Spiral sound beacon for simpler navigation of underwater robots
With the increased use of underwater robotics, underwater navigation becomes more and more important. A research team from Naval Research Laboratory, the University of Washington, and Naval Surface Warfare Center has developed and tested an underwater navigation system that uses a spiral shaped acoustic wave to determine aspect. The single stationary beacon can provide a navigation signal for any number of underwater vehicles.
Navigation by the satellite Global Positioning System (GPS) has become widely applied in modern life and many people have it in their cars and mobile phones. These systems can be accurate to within a few meters. Differential GPS (DGPS), which uses a fixed antenna as a reference, can be accurate to within a centimeter. Unfortunately, GPS signals cannot penetrate the water’s surface. Thus, various acoustic and inertial techniques have been developed for underwater navigation. Inertial techniques include accelerometers, like those popular in gaming consoles and gyroscopic compasses that can determine position by judging how the vehicle is moving relative to the earth. One acoustic technique available is known as “long-baseline”, which uses the distances to fixed sound sources, determined from the time it takes the sound to reach the receiver, to triangulate to the receiver’s position. Another popular acoustic technique called “ultra-short baseline” navigation measures the arrival of a single incoming signal using several hydrophones (underwater microphones) positioned on the same vehicle.
Under the water’s surface, sound waves can easily be made to form circular wave fronts, analogous to the pebble in the pond. The research team developed ways to produce sound with another type of wave front, a spiral wave front, where, instead of concentric circles, there is one continuous spiral shaped line emanating from the source. These can be stacked with two sources on top of one another to make a navigational beacon. When the team transmuted from both sources at the same time, the distance between the circular and spiral wave fronts does not change along a particular direction. Thus, if a hydrophone is placed at some position around the beacon, researchers could determine the aspect angle to the beacon by comparing the arrival of the different wave fronts. This navigation technique is also used by aircraft navigation and is called VOR (VHF Omnidirectional Range). However, VOR uses radio signals rather than sound waves.
Based on this concept, the research team had a beacon built by Thomas Howarth at the Naval Undersea Warfare Center in Newport, Rhode Island. To test the accuracy of the beacon, it was attached it to a dock on a pond about 3 meters below the water’s surface at the Naval Surface Warfare Center in Panama City, Florida. A remote controlled (RC) pontoon boat was equipped with a hydrophone below the water’s surface to determine aspect from the spiral wave front beacon and a GPS antenna above the surface to determine aspect using DGPS. The RC boat was driven around a pond and compared the aspect determination from the spiral wave front beacon to the DGPS result. Although not as good as the DGPS results, the results were quite accurate giving an error between 5 and 15 degrees across all of the data. The team also tested several different signal processing schemes, some of which worked better in different conditions than others.
The biggest advantage of this system over more traditional baseline techniques is its simplicity. A single stationary beacon can be used to navigate any number of remote underwater vehicles, and the remote vehicles need to have only a single hydrophone available. With future visions of swarms of underwater vehicles, this could provide significant advantage.