Feature ArticleSafe Navigation in Hazardous Areas: Forward-Looking Sonar for Submarines
Three-dimensional scene showing targets captured by the NDS SCOUT.
Submarine operations often transit through unknown and potentially dangerous waters. In addition to the anti-submarine warfare threat, risks of collision with underwater objects are ever-present, especially if detailed sea charts are not available. Air-independent propulsion technology for non-nuclear submarines enables travel with even longer transitions while submerged. To ensure a safe trip, high-accuracy obstacle avoidance sonar is required to prevent underwater collisions. Equally important is ensuring that navigational data and position information stay updated and precise.
With this in mind, L-3 ELAC Nautik (Kiel, Germany) developed the Navigation and Detection Sonar (NDS) SCOUT, which has three modes. While the submarine is cruising in dangerous or mine-laid waters, the obstacle avoidance (OA) mode detects objects within an area directly in front of the vessel to prevent collisions. As a navigational aid, the bottom-mapping (BM) mode provides depth information and creates sea-bottom profiles. For safe surfacing after long periods of submersion, the surface-mapping (SM) mode helps the submarine reach the surface without the threat of collision with drifting fishing boats or ships lying at anchor.
System Design and Signal Processing
The NDS SCOUT system consists of a Mills cross array for transmission and reception of acoustic signals. The transmit array is vertically oriented, while the receive array below is horizontally oriented. The transmit branch is built using a pulse-form generator that is implemented as a time-delay beamformer. Its output is amplified and sent to the transducers of the transmit array, which converts the electrical power into acoustical power.
In the receiving branch, the analog signals received by the transducers are amplified and then converted into digital signals. Signal processing begins with beamforming. Beam data are then post-processed according to the selected mode—OA, SM or BM—and the results are displayed on a human-machine interface (HMI). Information from motion sensors and sound-velocity sensors are used to receive and transmit processed data, making the system more stable in changing environmental conditions.
In OA mode, the emitted pulse consists of multiple pings using different frequencies that travel through the water at the same time. Each frequency is radiated to a different vertical angle, which is implemented by a time-delay beamformer. Vertical coverage can be increased by alternating the transmit angles for each frequency from ping to ping. A single plane has a vertical width of approximately 3 degrees and covers a horizontal swath of 90 degrees. NDS SCOUT provides a vertical coverage area of roughly 25 degrees.
To realize the full gain of this multifrequency transmission, the received signal is digitally filtered behind the beamformer with different bandpass filters. Each of the filters is designed for one of the transmission frequencies. Signal processing following the digital filters is performed separately for each of the frequencies. This would also apply for consecutive pings performed with only one frequency.
With the horizontally oriented receive array of the Mills cross configuration, it is possible to intersect each of the transmit planes with multiple receive planes by performing conventional beamforming. In doing so, a 2D grid of beam angles is created. The horizontal beam width of the receive beams is approximately 3 degrees, and the vertical coverage is large enough to cover the whole ensonified area. Taking the transmission and reception beam characteristics and the frequency separation into account, NDS SCOUT reaches a 2D beam resolution of about 3 degrees times 3 degrees. This new technology opens the door to high-precision direction of arrival finding for all detected objects. To continue this article please click here.
Martin Meister received his degree in electrical engineering from the University of Bochum in 2001. Since 2007, he has been working for L-3 ELAC Nautik's software department as head of the signal processing group. His main fields of work are active and passive sonars, algorithms and signal processing.
Gunnar Zindel received his degree in industrial engineering with specialization in electrical engineering from the University of Kiel in 2007. Since then, he has been working for L-3 ELAC Nautik's software department, with his main focus on designing active sonar systems utilizing systems engineering, algorithms and signal processing.