Please help me understand ADSB vs IFR GPS

While having a chat with friends in regards to ADSB vs GPS capable of making approaches one of the guys brought up that the GPS receivers used for ADSB are good enough to let everyone else know where you are (VFR or IFR, and will be the sole means for ATC to locate you in some areas after 2020) yet it is NOT good enough to let YOU know where you are in order to fly enroute or to shoot approaches.

So outside of the ole answer of they can't allow it because Garmin would be pi$$ed for all the money they had to spend to certify the GTN650 while some experimental joe schmoe is using a $500 gps to do the same thing, what exactly is the reasoning?

My understanding of the approved GPS recievers for ADSB is they can exclude satellites if they are giving bad info already so that checks that box. They are WAAS, so there's another box. The only thing I can see that is missing is a database that cannot be modified by the user. I'm sure Garmin can easily program that for my G3X (can vs would). Of course last but not least would be for the governing agencies to change their rules to allow for such a crazy thing to happen.

So again I must ask, why is it good enough for you to know where I'm at but not good enough for me to know where I'm at?

Much of it is probably related to map and nav data consistency and validation

GPS is just your position in 3D space over the Earth. You also need all of the route/approach guidance information and appropriate indications.

Here's an example: Just because you have a radio that can tune an ILS frequency doesn't mean you can shoot ILS approaches to minimums. You need more instruments than just the radio. GPS routes are more complicated than ILS (thus the software/data requirements).

agreed on the database. That was my reference in the original post to having garmin update a database for my G3X (or whoever your glass display is from) which cannot be modified by the user.

This question was asked on the Dynon forums and the responses were thorough and might answer your questions. It gave me a better understanding.

http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1362597418

GPS approaches

Hi Timberwolf, Along with the approved WAAS GPS for precision GPS approaches and an approved data base there are a whole bunch of additional Requirements on how the system presents the approach information to the pilot. And how any pilot flying errors affect approach presentation to include sequencing from initial to final and missed approach segments guidance.

And there are different equipment certification rules (&GPS requirements) between non-precision and precision GPS approaches. That's why non WAAS G430 has about 1/2 the value of a G430W

Cecil

The answer is that the FAA is pretty much insisting that approach GPS boxes conform to TSO 146. This TSO is huge; I've seen estimates of well over a million dollars in engineering time to document compliance. Look in the front of a 430W installation manual. You'll see two pages listing TSO requirements that Garmin could not or did not comply with, and their approved alternate means of compliance. That costs money. IMHO it's gross overkill, but the FAA is unlikely to back down now. So because, in part, of the huge up front development costs, there is limited competition, and pretty much none at the low cost end (e.g., like a 400W).