  In this paper we present the results of a $\sim$5 hour airborne gamma-ray
survey carried out over the Tyrrhenian sea in which the height range (77-3066)
m has been investigated. Gamma-ray spectroscopy measurements have been
performed by using the AGRS_16L detector, a module of four 4L NaI(Tl) crystals.
The experimental setup was mounted on the Radgyro, a prototype aircraft
designed for multisensorial acquisitions in the field of proximal remote
sensing. By acquiring high-statistics spectra over the sea (i.e. in the absence
of signals having geological origin) and by spanning a wide spectrum of
altitudes it has been possible to split the measured count rate into a constant
aircraft component and a cosmic component exponentially increasing with
increasing height. The monitoring of the count rate having pure cosmic origin
in the >3 MeV energy region allowed to infer the background count rates in the
$^{40}$K, $^{214}$Bi and $^{208}$Tl photopeaks, which need to be subtracted in
processing airborne gamma-ray data in order to estimate the potassium, uranium
and thorium abundances in the ground. Moreover, a calibration procedure has
been carried out by implementing the CARI-6P and EXPACS dosimetry tools,
according to which the annual cosmic effective dose to human population has
been linearly related to the measured cosmic count rates.
