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

${\displaystyle F_{D}={\frac {1}{2}}\rho C_{d}Av^{2}}$

where ${\displaystyle \rho }$ is the air density, ${\displaystyle C_{d}}$ is the drag coefficient, ${\displaystyle A}$ is the frontal area, and ${\displaystyle v}$ is the velocity.

The acceleration due to drag is then

${\displaystyle a_{D}={\frac {F_{D}}{m}}={\frac {\rho C_{d}Av^{2}}{2m}}}$

In aerodynamic theory, the factor

${\displaystyle B={\frac {C_{d}A}{m}}}$

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:

${\displaystyle B^{*}={\frac {\rho _{0}B}{2}}={\frac {\rho _{0}C_{d}A}{2m}}}$

thus reducing the expression for the acceleration due to drag to

${\displaystyle a_{D}={\frac {\rho }{\rho _{0}}}B^{*}v^{2}}$

As it can be seen, ${\displaystyle B^{*}}$ 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 0.1570 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