Current sonar and radar applications use interferometry to estimate the arrival angles of backscattered signals at
time-sampling rate. This direction-finding method is based on a phase-difference measurement between two close
receivers. To quantify the associated bathymetric measurement quality, it is necessary to model the statistical
properties of the interferometric-phase estimator. Thus, this paper investigates the received signal structure,
decomposing it into three different terms: a part correlated on the two receivers, an uncorrelated part and an
ambient noise term.
This paper shows that the uncorrelated part and the noise term can be merged into a unique, random term
damaging the measurement performance. Concerning the correlated part, its modulus can be modeled either as
a random or a constant variable according to the type of underwater acoustic application. The existence of these
two statistical behaviors is verified on real data collected from different underwater scenarios such as a horizontal
emitter-receiver communication and a bathymetric seafloor survey. The physical understood of the resulting
phase distributions makes it possible to model and simulate the interferometric-signal variance (associated with
the measurement accuracy) according to the underwater applications through simple hypotheses.
Concerning bathymetric multibeam echosondeur systems, the interferometric technique is widely used to get the altitude of an illuminated seafloor section beam. Classical methods only use the zero crossing instant of phase difference to obtain the sea depth. Nevertheless, the phase difference gives more information near the zero crossing. Using the idea of radar multilook, the seabed footprints of two close beams overlap and, consequently, it exists a common illuminated area. In this paper, we show that the mutual information between the two close beams is enough to merge them into one because of the coherently processing of the signals received from multiple sensors (that is, beamforming). This mutual information, set up by several beamforming methods, makes possible to take into account all points included in the beam footprint in order to rebuild more accurately the sea floor. Besides, considering a beamforming width between ±25° and ±60°, we can recreate a continuous phase difference by merging all phase differences. Beam angles close to nadir will not be considered because of their non acceptable performance in terms of interferometric quality. In addition, the effect of changing the interferometric spacing, commonly called baseline, is also studied. A correct baseline value plays an important role in high-resolution beamforming. Actually, the influence of the multibaseline causes an increase of the phase difference variance, and therefore, an increase of the measurement errors. Finally, we propose the fusion of the multilook techniques and the baseline effects to improve the multibeam
echosondeur bottom detection.