Real scale ray-tracing simulation of space earthshine measurement with improved BRDF model of lunar surface

The discrepancy in annual changes of Earth albedo anomaly among the Had3CM prediction, ground and low Earth orbit measurements attracts great academic attention world-wide. As a part of our on-going study for better understanding of such discrepancy, we report a new earthshine measurement simulation technique. It combines the light source (the Sun), targets (the Earth and the Moon) and a hypothetical detector in a real scale Integrated Monte-Carlo Ray Tracing (IRT) computation environment. The Sun is expressed as a Lambertian scattering sphere, emitting 1.626x10²⁶W over 400nm- 750nm in wavelength range. Whilst we are in the process of developing a complex Earth model consisting of land, sea and atmosphere with appropriate BRDF models, a simplified Lambertian Earth surface with 0.3 in uniform albedo was used in this study. For the moon surface, Hapke's BRDF model is used with double Henry-Green phase function. These elements were then imported into the IRT computation of radiative transfer between their surfaces. First, the irradiance levels of earthshine and moonshine lights were computed and then confirmed that they agree well with the measurement data from Big Bear Solar Observatory. They were subsequently used in determination of the Earth bond albedo of about 0.3 that is almost identical to the input Earth albedo of 0.3. These computations prove that, for the first time, the real scale IRT model was successfully deployed for the Earthshine measurement simulation and, therefore, it can be applicable for other ground and space based measurement simulation of reflected lights from the Earth and the Moon.