Page 52 - Winter 2020

FEATURED ARTICLE
Global Positioning Systems: Over Land and Under Sea
Lora J. Van Uffelen
Introduction
Imagine with me a pre-COVID world. We are at an Acoustical Society of America (ASA) meeting in, say, Chicago, IL. We’ve just enjoyed a stimulating afternoon session, and our brains are fried. We need to find a coffee shop for a chat and some caffeine. What’s the first thing we do? We quickly pull out our mobile phones and open a Yelp or Google Maps app to find a location within a five-minute walk of the conference venue with a four- or five-star review, and we are on our way, following turn- by-turn directions until we reach the destination. This
mapping solution is delivered courtesy of a Global Navi- gation Satellite System (GNSS).
It is hard to imagine a world without GNSS. Even during quarantine, when we cannot be out having coffee with our colleagues in a new and exciting city, the motivated among us still use mapping applications to chart out our neighborhood walk or bike ride to see how far we have gone. We can “drop a pin” or share our location with the push of a button and find a friend in a parking lot or in the middle of the woods.
This positioning has become indispensable in the land, air, and space domains; however, as the electromagnetic signals sent by satellite systems do not transmit well in water, they are not available for undersea applications. Acoustic signals, however, propagate very well underwa- ter and are commonly used for navigation of underwater vehicles as well as tracking marine mammals, fishes, tur- tles, and even lobsters. Typically, this tracking is done at short propagation ranges, but long-range signals can be used for positioning as well. Would it be possible to have a “Global Navigation Acoustic System” for the underwa- ter domain that would be an analogue to the GNSS that we have become so reliant on? To answer this question, let’s first familiarize ourselves with the GNSS.
Global Navigation Satellite Systems
Positioning from the GNSS is available all over the globe to provide localization, tracking, navigation, mapping, and timing, all of which are closely related but separate applications. Their availability and use have transformed the world in which we live. The most obvious relevancy of a GNSS is for the transportation industry. Mapping and route-planning applications include traffic avoidance features that have saved millions of dollars, reduced emis- sions, and limited time wasted in traffic. Aircraft pilots rely on the GNSS among other instruments when visual observations are not reliable.
Even the agriculture industry has been revolutionized by the GNSS. In the current age of precision agriculture, positioning systems in tractors and farm equipment can have centimeter accuracy to ensure that all of the fields are covered without driving over the same area twice. Snowplow operators also use the GNSS to locate edges of roads covered in snow.
Scientists who do field work, whether on land or at sea, would be lost without the GNSS. We rely on these satel- lite systems to locate our sensors and associate data with a position on the earth. Metadata for any type of dataset, acoustic or otherwise, typically contains time and loca- tion data provided by the GNSS.
The modern military uses the GNSS for guided missiles and drones to minimize collateral damage. In fact, the Global Positioning System (GPS), the US-based GNSS that we used to find coffee on our hypothetical trip to Chicago, was originally developed for military purposes. Before the development of the GPS, it was the task of several soldiers, sailors, or pilots to navigate the troops, ships, or planes. Tasking this to a remote and automated system minimizes the number of people involved in the
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52 Acoustics Today • Spring 2021 | Volume 17, issue 1
https://doi.org/10.1121/AT.2021.17.1.52