  Energetic particle instruments on board GPS satellites form a powerful global
prompt proton sensor network (GPPSn) that provides an unprecedented opportunity
to monitor and characterize solar energetic protons targeting the Earth. The
medium-Earth-orbits of the GPS constellation have the unique advantage of
allowing solar energetic protons to be simultaneously measured from multiple
points in both open- and closed-field line regions. Examining two example
intervals of solar proton events, we showcase in this study how GPS proton data
are prepared, calibrated and utilized to reveal important features of solar
protons, including their source, acceleration/scattering by interplanetary
shocks, the relative position of Earth when impinged by these shocks, the shape
of solar particle fronts, the access of solar protons inside the dynamic
geomagnetic field, as well temporally-varying proton distributions in both
energy and space. By comparing to Van Allen Probes data, GPS proton
observations are further demonstrated not only to be useful for qualitatively
monitoring the dynamics of solar protons, but also for quantitative scientific
research including determining cutoff L-shells. Our results establish that this
GPPSn can join forces with other existing solar proton monitors and contribute
to observing, warning, understanding and ultimately forecasting the incoming
solar energetic proton events.
