  The ionosphere has been normally detected by traditional instruments, such as
ionosonde, scatter radars, topside sounders onboard satellites and in situ
rocket. However, most instruments are expensive and also restricted to either
the bottomside ionosphere or the lower part of the topside ionosphere (usually
lower than 800 km), such as ground based radar measurements. Nowadays, GPS
satellites in high altitude orbits (~20,200 km) are capable of providing
details on the structure of the entire ionosphere, even the plasmasphere. In
this paper, a Regional Ionospheric Mapping and Tomography (RIMT) tool was
developed, which can be used to retrieve 2-D TEC and 3-D ionospheric electron
density profiles using ground-based or space-borne GPS measurements. Some
results are presented from the RIMT tool using regional GPS networks in South
Korea and validated using the independent ionosonde. GPS can provide
time-varying ionospheric profiles and information at any specified grid related
to ionospheric activities and states, including the electron density response
at the F2-layer peak (the NmF2) during geomagnetic storms.
