http://slashdot.org/articles/03/03/1...id=126&tid=103

 

And your average Boeing 757

 

Commercial aircraft don't primarily use GPS for the above very good reason.

 

gps is not certified for airline use so dont panic, the in flight displays may suffer, but not navigation as its radar.

apparently gps is not licensed by the federal aviation authority for use on commercial flights.

 

Their nav systems actually use VOR, but they also use GPS mapping for visual display. They can fly perfectly safely without GPS, before anyone gets worried.

 

If a GPS unit is multi channel, and can lock on more than 3 satellites (or 4 for a vertical fix too), the effects of sellective availability are reduced, as the errors are averaged out or ignored. Most new GPS units will lock on to up to 12 satellites.

SA was turned off largely due to the introduction of multi channel GPS units, which reduced its effect.

John.

 

Airliners use Inertial Navigation (gyroscopes basically) as their main source of navigation with GPS input in certain cases. The aircraft I fly has two INs and two GPSs which together generate two usable navigation solutions.

Certain GPS receivers are certified for enroute and terminal (approach) navigation and I have flown into many airfields in the USA and Canada that have published GPS approach procedures.

SA was turned off for many reasons but one of the main ones was pressure from the airline industry as it is such a good nav system that is unaffected by many of the things that upset INs.

VORs (ground based VHF radios that transmit their exact position) are now rarely used as a primary navigation tool. They are sometimes used as a backup where required.

I don't think the DoD will turn SA back on as they don't really need to. If they do do it it will be notified quite widely (in safety critical areas anyway) so that airlines etc can modify their procedures to accomodate it.