Software for determining static position from RINEX pseudorange data

I'm looking for programs (source) written in BASIC, FORTRAN, or
Scilab. They don't have to be fast - I'll be using data sets up
to five years old ;) Output in ECEF without any correction for
geoid(sp?) is acceptable (actually preferred).

Theoretically I should be able to do it myself. But I've not had
to solve solid geometry problems for 50 years, nor systems of

My goal is to plot position error vs time while varying:
1. how much of sky visible
2. with/without ionospheric/tropospheric correction of
propagation speed.

I will use data from CORS network. I have sufficient programing
skills to put the data in format assumed by the software.

TIA