Multibeam echosounders use split-aperture correlation for bottom detection. Split-aperture correlation divides the full receiver aperture into two sub-apertures that each observe an incoming echo independently. The baseline separation between the two receivers causes the outputs of the two receivers to have a phase difference. This phase difference will be zero when the incidence angle matches the steering angle of the receiver. A target angle estimate is taken where the phase difference is zero. An MBES makes the target angle estimate by analyzing all of the phase data, which is plotted as a “phase ramp.” Noise in the phase ramp results in uncertainties in the bottom detection. There are different sources of noise, but none of them adequately describe all of the noise present in phase ramps. This phenomenon was seen in data before this project started and in data analyzed from the Kongsberg EM2040P. For this reason, a new source of noise is proposed: transmission side lobe interference. Transmission side lobe interference is the result of the transmission beam pattern, which emits acoustic energy outside of the main beam. When the side lobe contributions return to the receiver, the receiver cannot differentiate between them and the signal retuning from the main beam. This increases the phase ramp noise and the uncertainty in the bottom detection estimate. Models were created that simulated a Mills Cross array and the Simrad ME70. Both models showed evidence that transmission side lobe interference significantly contributes noise in phase ramps. The ME70 simulation provides a framework for future field testing that can compare reality with the results of the model.