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Peter, you wrote:
" Technologies are just waiting for an excuse to not be reliable."

Yes, all technologies. Like chronometers, sextants, hell, the Nautical Almanac 
is calculated and printed by machines. Every celestial navigation book you've 
ever used has at least one error in it somewhere. Do we then say, "Celestial 
navigation, can't trust it, unreliable!" No, of course not. But the point is 
obvious enough. If you're going to claim that those waterproof bags are just 
another technology waiting to fail, then you have to apply that same standard 
to traditional methods of navigation. Do you store your chronometer in a 
waterproof bag? How about your spare chronometer?

What is the probability of a device-based problem leading to the failure of 
celestial navigation? Pretty low, right? Likewise, the probability of failure 
of a GPS receiver is quite low, too. How low? Well, let's suppose it's 
something like 1-in-500 on any given day at sea. That is, the vessel's main 
GPS system, either built-in or the expensive daily-use handheld has a 
1-in-500 chance of dying on any day when we need it. I think the numbers are 
actually better than that but let's go with this just for the sake of 
discussion. 500-to-1 against a failure is low enough that it might give us a 
false sense of security and high enough that we really should worry about it. 
So what to do... carry a cheaper handheld GPS that you can turn on when the 
main system fails. Since it's cheaper and probably tested by the owner less 
frequently, we should figure a higher probability that it will fail, let's 
say 1-in-100. So then what are the odds that your primary system AND your 
backup fail on the same day? The probabilities are multiplicative for 
independent events so we get 1-in-50,000. That means that you can expect to 
spend over a hundred years going to sea each and every day before you can 
expect even-money odds of having both fail at once. Those are damn good odds. 
As I noted in my previous post, it's important to ensure that the second GPS 
is not an exact duplicate of the first and not stored and powered in the same 
way. Otherwise you don't have independent probabilities. And of course, that 
backup should be stored to protect it from all those external injuries that 
might knock out the primary system. So keep it in a dry, shockproof, metal 
case.

Peter, you concluded:
"You're missing my point, Frank."

I have patience for your points, Peter, even when I think you're flat-out 
wrong, and I am always willing to reconsider them. So please try again if you 
feel like it. Can you explain your point from a different angle? Let me ask 
you this: if you met someone planning on doing some long-distance sailing, 
and they said to you, "I'm planning on taking a carefully stored handheld GPS 
for backup in case the main satnav system fails", would you advise AGAINST 
taking that backup GPS?? Do you not agree that the very great majority of the 
cases where the main satellite system fails can be answered simply by 
breaking out the spare and using it properly?

-FER



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