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Antoine,

I agree with Robert Bernecky that wolframalpha.com is probably the best place for you to find somewhat exotic calculation tools. Here's a link to his example again.

But now that you have lots of ideas, I have to ask the big question. You wrote originally:
"For Navigation purposes I need to transform radians into degrees with at least 16 significant digits (and - better - if possible to 20 or even 24 significant digits)."

Why? Are you navigating at Angstrom-level precision on the Earth? Measuring the distance from New York to Paris to the nearest atomic diameter? That would require 17 significant digits. Are you navigating across the Milky Way Galaxy at centimeter precision? That would require 24 significant digits. Clearly not... You don't need extended precision for any practical navigation problem, right? You don't even need double precision (16 digits). For normal celestial navigation, if you can convert degrees to radians and back with 7 significant digits, then you've got all the precision you need and some to spare. And that level of precision, of course, is available in nearly every handheld or online calculator.

Frank Reed

PS: Yet another way to do math to so-called "double precision" (16 digits --far more than you need) without using an online calculator, even when off-network in the middle of the ocean on any computer or even a smartphone, is to type javascript expressions into your internet browser's address bar. For example, type "javascript:document.write(Math.sin(Math.PI/4))" in your browser's address bar (not in search --you have to type it in the bar at the top where you normally see http: etc.), and it will display the result. It's all boiler-plate except the code inside the parentheses. In other words, if you type javascript:document.write(1), your browser will display the number 1. This isn't really all that practical, but it can be an interesting way to experiment with your browser's capabilities and security features. For example, you have to type "javascript:" manually in most browsers; copy and paste won't work. And it demonstrates that calculation capability is ubiquitous. A modern internet browser is a "Swiss Army knife". It can do anything.