The US government lets us as exp aircraft manufactures make many
decisions that are set in FARs for certified guys. flying with a rubber
band motor or a straw filled with water as an altimeter IMC is fine.
As long as they work (no reference to "safe") our government is protecting
the lives of others. (no TSO required)

GPS is different. Government describes not only its performance but "how"
it gets to those requirements. so while we could build a transponder
any which way to pass the performance tests, with GPS we must use their
method. So even if a all-in-the-sky position solution provides a more
accurate solution than the best 4 sats, we can't use it and are
locked out of the approach. This comes in the form of RAIM warnings.

so before you run out and buy that certified GPS, think about it. do you
want a tool that blanks out between Hawaii and CA after flying around
the world or the promise of updates 5 times a second when  you get no
updates for hrs if flying in the wrong direction?

http://mail.lancair.net:81/Lists/lml/Message/27405.html

yes, all problems will be solved... but think... what's best for you?

also, don't sell a baro corrected GS short... CFS offers a GS all the
way to dirt, below MDA for all approaches. not league, like that direct
without radar coverage, but very help full.

http://d2av.com/screen/lvk-faf.gif
http://d2av.com/screen/lvk-map.gif
http://d2av.com/screen/lvk-im.gif

that "B" for barro next to the height above terrain changes to a "G" for
gps alt above terrain with a certified gps...