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Old October 31st 11, 01:56 PM posted to sci.geo.satellite-nav,rec.aviation.ifr
Alan Browne
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Posts: 18
Default PRN133 ranging now useable for SoL, at non precision approachlevel

On 2011-10-30 20:08 , HIPAR wrote:
On Oct 30, 4:44 pm, wrote:

NASA JPL operates a Global Differential GPS system with worldwide
coverage. They claim 10cm performance.

http://www.gdgps.net/

'The NASA Global Differential GPS (GDGPS) System is a complete, highly
accurate, and extremely robust real-time GPS monitoring and
augmentation system'.

I believe the John Deere Starfire commercial service is based upon the
NASA system.

Why can't airplanes use it ?


First off it's proprietary - something WAAS/EGNOS avoid. Airlines are
loathe to add equipment and pay operating fees for it. (Not to mention
that JD would have to build (or have an avionics firm design, build and
certify) airborne Starfire receivers.

Secondly, Starfire receivers are L1+L2. More expensive than L1
receivers. L1+L2(codeless) phase comparisons provide for a lot of local
PR correction due to ionospheric delays. The data downloaded by
Starfire is thus limited to non-iono effects (ephemeris error, clock error).

So, an aviation certified L1/L2 antenna would be needed as well (I don't
know if any exist but surely a military antenna could be put through the
paces for DO-160D and whichever TSO applies to GPS antennas,
appropriately modified to cover L2 reception).

It doesn't provide (I assume) integrity signals - though likely it could
with little additional effort.

Aparently Starfire also uses the WAAS ephemeris/clock data even if it is
not as accurate as Starfire's own eph/clk data.

Finally, there may be various forms of liability issues both on the part
of JD and the national airspace services.

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