated errors for the respective position or velocity elements. In each off-diagonal field appearing in a row i and column j, enter a value r_{i j}, where

-1 <= r_{i j} <= 1

and represents the covariance between the errors for the elements heading the ith row and the jth column of the matrix.

When you set the satellite EstimateOrbit flag to true, ODTK uses the orbit uncertainty to compute the initial covariance for the satellite when you run the filter in initial mode and the simulator uses it to produce deviates of the initial orbit state. You can use the orbit uncertainty to specify a constant uncertainty in the RIC frame for a satellite using a reference ephemeris. For ODTK to interpret it in this manner, set the EstimateOrbit flag to false, specify a reference ephemeris, and set the CovarianceSource for the reference ephemeris to Constant.

Lander motion model option

When you select the Lander Motion Model, ODTK provides the initial uncertainty in the position of the landed spacecraft in terms of a covariance matrix for errors in south (S), east (E), and altitude (A) position.

S_sigma	SE_correlation	SA_correlation
...	E_sigma	EA_correlation
...	...	A_sigma

You can set the initial topocentric uncertainty to specify a constant uncertainty in the lander position when not estimating the lander location. To have ODTK interpret it in this manner, set the EstimateOrbit flag to false.

Surface vehicle motion model option

You can specify the initial position uncertainty of a surface vehicle in the same manner as that of a lander. You can specify the initial velocity uncertainty for each individual segment based on the stochastic motion model that the segment uses. The segments are located within the MotionModelSegments attribute of the surface vehicle.

A stationary segment has a small positive velocity uncertainty that is slightly greater than zero, to avoid numerical issues associated with covariance values of zero. A near-constant velocity segment has an initial velocity uncertainty that you can specify using either a topocentric representation or a heading-speed representation.

You can specify the initial velocity uncertainty with a topocentric representation in the terms of a covariance matrix, in sigma correlation form, for errors in south (S), east (E), and altitude (A) velocity, as shown in the table below.

Sdot_sigma	SdotEdot_correlation	SdotAdot_correlation
...	Edot_sigma	EdotAdot_correlation
...	...	Adot_sigma

When you set the surface vehicle EstimateTrajectory flag to true, ODTK uses the initial position uncertainty and the initial velocity uncertainty of the segment containing the epoch of the surface vehicle to compute the initial covariance for the surface vehicle when you run the filter in initial mode. The simulator uses it to produce deviates of the initial position/velocity state. If you set the EstimateTrajectory flag to false, ODTK will not use these values; the attributes will become unavailable on the white panel.