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