Ionospheric electron density profiles obtained with the global positioning system: Results from the GPS/MET experiment

The Global Positioning System Meteorology (GPS/MET) experiment, which placed a GPS receiver in a low-Earth orbit tracking GPS satellites setting behind the Earth's limb, has collected data from several thousands of occultations since its launch in April 1995. This experiment demonstrated for the first time the use of GPS in obtaining profiles of electron density and other geophysical variables such as temperature, pressure, and water vapor in the lower atmosphere. This paper discusses some of the effects of the ionosphere, such as bending and scintillation, on the GPS signal during occultation. It also presents a set of ionospheric profiles obtained from GPS/MET using the Abel inversion technique, and compares these profiles with ones obtained from the parameterized ionospheric model (PIM) and with ionosonde and incoherent scatter radar measurements. Statistical comparison of NmF2 values obtained from GPS/MET profiles and nearby ionosondes indicates that they agree to about similar to 20% (1-sigma) in a fractional sense. The high vertical resolution, characteristic of the occultation geometry, is reflected in the GPS/MET profiles which reveal ionospheric structures of very small vertical scales such as the sporadic E.