Signals from the Global Positioning System (GPS) satellites can be used to make measurements of absolute ionospheric group delay and relative phase advance. These ionospheric parameters have wide application in providing corrections for military systems requiring knowledge of ionospheric time delay and phase scintillation, such as satellite detection radars. Multipath effects from the local environment of the receiving antenna can cause severe contamination of ionospheric group delay measurements made with GPS pseudorandom noise receivers. Measured variations in the multipath effects in some typical clean and reflective environments are presented and shown to be consistent with signal analysis for pseudorandom noise receivers. Measured single reflector multipath is analyzed briefly and shown to correlate well with calculations. Since antennas must frequently be located in environments that are much less than ideal, data analysis procedures were developed for minimizing the impact of multipath on the group delay measurement. These include low-pass filtering, day-to-day correlation, and calibration to relative differential carrier phase advance measurements by using subsets of the absolute ionospheric group delay data. Results of these processes as applied to the test data from various multipath environments are presented.