From: Fredmoreno@aol.com
Message-ID: <5fcdc16d.24b99d35@aol.com>
Date: Sun, 11 Jul 1999 03:09:41 EDT
Subject: GPS antenna location on LIV
To: lancair.list@olsusa.com
Mime-Version: 1.0

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	I read Michael Smith's concerns about GPS antenna location.  Since I 
am planning to use the same radio and the same antenna location, I did some 
measurements.  Using the side view drawing of the Lancair IV I measured that 
the top of the fuselage rises about 8 degrees above antenna location.  
However, the key it seems to me is the blockage of the antenna's view above 
the horizon.  The top of the fuselage is about 11 feet from the antenna.  The 
half hemisphere of radius 11 feet that starts at the horizon has a surface 
area of about 760 square feet.  If we assume that the portion of the fuselage 
that sticks above the horizon and blocks the antenna is a semicircle of 
radius 2 feet (it does not actually stick up this high), the area blocked by 
the fuselage is 6.28 square feet (actually less).  That means the fuselage is 
blocking 6.28/769 = 0.008 or 0.8% of the view of the sky, and that is only a 
small bump next to the horizon where GPS geometry is least advantageous.  My 
guess is that the vertical stabiizer blockage is comparable, and also has a 
negligible effect.
	I would guess that biz jets with and King Airs T tails likely block 
comparable amounts of the sky for antennas mounted on the top of the 
fuselage.  MOreover, other LIV's are flying with certified GPS installations 
(other radio manufacturers) with the antenna location as described.  
	My guess is that there should be no problem making the case via 
calculation to the FAA that reception reliability is not degraded.  I would 
ask for clarification of the 5 degree interpretation, and support my inquiry 
with calculations before I started on the wild goose chase of running a 
comprehensive set of experiments.  
	Please let us all know how you choose to proceed and how it turns 
out.  

Fred Moreno
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