Combating low UHF satcom downlink margin

Relative comparison of the effective isotropic radiated power (EIRP) from Fleet Satellite (FLTSAT) against the UHF Follow-On (UFO) satellites generally shows equal or reduced downlink signal power from the UFO transponders, far less than the anticipated 3.5 dB increase in specified narrowband channel EIRP. Thus we have a relative measurement between what the warfighter sees today (FLTSAT) and what the future holds (UFO). Understanding of this measurement is key to the expected UHF SATCOM performance over the next decade as the UFO constellation replaces aging FLTSAT satellites. Certainly narrowband (5 kHz) channel spacing has improved with the advent of UFO, allowing an increase in transponder channel bandwidth. Wideband (25 kHz) channels have also increased in bandwidth, inviting the use of higher data rate modulation. Link margin is even more important today because of increased emphasis on higher data rates and the migration of terminals to on-the-move (OTM) operation with lower-gain, hemispherical-coverage antennas. Mobile SATCOM users however will not be able to take advantage of this increased data throughput, due to low link margins. Technological advances are required of SATCOM terminals to mitigate low carrier-to-noise (C/N) signals available at the antenna forward error coding (FEC) provides some improvement in data integrity at the cost of decreased data rates. However, if the link margins become too low during multipath fading, the terminal becomes synchronization-limited. Several techniques are available which provide significant improvement over the traditional Costas tracking and synchronization loops. These include mod-remod, data-aided, parallel loops and simultaneous clock/carrier synchronization. The MOST (multiple output SATCOM transceiver) terminal, for example, utilizes advanced software algorithms to provide acquisition and tracking for reduced C/N signals. Measured results show a 5 dB improvement in acquisition compared to traditional SATCOM terminals.