Positioning at sea

Any GPS will give your location at sea if you are with the unit, however if the GPS is to be left unattended, there are a few alternatives, you could use a GPS logger, to store the positions in memory so that you can examine them at a later date, if you require the GPS to remotely send you the positions so that you don't have to wait to recover the GPS logger, then you have a problem, you can get GPS trackers that will store positions in memory and then either you the call the tracker by cell phone and request the locations or the tracker calls you (via cell phone) every day and sends you it's location, the problem is that unless the tracker is on the coast, it will not be able to use it's built-in cell phone to send or receive calls because it won't get a cell phone signal while at sea. The only other way for you the receive locations remotely would be to use a satellite phone link (rather than a cell phone link), I don't have any experience of satellite phone links but I'm guessing they will be very expensive

 

It depends how long the tracker needs to run before re-charging and what the tracker is housed in, in a car the tracker will use the car battery and the GPS is 'always-on', more portable units with an internal battery will only run a few days. Trackers usually have a GPS, a Cell phone and timer circuity to keep the unit is stand-by most of the time.

Here is tracker that uses a satellite phone link :-
http://www.findmespot.eu/en/index.php?cid=130&from=frontpage_subbanner
This unit costs 125 Euros plus 100 Euros per year for the satellite link

 

You wouldn't fit the 'Spot' in a bottle anyway :-

I don't know of anything that would do that at any price