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With VHF a handy formula is miles = 1.25sqrt(feet)

So 25000ft gives a range of ~190 miles, 40000ft gives ~240 miles.

Thanks! The target area is approximately 250 miles equidistant from Iceland & Norway, so it's feasible that one of these boats could cover the entire area (under ideal conditions).

ADS-B is not on a VHF frequency.

It is indeed UHF.

I should've put "VHF/UHF" in the original comment as the formula is reasonable for both. They're both Line of Sight and that's really what the approximation is about.

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