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 a reference air density and the factor of two in the denominator, we get the starred ballistic coefficient:

$B^* = \frac{\rho_0 B}{2} = \frac{\rho_0 C_d A}{2m}$

thus reducing the expression for the acceleration due to drag to

$a_D = \frac{\rho}{\rho_0} B^* v^2$

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. The corresponding reference air density is given as 2.461e-5 kg/m^2/Earth radii.^[1]

References

1. https://celestrak.com/NORAD/documentation/spacetrk.pdf

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