Explorations | PHAEDRA 2006 | Logs

Where Are We?

June 30, 2006
36N, 25E

Brian Bingham
Professor

One of my colleagues likes to say, "start by figuring out where you are." Navigation, the art and science of determining were you are, is an important part of deep-sea science and exploration. Dr. Richard Camilli invents amazing new devices for sniffing out chemical traces, making it possible to see into a world previously unexplored. Knowing where the sensor is, putting his measurements onto a map of the seafloor, transforms experiments into systematic scientific exploration.

My job during this cruise on R/V Aegaeo is to help determine were 'we' are. When we arrive on station I work with the ship to deploy transponders - small automatic acoustic elements that serve as beacons. By knowing where these transponders are, we can use geometry to determine our location. This is very similar to the way GPS works, but unfortunately GPS does not work in seawater (the electromagnetic waves that carry the GPS signal only travel about 1 mm below the surface). Once our transponders are anchored on the seafloor we are ready to deploy a vehicle: the Thetis submersible, the Achilles ROV or the Seabed AUV (or maybe even more than one!). While we are exploring the seafloor these transponders constantly report our position relative to the fixed points. This information is accompanied by a Doppler velocity log (DVL) - a sort of underwater speedometer. If we keep track of our speed, our heading and our transponder messages, we can methodically and systematically survey the seafloor - creating detailed maps of our sensor data. The quality of this map is directly related to how well we know our position. When it is all over and time to move to the next stop we must collect these transponders. The transponders respond to a particular message, floating back to the surface for collection.

Navigation is one of the underlying technologies making possible all of what we do in exploring and understanding our oceans. The methods and instruments we use are constantly getting better, but it still takes patience and expertise to know precisely where you are. Just like the ancient mariners, relying on the location of the stars and environmental clues (cloud types, wave shape, and seabird presence), we use everything at our disposal to get better and better at knowing where we are.

Underwater Navigation Methods

Dr. Ryan Eustice
University of Michigan Assistant Professor

Since GPS does not work underwater, due to the attenuation of radio waves through the medium of water, engineers must resort to alternative methods for navigating the vehicles that we deploy. One standard method for navigation mentioned above has been the use of acoustic long-baseline navigation (LBL) - a technique developed in the early 1970's. LBL involves mooring acoustic transponder beacons on the seafloor, allowing each vehicle in the water to measure the round-trip travel time of sound signals transmitted and received between it and each beacon. With two or more beacons plus a measurement of vehicle depth, the position of the vehicle can be accurately computed. The major disadvantage of LBL, however, is that it takes time to deploy and calibrate the beacon network. Moreover, as the number of vehicles in the LBL network increases, the frequency with which each vehicle can communicate with the network decreases, meaning that LBL is only practical for a few vehicles at most. An additional limitation is that each beacon has a finite working range within which the vehicles can acoustically hear it; this limits the effective working range of the beacon network to one or two water depths.

On this cruise, we are experimenting with a new acoustic method for underwater navigation - a method that aims to eliminate the need for deploying acoustic beacons on the seafloor; this new method is called "Synchronous-clock one-way-travel-time navigation".  Our idea is to free the vehicle from a seafloor-anchored LBL beacon network by instead turning the ship into essentially a "moving beacon."  Each vehicle is equipped with an acoustic modem, a device which allows each vehicle to broadcast small bits of information between one another (you may be familiar with modems in your desktop computer, but the ones we use underwater are a lot slower due to the properties of transmitting through water).  By having very precise and synchronized clocks, on both the vehicleboard and shipboard modems, we can measure the one-way-travel-time (OWTT) from the surface ship to each vehicle in the network each time the surface ship broadcasts. Furthermore, since it's a modem, the ship can also use it to send its GPS position along with the OWTT packet, meaning that each vehicle now has a range to the ship and knows where the ship is according to GPS.  Based on this information, each vehicle can now compute an estimate of its own position by fusing other onboard sensor data, such as from the doppler velocity log that Dr. Bingham mentioned.  With this new technique we hope to free ourselves from the burden of deploying LBL transponders and enable us to more rapidly deploy and survey seafloor sites with multiple vehicles.

 

 

(top)

---

Sign up for the Ocean Explorer E-mail Update List.