First Fresnel ellipsoid’s height

The first Fresnel zone [10] must be free of any obstacle and consequently the level flight (h) needs to be higher than the first Fresnel zone radius R₁. At sea, under controlled maritime flight, the only possible obstacle between the AUT and the PA is sea surface. There are, accordingly, two possible options for flight level as depicted in Fig 4: either same as the AUT’s or higher:

The worst-case according to this criterion is when both antennas are at the same height and the first Fresnel ellipsoid’s height would be:

where d is AUT-PA distance (m) and f the frequency (MHz). As Fig 4 shows, this effect is only relevant for low frequencies.

The best-case scenario is for a UAV flying over AUT’s height, as depicted in Fig 5, as the ellipsoid is always above sea level.

Since the minimum flight level lowers as the frequency increases, communication antennas in HF, VHF, and UHF bands require a higher flight level than radar SHF antennas. For example, frequencies over 300 MHz need up to 16 m height for distances lower than 1 km; HF band, up to 1 km needs flight level in the 20–160 m range, while VHF ranges 10–50 m.