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The EM 121A Multibeam Echo Sounder is the first commercially available 1 x 1 degree bathymetric mapping sonar system in the world. The EM 121A, which operates at a frequency of 12 kHz with a variable transmit pulse length from 1 to 20 ms, provides precise mapping and calibrated backscatter measurement capability from 10 meters to full ocean depth (11,000 meters). When operated with maximum resolution this sonar transmits a beam measuring 1 degree fore-and-aft by 140 degree athwartships and forms 121 one degree (1 degree) receive beams covering a swath across the vessel track. The transmit beam is dynamically stabilized for pitch and the receive beams are dynamically stabilized for roll which is important in minimizing the position, depth and signal amplitude offset corrections. This feature extends the sonar’s full system performance to Sea State 4 and also contributes to the system’s typical depth sounding accuracies of 0.1% of water depth or 30 cm (whichever is greater) and the precise bottom reflectivity measurements. Bottom detection is based on combined amplitude envelope detection and phase detection (interferometric principle), a technique which is proven to be highly reliable and productive on previous designs including the EM 100, EM 950, EM 3000, EM 1000 and the EM 12 Multibeam Echo Sounders. Also included is full and automatic compensation for ray bending caused by varying sound velocities in the water column. The EM 121 multibeam system was in production from 1993 to 2003 and was replaced with the EM 120.