TERPEM and TGRAPH are the result of on-going algorithm and software development that began in 1989 with the release of PCPEM. PCPEM was the first commercially available software package that used the Parabolic Equation Model for modelling the effects of atmospheric refraction and ducting on electromagnetic wave propagation in the troposphere. PCPEM used a pure split-step model, and only modelled propagation over a flat surface. The FDPEM package followed in 1992 using a finite difference method and included the effects of terrain diffraction as well as atmospheric refraction.

PCPEM and FDPEM were both DOS-based programs and required an add-on numerical processing card in the PC to provide sufficient computational power.

We began working on hybrid algorithms in 1994 in order to extend their applicability and to speed up the PCPEM/FDPEM computations. Various DOS and 16-bit Windows packages have been developed for customers since then, but not released commercially. Version 5.1 of TERPEM was finally released as a commercial package in 1998 as a 32-bit executable targeted at Windows NT and Windows 95.