BSTAR

BSTAR is a way of modeling aerodynamic drag on a satellite in the SGP4 satellite orbit propagation model.

Traditionally, aerodynamic resistance ("drag") is given by

F_D = \frac{1}{2} \rho C_d A v^2

where \rho is the air density, C_d is the drag coefficient, A is the frontal area, and v is the velocity.

The acceleration due to drag is then

a_D = \frac{F_D}{m} = \frac{\rho C_d A v^2}{2m}

In aerodynamic theory, the factor

B = \frac{C_d A}{m}

is called the ballistic coefficient, and its unit is area per mass. Further incorporating air density and the factor of two in the denominator, we get the starred ballistic coefficient:

B^* = \frac{\rho B}{2} = \frac{\rho C_d A}{2m}

thus reducing the expression for the acceleration due to drag to

a_D = B^* v^2

As it can be seen, B^* has a unit of inverse length. For orbit propagation purposes, there is a field for BSTAR drag in Two-line element set (TLE) files, where it is to be given in units of inverse Earth radii.

References

  1. Frequently Asked Questions: Two-Line Element Set Format
  2. BSTAR Drag Term