Again for the gazillionth time
@Dan_T,
@JohnnyC . . .the Lost-at-Sea/Adrift/Missing Diver Scenario is a special case of the
classic Man Overboard contingency. Using MOB devices in the VHF radio frequency range designed for immediate recovery of blue water sailors and open ocean racing yacht crews like a Personal AIS Beacon (e.g. Nautlius Marine Rescue GPS, or any of their products) may not apply and might not be the best solution to the "delayed" problem of the ascending diver after an hour or more of bottom time and submerged drifting decompression as needed, then surfacing into a low visibility weather condition and/or sea state (i.g. Rain Squall; Fog Bank; Swells; Strong Surface Current; Low Angle Blinding Background Setting Sun etc), and losing sight of the Diveboat Vessel & Crew.
The best overall SAR utility, and last resort when there are no vessels left to be seen within line-of-sight for VHF beacon reception & visual signaling (Strobe Light, SMB deployment etc), is activation of a direct to COSPAS-SARSAT satellite 406 MHz PLB, and a designated Emergency Contact that can confirm you as owner of the PLB distress beacon as well as provide information to where in the world you were diving at, and emergency contact cellphone numbers for the Dive Operation or Liveaboard that you are using in order to relay your exact GPS location coordinates to effect a timely rescue.
This is the problem when you try to adapt a Man Overboard (MOB) protocol and technology from blue water open ocean yacht racing to the unique scenario of an isolated drifting Diver Missing/Lost-at-Sea:
Where divers get confused in the application, and assume "instant" local alert and rescue capabilities of an AIS/DSC VHF radio unit (i.g. current Nautilus Products like their Marine Rescue GPS), over a 406 MHz PLB is in the classic MOB (Man Overboard) Scenario:
Tactically this is fine for example, if you're crewing a sailing vessel in the biennial Transpacific Yacht Race from San Pedro Calif to Honolulu Hawaii, and you fall MOB during rough seas at night: Along with your crew, there will be anywhere from twenty to forty or so sailing yachts out there with AIS/DSC within VHF range in the otherwise traceless open Pacific Ocean ready to immediately assist in picking you up.
AIS/DSC VHF MOB devices are not meant to replace but rather augment PLBs. That said, since they alert nearby AIS-equipped vessels, such as the boat from which the person fell overboard, to assist in the search-and-rescue effort, these devices can reduce rescue times substantially. A PLB, on the other hand works worldwide -well beyond VHF range- to summon rescuers. It’s important to keep a PLB around if venturing far out to sea and away from other boaters.
A Lost-at-Sea/Missing Diver can be considered a special case of a "delayed" MOB Scenario: Delayed in the sense that an elapsed normal recreational depth dive time is about 50 minutes to an hour -->if the Diver surfaces after an hour and does not see any sign of the diveboat, then the Diver has to manually call or activate a VHF DSC Distress Beacon such as the Nautilus Lifeline first generation or current Marine Rescue GPS, but the caveat still being the limited range of the VHF signal along with hoping there are other boats or land stations in the vicinity with VHF transceiver reception capability. Again, if you're at a remote divesite hundreds of kilometers away from the commercial shipping lanes or land VHF receiving stations, then the direct to satellite PLB distress beacon is your last resort.
Again IMO/IME, a tropical squall & thunderstorm in zero visibility is the most common post-dive surfacing condition with a potential adrift lost-at-sea worst case scenario which you should smartly prepare yourself beforehand with at least a PLB:
A Nautilus Marine Rescue GPS/VHF Radio Transceiver Beacon may not save you in this instance due to "rain fade" signal attenuation, and the chances that any boats with marine VHF Radio DSC/AIS capability may be out of reception range of your distress signal by the time the squall clears and the weather & sea state moderates again. . .
