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  Figure 5. Global coverage of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) International Monitoring System (IMS), shown by a 3-dimensional model of low-frequency (<50-Hz) propagation. The property of reciprocity is invoked by placing sources at the locations of the six IMS hydrophone listening stations (red areas) from where the sound radiates. Colors represent transmission loss with a range of 70 dB. Figure created by Kevin Heaney and reproduced from Heaney and Eller, 2019, with permission.
Ocean as well. In a 2019–2020 experiment, 35-Hz signals were transmitted across the Arctic Ocean over the North Pole (Worcester et al., 2020).
TheelectromagneticsignalsbroadcastbyGNSSsatellitesare outsidethevisiblespectrum,sowedonotnoticethesignals that are continuously emitted by the satellites. In addition to the engineering challenges that would face continuous acoustic transmission, the frequency band of long-range propagation is within the hearing range of many animals, and the impacts to the environment, including potentially masking marine mammal vocalizations, would need to be considered. Long-range acoustic transmissions for scien- tific purposes go through an intense permitting process that takes into account the environment and the impacts on marine animals in the environment.
Each GNSS satellite broadcasts navigation messages that includes the date and time as well as the status of the satellite. It broadcasts ephemeris data that provide its specific orbital information for more precise localiza- tion of the GPS receiver. Localization using dedicated networks of sources, such as the example in the Phil- ippine Sea, which incorporates precise source position and timing as necessary for localization of an acoustic receiver as it is for GPS has been discussed. A vision for
a multipurpose acoustic observing system (Howe et al., 2019), would transmit this information as well to enable mobile platform positioning and navigation. Such a system could also provide ocean acoustic tomography measurements and passive acoustic monitoring for bio- logical, natural, and anthropogenic sources.
Final Thoughts
The GPS satellite constellation was originally designed to meet national defense, homeland security, civil, com- mercial, and scientific needs in the air, in the sea, and on land. The age of artificial intelligence and big data has made GPS data on land incredibly useful to all of us in our everyday life. Not only can we use information on our own location from our cell phone to find the near- est coffee shop, we can take advantage of the location information on many different devices to look at traf- fic patterns to gauge what is the best way to get to that coffee shop. It won’t be too long until we will be riding in self-driving cars, automatically taking the best route and precisely positioned relative to each other. All of this happened in just the last few decades because it has been only 25 years since GPS became fully operational.
An underwater analogue to a global navigation satel- lite system would revolutionize any operations in the underwater domain including oceanographic science, naval military applications, underwater vehicles, and even scuba diving. Acoustics is the most promising way to approach this on a large scale.
I extend my gratitude to Arthur Popper, Kathleen Wage, and Peter Worcester for their helpful suggestions and acknowl- edge the Office of Naval Research (ONR) for supporting my work related to underwater acoustic positioning.
Chamberlain, P. M., Talley, L. D., Mazloff, M. R., Riser, S. C., Speer, K., Gray, A. R., and Schwartzman, A. (2018). Observing the ice- covered Weddell Gyre with profiling floats: Position uncertainties and correlation statistics. Journal of Geophysical Research Oceans 123, 8383-8410.
Dall’Osto, D. R. (2019). Taking the pulse of our ocean world. Acoustics Today 15(4), 20-28.
Duda, T., Morozov, A., Howe, B., Brown, M., Speer, K., Lazarevich, P., Worcester, P., and Cornuelle, B. (2006). Evaluation of a long- range joint acoustic navigation/thermometry system. Proceedings of OCEANS 2006, Boston, MA, September 18-21, 2006, pp. 1-6.
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