Verifying COSMIC GPS occultation inversion profiles with the ALTAIR radar

T. Beach1, F. Centinello1, R. Caton2, D. Sponseller3, K. Groves1 

1Space Vehicles Directorate, Air Force Research Laboratory, Hanscom AFB, Massachusetts, US
2Atmospheric & Environmental Research, Inc., Mansfield, Texas, USA
3Kwajalein Range Services LLC, Kwajalein Atoll, Marshall Islands

In September 2007, we conducted several incoherent scatter radar (ISR) data collections at Kwajalein Atoll
using the ALTAIR facility.  Some of these measurements corresponded to COSMIC/FORMOSAT-3 satellite GPS 
occultations that occurred within the ALTAIR field of view.  Of six targeted occultations during the 
experiment period, three yielded simultaneous ISR and occultation-derived ionospheric electron density 
profiles (EDPs) to be compared.   The novel features of this set of EDP comparisons are at least 
three-fold.  First, we specifically targeted occultations where the tangent point remained within 
the ISR’s field of view for an extended period.  Second, we conducted further radar scans around 
the time of the occultation to establish additional geophysical context.  Finally, Kwajalein Atoll 
lies between the magnetic equator and the northern equatorial ionization anomaly crest, one of the 
most challenging regions for ionospheric occultation inversion due to the presence of horizontal 
gradients in electron density and the potential for equatorial spread-F (ESF) disturbances.  In 
particular, the presence of moderate ESF significantly distorted the occultation-derived EDP on 
16 September 2007.  In order to validate GPS radio occultation as a technique for monitoring the 
global ionosphere it will be necessary to understand its limitations.  The present case study 
provides additional insight into the behavior of occultation-derived EDPs in near-equatorial regions.