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Poly doesnt explain all the issues with phones and triangulation.
In a town the cell size is small perhaps 1km. In rural areas it can be 25km maybe more. So in town your phone can reach its nearest tower and others. Triangulation joins that data together, and uses signal strength to get an estimate of position. Switch off WiFi and GPS and your smart phone may be able to locate you at least to town level. 999 services are not routinely supplied that...
If you go rural the accuracy of that drops. Even in a village near me I can show as being in 5 different locations all within about 7 miles if where I actually am.
At sea you don't have masts all around they are all on one side that makes the algorithm dodgier, if you are lucky enough to be hitting more than one tower. In reality even 2 towers won't give a fix.
Then the other issue is best highlighted by a mayday from a canoe at the entrance to the Tyne yesterday. Their call was overheard by Tyne Pilot vessel so it responded as well as two RNLI boats and I suspect a chopper was probably launched and then cancelled. There will be a mayday relay following a phone call but there will be a delay.
On that same incident I'm not 100% sure if you made that call you'd need a GPS position. Yes if I had DSC I'd press it, if I had HH GPS I'd give it, if I had PLB I'd activate it. But 'I am 200m east of the North Pier at Tyne Harbour' would be good enough. If you could describe your pos with that accuracy and are sure you won't be out in zero vis' then GPS wouldn't be my priority.
DSC will increase range. 5w Handheld will do 3-5miles to another handheld. Square the power and you double the distance so a 25w radio on the same aerial might reach 6-10miles. Stick it up a yacht mast and you can increase 25W to 25W to 20-30miles. To a CG tower people say up to 50miles may be reasonable from a yacht mast and its common in the right conditions to get further than that. I've heard Swansea in the Solent! 70 miles is often quoted for DSC to CG tower from (upright) yacht mast.
You might achieve 40% more distance than voice on a HH DSC. *but* your biggest challenge on a small boat may be cliff shadows rather than distance out. If I was heading out and relying on a data message because i thought voice might fail when the sh*t hits the fan I think I'd prefer to use a PLB.
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