BSTAR
BSTAR is a way of modeling aerodynamic drag on a satellite in the SGP4 satellite orbit propagation model.
Traditionally, aerodynamic resistance is given by
where
is the air density,
is the drag coefficient,
is the frontal area, and
is the velocity.
The acceleration due to drag is then
In aerodynamic theory, the factor
is the inverse of the ballistic coefficient, and its unit is area per mass. Further incorporating a reference air density and the factor of two in the denominator, we get the starred ballistic coefficient:
thus reducing the expression for the acceleration due to drag to
As it can be seen, has a unit of inverse length. For orbit propagation purposes, there is a field for BSTAR drag in Two-line element set files, where it is to be given in units of inverse Earth radii. The corresponding reference air density is given as 0.1570 kg/.
