My friend and Lancair Legacy 2000 builder, Rob Logan, posted the following on a Lancair Forum and encouraged me to cross-post it here to get folks’ comments…
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From: Rob Logan Rob@Logan.com
Subject: WAAS or not.
Date: Sun, 02 Jan 2005 00:09:24 -0500
WAAS corrected GPS are all the rage. It offers:
- takes GPS from 6.0m to 3.2m Horizontal accuracy with 95% confidence.
- promise of newly written vertical LPV guided approaches.
- promise of maybe getting “Sole means” en route blessing.
It’s true WAAS improves accuracy:
http://gpsinformation.net/exe/iono-day.gif Ionosphere temp
Two of them with lower mins! so one does get greater capability today
with WAAS corrections. But this way over budget, way late government
project also came with a mild threat by Administrator Blakey at the AOPA
convention: “if you are slow to equip [with WAAS], there will be folks
who will say there aren’t enough users in the system and push to move
the money away from developing and maintaining procedures for all those
airports I talked about.”
Usershttp://www.freeflightsystems.com/ the first WAAS corrected GPS,
started complaining about losing position reports. Then CNX80 (second WAAS
corrected gps) users also started noticing failed in flight position
reports. When I started playing with a gps18, it worked fine on the
ground, but when I turned away from the southern sky it would freeze its
What’s going on here? While I’m not saying GPS is junk
transmitting corrections to (Inmarsat IIIs: POR (Pacific Ocean Region)
These satellites are 22,300 miles above the equator vs 12,600 miles
mostly over our head. If we look at the angle of the satellite TV
dishes in our neighborhood, they point low on the horizon to get a
satellite above the equator. That one signal is going twice as far,
through more ionosphere. And unlike your satTV dish, the antenna must
be omni directional, making it impossible to achieve any forward gain.
When we lose this one signal, we lose the whole position solution.
So is going from 6.0m to 3.2m with 95% accuracy worth the increased loss of
availability? Wouldn’t we increase safety by providing some information
rather than no information? (this is a big one for the FAA)
Perhaps with some new antenna technology, my point will be mute, but the
3 WAAS receivers I’ve tried, and all unlocked without southern sky.
Heck, this new magic antenna will increase the reliability of
wsi and xm weather service, as they face the same weak signal challenge,
and are unavailable in an uncorrected way. (the weather service, Merlin,
failed because they couldn’t make an antenna that worked)
Is accepting the added risk of a more complex system worth it?
FADEC vs mags
EFIS vs HSI
WAAS vs GPS without corrections.
Bonus question: is a TSOed C129A receiver with Receiver Autonomous
Integrity Monitoring (RAIM) better than a 12 channel all in the sky
BTW, CFS has been offering barro corrected GS on all approches for years,
without WAAS or a TSOed GPS.
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So what do you folks think?