WAAS - Good or Bad ?

weinhous · January 2, 2005, 10:21pm

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…

============= starts here ==================
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://www.navcen.uscg.gov/Ftp/gps/status.txt
http://users.erols.com/dlwilson/gpswaas.htm
http://waas.stanford.edu/metrics.html
http://www.gpsinformation.org/dale/dgps.htm
http://www.edu-observatory.org/gps/gps_accuracy.html
http://gpsinformation.net/exe/iono-day.gif Ionosphere temp
http://www.igeb.gov/sa/diagram.shtml
http://www.montana.edu/places/gps/lres357/slides/GPSaccuracy.ppt
http://www.montana.edu/places/gps/lres357/slides/GPSstatus.ppt

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.”
http://www.faa.gov/newsroom/speeches/Blakey/2004/speeches_blakey_041021.htm

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
position. http://rob.com/lancair/flights/kirk/1gps.gif

What’s going on here? While I’m not saying GPS is junk
http://www.gtwn.net/~keith.peshak/SatNavStatus.htm http://gpsinformation.net/exe/waas-coverage-dale.gif
transmitting corrections to (Inmarsat IIIs: POR (Pacific Ocean Region)
http://www.lyngsat.com/tracker/inmar3f3.shtml http://www.lyngsat.com/tracker/inmar3f4.shtml

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
receiver?

BTW, CFS has been offering barro corrected GS on all approches for years,
without WAAS or a TSOed GPS.
============= ends here ==================

So what do you folks think?

jtberlin · January 2, 2005, 11:22pm

While I’ve yet to see WAAS in action, a friend of mine who writes for a prominent aviation magazine that begins with “F” went out the other day and flew a bunch of WAAS approaches in an a/c with a GNS480 and raved about it to me. From what I’m hearing, I expect WAAS to be a big deal in the coming years as more WAAS-capable navigators and ground stations enter service.

Jeff

timg · January 3, 2005, 3:13am

Marty:

I cannot fathom how WAAS could possibly reduce availability of the existing GPS signals, all other things being equal, so I question the premise of WAAS having a reduced availability ‘cost’ associated with it based on anecdotal experience with new model GPS units. Is the WAAS signal coming from a different satellite or on the same satellites? Even if it’s a separate sat, I would expect a decent GPS to fall back to non-WAAS mode without affecting availability.

It wouldn’t surprise me a bit if early implimentations of WAAS were full of bugs and problems; expressing an opinion on the whole concept based on tests of new units in the early days of the system being on line seems premature unless there’s a solid theoretical basis for concern.

Tim

gordonf · January 4, 2005, 5:49am

In reply to:

promise of maybe getting “Sole means” en route blessing.

That’s a reality now. If you have a certified WAAS box you can use GPS as “sole means” and you can file as an alternate an airport that only has GPS approaches.

edboland · January 3, 2005, 1:37am

In reply to:

While I’ve yet to see WAAS in action, a friend of mine who writes for a prominent aviation magazine that begins with “F” went out the other day and flew a bunch of WAAS approaches in an a/c with a GNS480 and raved about it to me. From what I’m hearing, I expect WAAS to be a big deal in the coming years as more WAAS-capable navigators and ground stations enter service.
Jeff

Check out the video on the AOPA site.Compares the ILS and the WAAS side by side. Saw it at the AOPA convention and can not wait to have WAAS…Ed