TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2923 SUBJECT: Detection of the SGR 1806-20 giant outburst back-scattered by the DATE: 04/12/29 19:32:36 GMT FROM: Dmitry Frederiks at Ioffe Institute S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks on behalf of Konus-Wind and Helicon/Coronas-F teams, and T. Cline on behalf of the Konus-Wind team report: We present an evidence of Helicon-Coronas-F detection of the giant outburst from SGR1806-20 which was scattered back from the Moon. At the time of the outburst SGR1806-20 was occulted by the Earth for Coronas-F. A short burst triggered Helicon at 21:30:29.303s UT on Dec 27. A time delay between Konus-Wind and Helicon-Coronas-F detections is -7.70 s. This value corresponds exactly to burst travelling time from the Wind to the Moon and back to the Coronas-F. The spectrum of the event detected by the Helicon is highly unusual. It looks like a broad assymetric line peaked at ~100 keV. Apparently such a shape corresponds to back-scattering peak of a huge initial pulse of the outburst. The fluence of event is about 7.5x10^-7 erg cm-2 in 25-400 keV range.