Many multibeam echosounders use split-aperture correlators to generate phase ramps and, subsequently, form bottom detection estimates. The noise present within the phase ramp couples directly into bottom detection estimates. This results in increased bottom detection estimate uncertainty. Known sources of phase ramp noise are additive random noise and baseline decorrelation. Anecdotally, these known sources of noise do not appear sufficient to explain phase ramp noise in several(though not all, e.g. the Simrad ME70) bottom mapping multibeam echosounders. The goal of this thesis was to explore a hypothesis for where additional phase ramp noise might be generated. In pursuit of this goal, data from the Kongsberg EM2040P was analyzed to see whether the noise models for the known sources of noise matched the phase ramp noise. When it did not, a new source of phase ramp noise was proposed: transmission side lobe interference. This source of noise is introduced by side lobes from the transmitter when they are incorporated into the split-aperture correlator by the receiver. A computational model of a Mills Cross array was created to quantify this effect in an idealized system. An approach for further field testing of the ME70 was developed, which takes advantage of the ME70’s ability to be configured and using it in a non-standard mode.