Abstract:
Underwater target tracking has vital importance in a variety of applications, most notably military surveillance and defense. This work is focused on underwater target tracking
using active sonar. In active sonar, sound pulses are transmitted in water, and an array of
hydrophones receives echoes produced by the target and any other obstacles. The received
echoes are processed to obtain estimates of object location and radial velocity, which form
the input to a tracking filter for estimating the object's motion over time. The object location is given in terms of range and bearing (angle); the object's radial velocity, also known
as range-rate, is obtained from the estimated Doppler shift.
In active sonar, the conventional sound pulses are the single-frequency continuous wave
(CW) pulse and the frequency modulated (FM) pulse. These pulses differ from one another
in terms of the range resolution they provide and whether or not they allow the system
to measure Doppler shift. CW pulses allow for measurement of Doppler shift but have
relatively poor range resolution. In contrast, a broadband FM pulse is insensitive to Doppler
but has a better range resolution than a CW pulse.
Reverberation is defined as the rejection of sound energy toward the sonar system by
objects other than the target of interest. Most active sonars operate in a reverberation-
limited environment where an appropriate choice of sound pulse plays a key role in target
tracking. If the target exhibits high Doppler at the time of sound pulse impingement, a
CW pulse provides better tracking capability, while an FM pulse is more effective for a
low Doppler target. A challenge, however, is that the target Doppler changes with time
and is unknown at the time of waveform impingement. At present, the complementary
strengths of CW and FM waveforms can be exploited only by an operator who actively
chooses between them. For the operator, to choose the transmission waveform for each ping
presents an unreasonable burden.
In this thesis, we devise and evaluate a decision algorithm that decides which waveform
to transmit based on the target's predicted state. Building on the intuition of selecting a
waveform based on the target's Doppler, we propose the Predicted State-Based Selection
(PSBS) algorithm, which uses an estimate of the target's Doppler, derived from a state
prediction produced by the tracking filter, to select the waveform for the next ping. A sys-
tem consisting of a target path simulator, measurement data simulator, and tracking filter
is modeled to conduct Monte Carlo simulations of PSBS. Its performance is evaluated by
comparing it to the performance of transmitting either CW always or FM always. Simula-
tion results show that the PSBS algorithm improves target localization estimates by 7.7%
on average from the next best performing waveform selection approach, over all target turn
rates considered.
While PSBS improves significantly upon using only CW or only FM waveforms, it
suffers substantial performance degradation for highly maneuvering targets. To address
this shortcoming, this thesis suggests an enhancement to the PSBS approach: EPSBS
(Enhanced PSBS) takes into account the number of measurements being considered by the
tracker when making a waveform decision. Simulation results show that EPSBS eliminates
the performance degradation observed for PSBS when targets undergo rapid maneuvers.
Based on results obtained in this thesis, we conclude that intelligent selection of transmitted
waveforms` can significantly improve tracking performance in active sonar.