Digital-signal processing helps track underwater vehicles
In the past, tracking underwater vehicles with precision was a difficult task. Systems had to use short acoustic signals or "pings" to avoid corruption of the received signal from multipath transmission. As a result, signal energy was limited, and fairly high-power signals were required to obtain adequate resolution.
Now, Roger Stokey and Tom Austin, research engineers at the Woods Hole Oceanographic Institute (Woods Hole, MA), have used a spread-spectrum-signal approach to solve the problem. Using off-the-shelf A/D boards and DSP processor, the system receives signals from an acoustic transducer on the underwater vehicle that transmits spread-spectrum-coded acoustic signals on a 70-kHz carrier. These pings are received by an array of eight underwater hydrophones and associated preamps. Arrival times and the phase differences among the outputs of the hydrophones are then measured and used to compute the range and bearing of the vehicle.
Multipath signals, which arrive at the receiver delayed in time, are uncorrelated with the direct path and can be filtered out. Also, signal duration is no longer limited by multipath delay, and signal-to-noise ratio is improved by increasing ping duration rather than power. Because of this, the underwater acoustic tracking system can track tethered underwater vehicles with precision at ranges up to 1000 yards.
In the design of the system, amplified outputs of the eight hydrophones are sampled with the 4248--a 32-channel A/D converter board from Pentek (Upper Saddle River,NJ). "We selected the board because it has a fast sampling rate and simultaneous sample-and-hold on all channels-essential for accurate phase measurements," says Stokey.
Digital outputs from the A/D are then transferred over the MIX bus to a Pentek 4283 TMSC30-based pro cessor board, where the tracking algorithms are executed to compute range and bearing of the vehicle (see figure above). These algorithms were implemented using the SPOX DSP library from Spectron Microsystems (Santa Barbara, CA).
Through the vehicle telemetry interface, a transmitted coded signal is generated by a Pentek 4249 D/A converter board that drives a power amplifier, which in turn drives the acoustic transducer. The user interface is implemented on a workstation from Sun Microsystems (Mountain View, CA) running OpenWindows. This workstation communicates with a VMEbus 68030 host CPU through an Ethernet link. The control system allows the user to select different processing options and view selected waveforms for analysis.
According to Stokey, the system has completed tests in a water tank and is currently undergoing static trials in the harbor at Woods Hole. When these are completed, the system will be installed aboard one of the Woods Hole oceanographic vessels.
