  This paper describes the Fortran 77 code SIMU, version 1.1, designed for
numerical simulations of observational relations along the past null geodesic
in the Lemaitre-Tolman-Bondi (LTB) spacetime. SIMU aims at finding scale
invariant solutions of the average density, but due to its full modularity it
can be easily adapted to any application which requires LTB's null geodesic
solutions. In version 1.1 the numerical output can be read by the GNUPLOT
plotting package to produce a fully graphical output, although other plotting
routines can be easily adapted. Details of the code's subroutines are
discussed, and an example of its output is shown.
