Wednesday, April 19, 2017

Boaty's second big adventure in the Orkney Passage (from Eleanor Frajka-Williams)

Here is the promised post about Boaty's second mission in the Orkney Passage, which took place during 12-14 April. This post was written by Eleanor Frajka-Williams of Southampton University, with some editing by Stephen Griffies of NOAA/GFDL and Princeton University. It should appeal especially to those interested in details of Boaty's engineering feats and some of what it does while beneath the ocean surface. 

ALR-1 (aka Boaty McBoatface) makes its second Southern Ocean mission!

Last week, ALR-1 completed its second mission in the Orkney Passage area. Now that we have had some time to go through the data recovered and navigation logs, we can update you on the progress.  But first, here is a bit of background on Boaty for those joining this blog a bit late.

Boaty slowly making its way into the abyss. A curious sea bird examined Boaty as it cruised on its way from the ship.  We have found sea birds to be forever curious of any large object, including whales, ships, and auto-subs!

What is an Autosub Long Range (ALR)? 

ALR-1 (Autosub Long Range, aka Boaty McBoatface) is an autonomous submersible that is roughly 4 m long and contains a mass of roughly 800 kg.  It has been developed at the National Oceanography Centre (NOC) in Southampton, UK. We have two NOC/MARS (marine autonomous robotic systems) engineers (Steve McPhail and Rob Templeton) on the cruise to build, program, deploy, troubleshoot, reconfigure, and generally make Autosub go. 
ALR-1 is capable of motoring around the deep ocean on its own, carrying oceanographic sensors to make measurements.  The platform itself (Boaty) must overcome several challenges with the conditions in the deep ocean, including extreme pressures, navigation without connection to GPS (global positioning system), climbing up and down underwater mountainous terrain without hitting the bottom, and doing this all powered by a few (well, quite a few) D-cell batteries.  Let us look at each of these challenges in turn.

Guiding the auto-sub Boaty into the waters of the Orkney Passage region of the Southern Ocean. That is Andy and Povl with arms outstretched, and Rob with the red line.

Challenge #1 - Intense pressures in the ocean abyss

Anything that goes to the deep ocean must withstand incredible pressures.  For the DynOPO project, Boaty ventured to 3000 m on its second mission, and plans to go to 4000 m on its third.  Three or four kilometres of water sitting above you offers a tremendous weight!  Seawater has a density of about 1030 kg per cubic meter (about 3% more dense than fresh water). Consequently, every component of ALR-1 must be designed with pressure hardiness in mind.   

The nice yellow exterior of Boaty is a simple fibreglass fairing.  It gives the ALR a hydrodynamic shape. But underwater, it is “flooded” meaning the water can pass into the yellow body.  Within the fairing are two anodised aluminium spheres that are pressure-housings, meaning that objects contained within the spheres are protected from the intense pressure and corrosion from seawater. The battery supply is within one of these spheres, and the electronics or “brains” of Boaty are in the other.  

The two spheres are built from two hemispheres, bolted together across a reinforced rim with an O-ring between them to ensure it remains water tight.  The hemispheres are then closed with a vacuum seal that is measured prior to deployment to ensure there are no leaks. The power cables and electronics cables are piped through the reinforced rim and are “wet-pluggable”, meaning that the cables can get wet, and the connectors are designed to keep water away from the important inner bits.  

Outside these two spheres, any other components (including specially designed foam for buoyancy and the various sensors) must withstand the pressures.  The vehicle’s control surfaces (i.e., the moving parts that enable control on the flight direction and speed) including the two stern planes, rudder and propeller, need to have the compressibility of their components accounted for so that differential rates of compression do not lead to unintentional “squeezing” or friction between parts that could impede operation at depth.

Rob and Billy preparing Boaty for a mission while it its container shed. Boaty has its cover removed, thus revealing some of the internal instrumentation, such as the acoustic transducers (brown circles in green housing) and CTD (silver/yellow cylinder).

Challenge #2 - Navigation

The next challenge is underwater navigation.  At the surface, ships and other platforms can use GPS (like your phone, or satnav).  However, GPS signals from satellites do not reach underwater, so anything underwater needs an alternate means of navigation. The simplest means of navigation is dead reckoning, where you use a compass to determine which way to go. If you know how fast you are going, then you can work out when you have arrived.

Boaty does an improved version of dead reckoning by using an onboard velocity meter called an ADCP (Acoustic Doppler Current Profiler) essentially the same as the one installed on the ship's rosette and discussed in an earlier post to this blog.  When it is operated in “bottom track mode”, Boaty can accurately measure its speed and direction relative to the bottom. In this way, Boaty can integrate the speed against time to determine its location. There is some error in the calculation (measurement uncertainty, heading misalignment, and otherwise), which are expected to result in a 1% position error relative to distance traveled. 

Over Boaty's Mission #2, it went 111km (about 71 miles—the distance between New York City and Philadelphia, or between London and Southampton). At the end of this exercise, it should have been missing its waypoints (points where navigation checks are made) by roughly a kilometre. However, it did far better, by passing waypoints within 200 m (less than half-a-percent error).  

Tracking error is also accrued if the vehicle loses information about where the bottom is located.  ALR was programmed to drive along bathymetry (i.e., bottom topography) at 100m off the bottom or 4000 m, whichever is shallower. Hence, if the ALR ever found itself over 4500 m of water, instead of continuing to track the bottom, it would rely on the compass and “water track” velocities only, skimming along at 4000 m.  Likewise, if the ship is in 2000 m of water when ALR is deployed, and it spirals down to the bottom over a couple hours, then during this time its navigation will have drifted due to movement with ocean currents.  For this reason, after deployment and when the ALR has reached the bottom, it can be told to maintain a circle around a waypoint.  We can then acoustically communicate with the ALR using “the fish” (a transducer that is lowered 10 m down), which allows us to receive diagnostic information and to send navigation updates.  

Navigation updates consist of telling the ALR that it is actually X metres east and Y metres north of where it thinks it is.  Once an update is sent, the ALR will zoom towards the actual waypoint and then continue along. During the first ALR mission on this DynOPO cruise, we updated navigation twice using the fish.  After the first mission, Steve made a correction to the navigation using the actual positions versus where the ALR thought it was. For the second mission, an update was only needed immediately following deployment, after the vehicle reached the bottom.

Brief aside

Tracking Boaty has been a fun exercise on the ship.  We have acoustic tracking on the ALR, meaning that there is a beacon installed that pings back when it hears a ping from the ship.  When Boaty is in the water, and we are stationary, such as when we are at a CTD or VMP station, we will generally have the pinger on. The fixes are displayed on a monitor in the UIC lab, and recorded onto a network drive on the ship.  I have been using Matlab to pull down the fixes in real time and plot up a bulls-eye with the nearest waypoint at the centre.  If ALR comes within range (2-3 km), it will display the position and range to waypoint. When the ALR is within a certain threshold of the waypoint (100 or 200 m), it will sound a gong.  It also computes repositioning information in the case ALR needed an update to its navigation.

An example of the waypoint software from Matlab, this one taken from Boaty's third mission which is now ongoing. The bull's eye centre is the pre-specified waypoint (this one is ''Waypoint 2'').  The red dots monitor the track of Boaty as it approaches and leaves the waypoint, with the blue triangle marking the latest position where the ship's USBL (acoustic tracking device) found Boaty. In this case, Boaty had lost bottom track mode and the navigation was out by about 100 m (hence the overshoot of the waypoint).  This issue was identified, and a correction was acoustically transmitted to Boaty. You can see the southward jog in the track after the waypoint where Boaty had incorporated the navigational update.

Challenge #3 - Steeply sloping bathymetry

The Orkney Passage (OP), where the DynOPO projected is located, is an area of very steeply sloping bathymetry.  The OP is like a mountain pass through a mountain range which separates the Weddell Sea to the south from the Scotia Sea to the north.  Along this pass, which winds between peaks (including islands), the slopes along the sides can be up to 1 in 2, which is a 45-degree angle.  It is hard to imagine how steep this is, but perhaps you can find an equivalent on land somewhere?  Perhaps the stairs to your attic?

This underwater mountain range forms a barrier to the waters flowing cyclonically (clockwise; in the same sense as the earth rotates when viewed from above the South Pole) around the Weddell Sea, hindering these waters from reaching into the rest of the world's oceans. Yet some water does escape.  When it does, and it encounters the relatively shallow bottom depths in the Orkney Passage (3500 m)this narrow channel concentrates the currents resulting in persistent deep flow of 30 to 40 cm/s (which is very very fast for the deep ocean).  

When the ALR waypoints were given, we had several discussions back and forth between the ALR engineers (Steve and Rob) and the science party in order to optimise the data without putting the ALR in danger.  In the end, it turned out that ALR had little trouble navigating against current speeds of 50 cm/s and moving up and down steep bottom slopes. Our only real concession was to choose paths to avoid sending it near to one of the underwater waterfalls in this region.

Kurt Polzin next to Boaty, examining test readout from the microstructure device that measures ocean turbulence. Checks and double checks are essential to ensure reliable data is returned from Boaty's mission. 

Challenge #4 - Collecting oceanographic data

The final engineering challenge for this observational platform is how to record useful data about the ocean.  For the DynOPO project, the instruments that ALR carries include familiar oceanographic instruments, such as a CTD to measure temperature, conductivity and pressure, and an acoustic velocimeters to measure water currents and the speed of the vehicle over ground. It also carries more specialized equipment such as a fluorimeter to measure biological activity of small, single-celled organisms, as well as a Rockland microRider to measure tiny fluctuations in the ocean temperatures and currents (like the VMP) in order to map ocean turbulence.  

During its first mission in the Orkney Passage, there were some electrical contamination problems due to the microRider being energized along with Boaty's communications.  After these issues were identified, Steve and Rob electrically isolated the power and ground for the microRider, and operated it by setting it to run in a self-logging mode. The second mission produced excellent turbulence data.  Indeed, we have a preliminary estimate on the noise floor of 10^(-10) W/kg, which means there is lots of room for mixing signals above this rather low noise floor. 

 The first mission also had an issue with the ADCPs. Hence, these were reprogrammed for the second mission, with a resulting retrieval of 100 m ranges of velocities around the instrument.  We were able to use this velocity data to map out the bottom currents and bottom temperatures along the western flank of the Orkney Passage. We have used that information to inform more targeted surveys with the CTD and VMP profilers.

Some data returned from Boaty's mission #2. The red and blue path follows Boaty, with colours indicating the temperature along its path.  The vectors are the ocean currents.  The background green/yellow is the bottom depth. Boaty zig-zagged from a shallow region of the Orkney Passage, around 500m deep, downslope towards the Orkney Deep.

Challenge #5 - Interpreting the oceanographic data

The data from Boaty are unlike most other oceanographic datasets. Rather than moorings (which offer time series at a fixed point in X-Y-Z) or CTDs/VMPs (which provide vertical profiles at a fixed location in X-Y), the ALR retrieves properties along its moving position roughly 100 m off the bottom. The data is relatively high frequency sampling (1 second sampling on CTD and ADCP, and 1 second bins on turbulent microstructure).  While the data are limited to a single depth (in the case of the CTD and  microstructure) and a roughly 100 m depth range for ocean current velocities, the data allows us to map out the evolution of properties at very fine spatial scales in a region of particular interest for our science. For the purposes of the DynOPO project, which is focused on the deep overflow waters through Orkney Passage, the data offer us a hugely valuable and unique perspective on dynamics in this region of the deep ocean.

Preliminary assessment after two missions to the Orkney Passage

ALR-1 is a complicated piece of machinery working in one of the most extreme places on this planet. There are numerous opportunities for things to go wrong. Troubleshooting options are limited to what can be found onboard, as we have no way to ''FedEx'' a spare part. This sort of work offers an ongoing challenge, taking years of patience, creativity, and persistence in order to realise its full capabilities. Even so, both Orkney Passage missions for ALR-1 Boaty McBoatface have been amazing successes for both the science and engineering. We therefore trust that Boaty, and its siblings, will soon become routine and trusted tools for seagoing oceanographers in the 21st century.

A curious sky one afternoon while deploying Boaty in the Southern Ocean.  This was the day when a number of whales were feeding near the ship. Note the sun rays lighting up the distant sea on the horizon.

An anecdote from Stephen 

Stephen here again: In closing this post from Eleanor, let me share one interesting anecdote about what can happen doing science at sea.  Prior to Boaty's third deployment just yesterday (18 April), many of us noticed an unusually large number of southern right whales feeding near to the ship. Whilst Steve and Rob were busy preparing Boaty for its mission, others were gawking at the whales. One of crew members said something like: ''I wonder if Boaty is able to navigate around so many whales?''  As it turned out, Boaty had no problems with whales, largely since whales are quite aware of their own huge presence even around such an odd looking mechanical fish like Boaty. But what did catch Boaty by surprise was the massive amounts of krill in the water. Boaty's acoustic signals are meant to detect the ocean bottom. Instead, when surrounded by so many swimming krill, the acoustic waves scattered off the krill and told Boaty to surface since it thought the krill was an unexpected bottom!  After a bit of troubleshooting, Steve and Rob instructed Boaty how to distinguish the bottom from krill, after which point Boaty was soon off into the abyss for Mission #3. 

A cartoon sketch from artist Susan Davies, the mother of mooring expert Andy Davies from Woods Hole. Her sketch is based on a photo taken from the first Boaty mission (see the 5 April blog post). Rest assured, Boaty was securely tethered to the winch cable, and only flew after being well within the ocean waters. 

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