AMSAT-OSCAR 51 or 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.

AO-51 had four VHF receivers, two UHF transmitters, six modems, and 56 channels of telemetry.

The two UHF transmitters were 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.

[3] By September, a work around for the battery issue was found, bringing the repeater back in use.