A prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Global Positioning System (GPS) receivers. Case studies in this paper indicate that GPS receiver loss of lock of signal tracking may be associated with strong phase fluctuations. It is shown that a network-based GPS monitoring system will enable us to study the generation and evolution of ionospheric irregularities continuously around the globe under various solar and geophysical conditions, which is particularly suitable for studies of ionospheric storms, and for space weather research and applications.¿ 1997 American Geophysical Union