Impact of FM pulse compression sidelobes on multibeam bathymetry measurements
Résumé
The latest generation of bathymetric multibeam echo sounders (MBES) uses frequency modulated (FM) signals with matched filtering using pulse compression in order to reach wider swath (by increasing the transmitted energy via pulse duration, resulting in a better SNR) while keeping a high accuracy (shorter efficient pulse duration).
In practice, this makes it possible to get measurements from deeper seafloor or wider swath where classical pulse shapes (CW) cannot do it. However, the comparison between bathymetric data from FM and CW systems shows that data from FM signals are often noisier than the one using CW signals. Why this paradoxical observation, since in theory the signal to noise ratio (SNR) is increased by the matched filtering when using FM?
This paper will show that the coherence level between the subarray signals decreases when using FM rather than CW. Indeed, a common mistake is to consider that FM pulse compressed signal is equivalent to CW whose duration equals its efficient duration; however, because of its side lobes, its real duration is longer. In a given time, the considered footprint is thus wider using FM than CW. The larger the footprint is, the more fluctuated the emission pattern of it is, resulting in a decorrelation of the two interferometric signals (angular or baseline decorrelation). This phenomenon has been studied theoretically. In addition, the impact of the side lobes on bathymetric measurements has been comforted on field and simulated data.