AO-51 is the in-orbit name designation of a now defunct (following battery failure) LEO amateur radio satellite of the OSCAR series; formerly known as ECHO, built by AMSAT. It was launched on June 29, 2004 from Baikonur Cosmodrome, Kazakhstan on a Dnepr launch vehicle. It is in sun synchronous low Earth orbit.
AO-51 contains an FM repeater with both 144 MHz (V band) and 1.2 GHz (L band) uplinks and 435 MHz (U band) and 2.4 GHz (S band) downlinks. It also contains a digital subsystem that transmits telemetry on 70 cm and provides a complete PACSAT BBS that can be configured on both V band and S band uplinks. As well, there is a 10-meter PSK uplink.
AO-51 has four VHF receivers, two UHF transmitters, six modems, and 56 channels of telemetry. The two UHF transmitters are connected to four phased antennas, yielding right-hand circular polarization for the 435.300 downlink and left-hand circular polarization for the 435.150 downlink.
The AO-51 FM satellite was easily workable with an amateur radio VHF dual band hand-held radio, as long as you knew when the satellite's footprint was within reach. Transatlantic contacts had been made without much effort, as long as the satellite was approximately mid-Atlantic so that the edge of the satellites footprint was within reach on either continent.
As of May 2011 the satellite faced problems with the battery. By September, a work around for the battery issue was found, bringing the repeater back in use. On November 29, 2011, the AO-51 Command Team announced that AO-51 has ceased transmission and is not responding to commands.