This document describes the software design of Demographic Distribution Database (D3) exploitation and integration into the Greener Aircraft Trajectories under ATM Constraints (GATAC) optimisation framework. This model has been developed by GSAF (Cranfield University) to support the use of existing in-house built noise models developed within WP 3.1, namely INMTM and Doc. 29. This document covers the design description of the model and GATAC user instructions.