HOW DO I BUILD A AVIATION GPS UNIT?

The answer is "not easily" but you can easily build a flight usable one. That may sound strange but an aviation GPS is simply GPS with aviation maps. An aviation grade GPS is one that has been approved by the FAA, CAA or other aviation authority and is approved for use in aircraft. The approval process is really what makes any GPS aviation grade.

As far as making one that works, like all the portable GPS units with aviation maps, sure, in fact there are tons of map programs out that include full aviation charts/maps/terrain and even XM weather. There is no portable unit that is "approved" for aviation use. People can use them but they are NOT approved for aviation at all. Check out "True Flight" from Aviation Safety http://www.aviationsafety.com/index.html

The program is very complete, it is not an "approved for flight" GPS at all although it is used in flight.

All Portable GPS units even those from Garmin, King, etc make can be used in flight but are not approved aviation gps units. Garmin and King both make approved units BUT all approved GPS units MUST BE Permanently installed and have full FAA/CAA etc approvals on them. The approval process is very time consuming and dollar intensive! Every item used has to meet very specific requirements and the completed unit has to "prove itself".

PS a small tablet UMPC (Sony, Samsung, Fujitsu, Viliv, etc) work great for the purpose.