## Legacy implementation in ASTRO

In the Legacy implementation of the planet flux computation, the basis is a reference brightness temperature at 100GHz, derived from radio observations.

For Mercury, Jupiter, and Saturn the brightness temperature is assumed to be constant over the spectrum

For Venus, Uranus and Neptune, the variation of the brightness temperature with frequency is accounted for using a spectral index.

For Mars, the brightness temperature is scaled according to the square root of the heliocentric distance of the planet.