The aim of the ESA-funded TREND (Trapped Radiation Environment Development) project is to evaluate and extend existing models of the Earth's trapped particle environment, and, where appropriate, to develop new models. At low altitude, protons interact with the atmosphere whose neutral density is a function of solar activity. Therefore the low-altitude cut-off of proton fluxes by atmospheric absorption is solar-activity dependent. The cut-off is a very important feature and explicit modelling of the geomagnetic-atmospheric control of particle fluxes is difficult. We report on work done for the low altitude region, including study of coordinate systems, tracing drift shells to deduce atmospheric control, and atmosphere-induced flux anisotropies. The construction of new a trapped proton model based on data from the AZUR particle telescopes is described. The new model is being included in the UNIRAD software package [Heynderickx et al., 1996c].