Access Coordinate Frames

Access calculations compute azimuth, elevation, and range between the primary and associated objects based on the local coordinate system of the primary object. AER reports for accesses are based on a default local coordinate system for every type of STK object for which access is possible. The following table lists the default local coordinate frame for the given objects:

Objects Coordinate Frame Az El Computation
Facility, Place, Target

Local Horizontal (LH)

  • X axis = constrained toward local north
  • Y axis = local east
  • Z axis = aligned with local geodetic nadir

Azimuth is referenced to X, positive toward Y (clockwise from above).

Elevation is referenced to the X-Y plane, positive along negative Z.

Satellite, Missile, Launch Vehicle

ECI Vehicle Velocity, Local Horizontal (VVLH)

  • X axis = constrained toward velocity vector
  • Y axis = perpendicular to X and Z
  • Z axis = aligned with geocentric nadir

Azimuth is referenced to X, positive toward Y (clockwise from above).

Elevation is referenced to the X-Y plane, positive along negative Z.

Aircraft, Ship, Ground Vehicle

ECF Vehicle Velocity, Local Horizontal (VVLH)

  • X axis = constrained toward velocity vector
  • Y axis = perpendicular to X and Z
  • Z axis = aligned with geocentric nadir

Azimuth is referenced to X, positive toward Y.

Elevation is referenced to the X-Y plane, positive along negative Z.

All other objects local coordinate system of the object's parent (or grandparent), as applicable Depends on the coordinate system.

When using the ECF VVLH frame, if the primary velocity is near zero, STK may switch to the LH frame to avoid an error condition.

Local Horizontal (LH) coordinate system

Vehicle Velocity Local Horizontal (VVLH) coordinate system