  Flight height is a fundamental parameter for correcting the gamma signal
produced by terrestrial radionuclides measured during airborne surveys. The
frontiers of radiometric measurements with UAV require light and accurate
altimeters flying at some 10 m from the ground. We equipped an aircraft with
seven altimetric sensors (three low-cost GNSS receivers, one inertial
measurement unit, one radar altimeter and two barometers) and analyzed $\sim$ 3
h of data collected over the sea in the (35-2194) m altitude range. At low
altitudes (H $<$ 70 m) radar and barometric altimeters provide the best
performances, while GNSS data are used only for barometer calibration as they
are affected by a large noise due to the multipath from the sea. The $\sim$ 1 m
median standard deviation at 50 m altitude affects the estimation of the ground
radioisotope abundances with an uncertainty less than 1.3%. The GNSS
double-difference post-processing enhanced significantly the data quality for H
$>$ 80 m in terms of both altitude median standard deviation and agreement
between the reconstructed and measured GPS antennas distances. Flying at 100 m
the estimated uncertainty on the ground total activity due to the uncertainty
on the flight height is of the order of 2%.
