MARECS B is a telecommunications satellite intended to serve as a relay between boats and ground controllers. It is equipped with three repeaters and an antenna. The satellite did not reach orbit because of the failure of the launch.

The cylindrically shaped spacecraft measured 144 cm in diameter and 240 cm in length including the apogee boost motor. The primary structural members are equipment platforms and a central thrust cone carrying conventional housekeeping electronics in addition to payloads. Attitude, orbit, and spin rate control were performed using a hydrazine propulsion system including four thrusters. The spin rate would have been maintained at 90 rpm throughout the two-year nominal lifetime. A mechanically despun antenna relayed S-band telemetry (meteorological and housekeeping), while a traditional omnidirectional turnstile system supported VHF ranging and telecommands, as well as housekeeping telemetry in transfer orbit and as backup geostationary orbit.