Tracking Reference Objects

Tracking Reference Objects define the mode alignment and clocking targets for the spacecraft while tracking. Alignment is done in the Mode Controller, while clocking is done at run time (see Initial Conditions or Sequence Editor).

There are a variety of options for an axis of the spacecraft to point toward. Below are descriptions of all of the Targeting Reference Objects available in SOLIS.

Central Body

Object	Description
Sc -> Sun	Points the spacecraft body (align or clock) axes with the spacecraft-to-Sun vector.
SC -> Nadir (detic)	Points the spacecraft body (align or clock) axis along the vector from the satellite to the local surface-normal of the selected central body.
SC -> Nadir (centric)	Points the spacecraft body (align or clock) axis along the vector from the satellite to the center of the selected central body reference frame.
CB Orbit Normal	Points the spacecraft body (align or clock) axis along the vector of the specified Central Body's orbit normal vector. This orbit normal vector is computed by taking the cross product of the Central Body's position and velocity.

Target

Object	Description
SC -> Target Object	Points the spacecraft body (align or clock) axis along the vector from the spacecraft to the specified Target Object. Target Objects can be locations on a central body, scans, or orbiting objects. You can change the definition of the target via the EPH_SET_TGT, EPH_SET_TGTSCAN, or EPH_ADD_ENTRY (for orbiting targets) commands.
True North at Target	Points the spacecraft body (align or clock) axis along the vector of True North at the location of the specified Target Object.
Target velocity	Points the spacecraft body (align or clock) axis along the vector of the specified Target Object inertial velocity. This is useful for scans to clock relative to (or orthogonal to) the scan direction.
cross(SC -> TgtObject, SC Vel)	Points the spacecraft body (align or clock) axis along the vector from the spacecraft to the vector cross product between the Target Object pointing vector and the spacecraft velocity.

Spacecraft Parameters

Object	Description
SC Velocity	Points the spacecraft body (align or clock) axis along the vector of the SC velocity.
SC Mag Field	Points the spacecraft body (align or clock) axis along the vector of the magnetic field at the location of the spacecraft.
SC Orbit Normal	Points the spacecraft body (align or clock) axis along the vector of the spacecraft's orbit normal vector. This orbit normal vector is computed by taking the cross product of the satellite's position and velocity.
SC Relative Velocity	Points the spacecraft body (align or clock) axis along the UCBF velocity vector. This differs from the UCBI velocity vector (SC Velocity) by incorporating the angular velocity of the central body axis rotation.
True North	Points the spacecraft body (align or clock) axis along the vector of True North at the subsatellite point.

Inertial

Object	Description
Inertial Vector	Points the spacecraft body (align or clock) axis along the inertial vector specified.
Inertial Fixed	Orients the spacecraft in a specified inertial (True of Date) attitude using a quaternion or direction cosine matrix (DCM).

External

Object	Description
External Guidance	Points the spacecraft body in a quaternion orientation (CBI2BDY) defined over the ExternalGuidance interface connected to the FOB_ModeType_Tracking. The interface also provides the BDY angular velocity command and the BDY angular acceleration command. This allows you to fully determine the guidance commands using any custom logic you desire. This Reference Object is not valid as a Clocking Option.
External Vector	Points the spacecraft body (align or clock) axis in the UCBI vector specified by the ExternalAlignVector or ExternalClockVector interface connected to the FOB_ModeType_Tracking object. This enables you to define any logic you desire to create the UCBI vector. You must make the interface connection via Connection_FSC_Gui_Vec_A.