STK points sensors using the parent's body coordinate systems, except for facilities and targets. STK does not use the Vehicle Velocity, Local Horizontal (VVLH) frame for sensor pointing, but does use it when generating an Azimuth, Elevation, and Range (AER) report. The VVLH coordinate system is defined as follows:
Ground vehicles, launch vehicles, missiles, and ships:
x-axis: aligned in the direction of the velocity vector
y-axis: to the left from the velocity vector, parallel to the local horizontal
z-axis: to comply with a right-handed coordinate system; for Ground Vehicles and Ships, this is usually up
Aircraft:
x-axis: aligned in the direction of the velocity vector
y-axis: to the right from the velocity vector, to comply with being a right-handed coordinate system
z-axis: down (constrained with nadir)
Facilities, places, and targets:
x-axis: north
y-axis: east
z-axis: down (geodetic); however, for sensor pointing, up is positive, resulting in a left-handed coordinate system
You can learn more about how STK uses the VVLH coordinate system for AER calculations at: STK Help > How to Use STK > Analysis > Access
