UHF SATCOM downlink interference for the mobile platform

UHF satellite communications (SATCOM) aboard mobile platforms are difficult to achieve compared to stable platforms. Mobile platforms use hemispherical-coverage or quad diversity antennas. Hemispherical antenna coverage is required because of continuous changes in path geometry. Mobile antennas are limited both in size and gain. Selectivity requirements for narrowband (5 kHz) and wideband (25 kHz) SATCOM channel operation are about 10 dB more difficult for the mobile platform. SATCOM receiver selectivity is needed to limit AGC, control intermod products and allow quad diversity reception. The design of a UHF SATCOM terminal should consider worse-case narrowband and wideband channel downlink interference for the mobile platform, assuming the advent of the UHF Follow-On (UFO) satellite. Presently many of FLTSAT and LEASAT satellite narrowband channels are spaced on 5 kHz centers, which impose very tight selectivity requirements on SATCOM terminals. UFO narrowband channel spacing has been increased to 10 kHz easing selectivity requirements. Even though the wideband channel spacing for UFO remains at 100 kHz, half of the channels appear to the hemispherical-coverage antenna user to be spaced 50 kHz apart. Wideband selectivity requirements for the SATCOM terminal are dictated by the high dynamic range for adjacent channel interference with 50 kHz spacing for line-of-sight operation, whereas narrowband selectivity requirements result from consideration of noise interference from adjacent idle SATCOM channels. Narrowband selectivity can be upgraded in firmware as FLTSAT/LEASAT satellites are transitioned to UFO.