EXPERT REACTION: Submersible visiting Titanic wreck reported missing

1.       GPS doesn’t work underwater, and even if it did it wouldn’t help.
The GPS signal doesn’t travel though salty water, so even if Titan has a GPS receiver it won’t be able to detect the GPS signal if it is submersed. If Titan is the not submersed and it has a GPS receiver as its communication systems are down, it won’t be able to report its location.

2.       If it is on the surface, can we observe it from the air?
Even in good light it still may be hard to see Titan. Its white colour could easily be lost in the waves, depending on how rough the see is. Furthermore, as the search area is large it will take a long time to survey the search area.
Airborne radar could be used to try to detect Titan but if it has a low radar cross section it will be hard to distinguish radar signals returned from waves with those returned from the vessel itself, especially if the seas are rough. Again, as the search area is large it would take a long time to survey all possible locations of the vessel. The search area will expand as time goes on as the vehicle will drift.

3.       If it’s below the surface, can we use sonar?
Yes sonar is being used. Sonar relies on sound waves being sent and the “echo” of the vessel be heard sometime later. The time delay between to the return of the sound wave tells us how far away the vessel is. Using a bunch of sonar receivers, we can determine the location of the object that the signals bounce off. The problem is we must be close enough to “hear” the echo and we have to be able to distinguish the echo of Titan from marine life and other objects, such as shipping containers.
We assume that the submersible has an acoustic pinger to locate it, this is a mandatory requirement. Given the depth, it is possible that it may require a deep hydrophone to pick up the pinger signal. This may be delayed by the bad weather conditions up top.

4.       Are there other ways?
If Titan is sending out any signals we can try can detect them and locate the emitters and hence the vessel. This is called passive localisation.
If Titan is making noise, either by a propeller or people talking it may be possible to pick up the sound and locate the vessel in the same way that sonar works without the need to send out a signal.
If the vessel has radio transmitters onboard (such as data links, mobile phones or CB radios) it may be possible to detect these signals from an antenna array and locate the source. This will not work if Titan has “gone dark”.

This is a needle in a haystack situation but by combining, visual observations, radar, sonar, and passive techniques hopefully Titan will be found soon.

If they find it below the surface, they could use a deep submergence remotely operated vehicle to hook a tether on to the submersible and then lift it up using a surface ship.

Using sonar to locate submersed objects up to 3 km underwater can be challenging, considering the signal loss and complex environment. The best approach would be to employ a remotely controlled undersea unmanned vehicle with active sonar capability to detect reflections from potential objectives and to identify them. Sea surface scanning by airborne radar and the use of sea surface sonobuoys is definitely helpful. However, the coverage will be limited to objects near the sea surface. A search effort combining all of these systems would give the best outcome. Detecting submersed objects such as submarines from airborne platforms has been one of the major research challenges for defence.

Why is it difficult to localize and communicate the Titan?

As there is no tether between the Titan and the support surface ship, the only way to communicate or detect from the surface is through the sea water.
 
In the atmosphere, detection is performed by radar and communications are instantaneous thanks to electromagnetic waves across long distances (several kilometres). Unfortunately, under the surface, the sea water is blocking propagation very quickly (see image): no radar, no GPS, and spotlight or laser beams are absorbed within a few meters.

A submersible like the Titan can mostly use acoustic (sound) sensors and communication equipment, and the submersible crew, under normal circumstances, can text messages transmitted via an ultra-short baseline (USBL) acoustic system.
 
What could possibly have gone wrong with the Titan?
 
Four major risks could have caused a loss of communication:

1. Power blackout: the Titan runs on batteries and a loss of power would means loss of communications. It is unclear if power back-ups (Uninterrupted Power Supplies) are on the Titan at this point in time. This is why some submersibles have two independent sources of energy (electrical and other such as compressed air or hydraulic) to power safety systems.
2. Fire: potentially from electrical origin (short circuit) that would cause a fire onboard. On top of ruining the submarine systems, the immediate danger is creating toxic fumes that would poison the atmosphere and intoxicate the passengers, potentially incapacitating them, unless they have access to Built-In Breathing Systems (sorts of oxygen masks)
3. Flood: submersibles are facing increasing pressure the deeper they dive. At 4,000 m, that is the equivalent of more than 4,000 tonne applied on area 1 square meter. Titan has a composite hull with inbuilt sensors that can withstand such pressure, but it requires a very accurate round shape. Any out of circularity defect could result in a near instantaneous implosion (less than 40 milliseconds) at such depths. We don’t know yet if any seismometers have detected such a phenomenon should the Titan have suffered that fate. Should some system failure cause the Titan to start to sink, systems such as safety leads (if fitted on the submersible) can be dropped to gain instant lightening and raise to the surface. It is a long climb though.
4. Entangling: strong currents can occur undersea, and submersibles stay away from anything that could entangle them and block them. A wreck, such as the Titanic, can present tricky spots if coming too close, where the Titan could get trapped and struggle to free herself.

 
What happens next for the Titan?
 
Priority is to localize and communicate prior to rescue.
 
It would require a submarine with both the ability to dive as deep as the Titan and, or, some active sonar to sweep the area.
We are not sure how close and fast such means can come to the area where the Titan dived.
 
Even military nuclear-powered submarines are limited to depths between 0-500 m and could only use their sonars (active or passive) to detect any sign of the Titan, without a possibility to get closer.
 
If the Titan were to be fitted with a distress acoustic beacon or signal ejector (flares with coloured fumes/ink floating to the surface), potentially coupled to a “dead man-switch” (if the operator does not consciously respond to the computer solicitation regularly, every hour for instance, the submersible automatically sends a signal to the surface), this would help the rescue mission.
 
The clock is ticking, and any submariner/submersible deep divers know how unforgiving the Abyssal domain is: going undersea is as, if not more, challenging than going into space from an engineering perspective.
 
Let’s hope for the Titan and her passengers that they can come back to the surface safely.