First, WAAS has nothing to do with anything in ADS-B. As pilots we got used to WAAS as a big deal because Garmin updated their GPS' from non-WAAS to WAAS and called them the 430W and 530W. These new units could do precision approaches, and everyone associated that with WAAS. The reality is they were designed under a whole new TSO, which WAAS allowed them to meet, but was not required. Your cell phone has WAAS. It's not the thing that makes a GPS aviation legal.
What ADS-B needs is a GPS device of sufficient accuracy and integrity. Now WAAS is one of the technologies that allows that at a reasonable cost, but you need a lot more than just WAAS, and it is also possible to build an ADS-B legal GPS without WAAS.
Nothing in an experimental needs to be certified. However, a few things need to "meet the performance requirements in the TSO." Those would be your transponder, your GPS navigator if used as primary navigation in IFR, and your ADS-B OUT
system. The system is both the ADS-B transmitter and the GPS position source.
The FAA made this very clear. They updated the FAR in Febuary with a statement that no TSO or other certification was needed. This doesn't mean you can use anything you want. Check out the FAA's FAQ on ADS-B installs:
https://www.faa.gov/nextgen/equipadsb/faq/
Specifically, it says:
The ADS-B Out equipment installed in an aircraft must meet the performance requirements of the ADS-B TSOs. A TSO authorization, issued in accordance with 14 CFR 21 subpart O, is not required. However, ADS-B Out systems and equipment installed or used in type-certificated aircraft must have a design approval issued under 14 CFR 21 (or must be installed by field approval, if appropriate).
The performance requirements include those requirements referenced in section 3 of the applicable TSO (UAT or 1090ES), including considerations for design assurance and environmental qualification. Deviations to the requirements can be approved for equipment which does obtain a TSO authorization, as identified in 14 CFR 91.227.
For experimental category aircraft there is no FAA approval required for the ADS-B Out system installation. Owners of these aircraft may elect to install equipment authorized under a TSO, in accordance with the installation instructions provided by the manufacturer. Alternatively, owners of these aircraft may elect to purchase uncertified equipment. For uncertified equipment, the owner should obtain a statement of compliance from the supplier, along with installation instructions, that identifies that the ADS-B equipment complies with section 3 requirements of the applicable TSO and that, when installed in accordance with the installation instructions, complies with the aircraft requirements of 14 CFR 91.227. The FAA expects manufacturers to perform appropriate engineering efforts to ensure the equipment complies with all requirements of Section 3 of the TSO before issuing their statement of compliance, and expects installers to consider the guidance in the current version of AC 20-165 when performing the installation.
Owners of experimental aircraft should retain the statement of compliance from the equipment supplier in the aircraft records to assist in resolving in-service issues, should they arise. The FAA monitors compliance to the ADS-B Out requirements, and if the equipment, or an installation, is determined to be noncompliant the operator may not be able to enter the airspace designated in 14 CFR 91.225 until the equipment or installation is brought into compliance.
The FAA expects manufacturers to provide a clear install manual, and state that if installed as instructed, the system is compliant with all performance requirements in the 91.227 and the TSO. Without that, there's no way for you to have any idea that it complies. Even with that, tn the end, it's up to you, the installer to decide if the information provided is sufficient to be legal. Note that this doesn't say it just needs to transmit a SIL of 3 and SDA of 2. It needs to have actually been fully tested and verified to comply, and the company needs to be willing to provide a statement as of such, in lieu of a TSO from the FAA.
As per an EFIS system adding a $600 GPS position sensor that makes the system legal for IFR navigation, Dynon has a statement on the feasibility of this, which you might want to read. The issue is not the "GPS" it's the rest of the system which also has to meet all the requirements:
http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1362597418
--Ian Jordan
Dynon Avionics