A scheme to mitigate interference from Iridium satellite’s downlink signal captured by omnidirectional antenna array

In recent years, radio astronomy has faced a growing interference problem from commercial communications. Relatively strong signals from different communication devices can cause severe interference to weaker radio signals. A prime threat comes from transmitters in orbiting Earth satellites, since those transmitters are located overhead, precisely where radio astronomers must aim their telescopes to study the universe. For example, the downlink (space-to-Earth) signals from the Iridium satellite system span the range 1621.35-1626.5 MHz (L-Band) and are quite strong. Besides, significant sideband emission from Iridium can sometimes be seen as low as 1619 MHz and can cause harmful interference of up to 30 dB above the levels deemed harmful by ITU. The important OH transition at 1612.231 MHz lies very close to the Iridium downlink signal. The transmissions from Iridium cause disruption to searches for OH megamasers because their signals are nearly always present, and they cover a wide range of frequencies. The scope of this paper is to mitigate the interference caused by the downlink signal of Iridium. An algorithm is presented here to detect the signal from Iridium, estimate the signal, recreate it using estimated parameters and finally subtract it from the radio telescope. The Iridium data used in this paper is obtained from the Argus, an experimental omnidirectional radio telescope developed to detect the L-Band signals (S.W. Ellingston et al.) .