NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bowditch tables 12 and 13
From: Lu Abel
Date: 2012 Aug 14, 13:08 -0700
From: Lu Abel
Date: 2012 Aug 14, 13:08 -0700
Electromagnetic waves will "cling" to a conductive surface. The longer the wavelength (ie, the lower the frequency of the radio signal) the more the waves cling.
Our visible horizon is a tad further than pure geometry would indicate because light waves cling a little longer.
Radar clings even more, hence the few extra percent we get in radar's distance to the "horizon" than light waves.
The ultimate, of course, is ultra-low-frequency radio waves which go all the way around the earth. These were used in the late OMEGA system.
"Clingy" waves also penetrate the surface of the
water. Extremely-low-frequencies (even lower than ULF) are also used to communicate with submarines while they are submerged.
From: Jeremy C <jcaoy@yahoo.com>
To: NavList@fer3.com
Sent: Tuesday, August 14, 2012 7:18 AM
Subject: [NavList] Re: Bowditch tables 12 and 13
Notice that both formulae are exactly the same. If you look at the radar navigation manual (pg 12-13), it gives a formula and diagrams for radar waves (much longer wavelength than light) which is different. The visible light (optical horizon) is 1.17 verses 1.22 for radar (SHF radio).The distance derived from these formulae is actually a great circle path, so curved. If it was straight line (geometric horizon) the contant is 1.06.This is how I understand it. This stuff is why the radar picks up ships well before I can see it from the bridge, even with 50+ nm visibility.Jeremy
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