How Does Sentry Work?

As an AUV, Sentry is given a mission before each dive that contains lines of code telling Sentry where to go, what to expect, and a timeline for the dive. The expedition leader works with the science leads to create maps and plans for sampling. With four rotating fins each with a propeller, Sentry has a wide range of motion that allows it to traverse the deep sea. Throughout each dive, Sentry communicates with the vessel using an Ultra Short Base Line (USBL) system and a WHOI Micromodem for acoustic communication. Sentry sends acoustic messages (“squawks”) relaying data relating to position, velocities, charge, and error messages to a receiver, the USBL pole, that sticks through the ship from the deck to under the vessel (you can hear these squawks from the ship!). Sentry also has a camera on the underside with a strobe that takes pictures consistently throughout the dive, which are retrievable once recovered. Sentry follows a rectangular path back and forth, “mowing the lawn” around sample sites, and when ready to surface, follows a denser path to provide an accurate resurfacing location where the research vessel may retrieve it.

What Does the Sentry Team Do on Board?

Before deployment the Sentry team does a deck test – making sure all the fins and propellers are moving properly, the SyPRID collectors open and close, verify times are synched, acoustic communications are working. In addition, before deployment, the new mission is uploaded to Sentry, the navigation system is set up, weights are attached, and data from the last dive is extracted and cleared. Once checked, the Sentry team detaches all cords, pulls off covers, and makes sure Sentry is ready to be in the water. Then, with the ship’s crew, Sentry is hoisted off the deck with a crane, ropes and taglines, and expertly transferred into the ocean. During deployment, the Sentry team has 4-hour shifts watching and communicating with the AUV. During these shifts the watch listens to Sentry and notes positions and velocities regularly, while staying alert for error messages and ensuring systems are working properly. While Sentry drives itself, manual movements can be inputted including slight changes in positions, noting areas to avoid, or when to abort the missions and resurface. Once resurfaced the team helps recover Sentry on board, unload the SyPRID samplers, ensures Sentry is supported properly, and conducts another deck test.

How Are We Using Sentry?

 On this cruise we have attached the SyPRID samplers that allow us to take plankton samples at different depths on and off-site. Sentry takes two samples per dive, one just above the bottom, and another at a predetermined depth. We conduct two Sentry deployments per site, one on-site and one off-site to compare larval compositions near our seep sites to a control. Sentry has been programmed to open one of the SyPRID samplers, “plankzookas” at each depth, which closes before moving to the next depth. This ensures we have an accurate sampling of only the target depth and site.

Meet the Crew

We love Sentry and their team, not only for assisting with our scientific endeavors and being fantastic lab mates, but also for allowing us to personally interact with Sentry. Sentry usually adds characters to the sides of Sentry using electrical tape, and the team allowed us to get involved on this cruise! As the glorious SyPRID plankton collector is named after the barnacle cyprid larva, we decided to decorate Sentry with larvae! Flip through our process here and the finished product! Sean ended up making the cyprid, while our team created the pluteus, bathymodiolus, and pelagosphera depictions.