Study On Digital Noise Targets Passive Ranging Sonar

Active SONAR provides signal for enemy when ranging by transmitting pulse and this is very dangerous for submarine. Therefore, passive SONAR, aiming at universal noise targets, becomes the dominating SONAR of submarine. As we know, noise target ranging technology is one of its main problems. The purpose of this thesis is to study and design a digital noise targets passive ranging system which will be used to overcome the imperfectness of conventional SONAR, to track the whole course of a noise target, to follow a fast moving target, as well as to adapt the system to different flank three sub-array types without the limitation of installation place.In noise target passive ranging SONAR (NTPRS), target range finding is accomplished by estimating the time delay between signals emanating from a remote source and received by three spatially separated array elements. With the help of passive ranging and bearing formula, which is derived for arbitrary linear array type, general characteristic of the time delay is analyzed in detail in the thesis. To higher the ranging accuracy, first of all, a bigger array aperture is required to effectively estimate the range of target far away. In addition, singnal processing measures are also indispensable, which leads to different time delay estimation (TDE) methods acceptable for far and near target status. For far targets, based on azimuth word from tracking system, K parameter distribution method (K-PDM) is used to estimate the time delay because of the lower signal to noise ratio (SNR) and the vague correlation summit. However, for near targets, K-CDM has the problem of range blurring, then direct measurement method (DMM) is used instead the higher SNR and the clear summit. The problem of whole course ranging is accomplished by switching these two methods appropriately.Though simple the passive ranging model in theory, the practical implementation detail is very complex because the time delay needed is of the order of micro-second, which is the same or less than the propagation time fluctuation of underwater channel. Consequently, some signal processing andreal-time correction algorithm must be conceived for passive ranging. This thesis discusses the general design principle of passive ranging system, including pre-processor, beamformer, generalized cross-correlator (GCC), ranging time delay estimator, and post-filter. The proposed estimator composed of GCC and adaptive noise canceller has advantage over other ones and has very excellent tracking ability, especially under the circumstance of complex target course and fast azimuth changing ratio.NTPRS is implemented digitally. As a real-time signal processing system, it is designed with an eye on software and hardware co-work, real-time and reliable running. To reduce the computation burden, beamformer is designed in FPGA digitally. As the main processor, DSP from TI Corporation bears the responsibilities of implementing other signal processing. This thesis also discusses the problem giving arise in this implementation accordingly.NTPRS has passed the validation runned by simulator in laboratory. For symmetric and asymmetric arrays, tests are carried out to assess the ranging capability of the system with versatile test condition. It has proved that NTPRS has higher ranging accuracy and better reliability independent of the array types. Compared with similar conventional SONAR, NTPRS improves the passive SONAR performance.