Tom Gourley writes:
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The Nav42x series has an integrated WAAS compatible GPS.
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Wide Area Augmentation System (WAAS) uses a series of strategically placed ground stations that receive the GPS signal from the GPS constellation, compare the reported position to the known position and calculate the difference. This difference between the two is used to infer the signal propagation delay through the ionosphere. The ionosphere is very dynamic and not very predictable. The delay "coefficients" are then transmitted to three (two currently in service) specialized WAAS satellites that then transmit the correction data back to earth.  (simplified explanation).

"WAAS Compatibility" simply means that the GPS receiver can receive the signals from the three WAAS satellites. Most GPS receiver can do this. The trick comes in being able to process the WAAS data and use it to correct your position information. There is a lot of math involved and it must be done in real time so it requires a beefy Digital Signal Processor (DSP).

Think of it this way, your telephone is "FAX Compatible" just like most GPS receivers are "WAAS Compatible".  You telephone can dial a FAX number and receive the pops and screeches but it has no way to process the data into a facsimile of a document. 

WAAS Capable GPS receivers (like the FreeFlight) have an internal DSP in addition to the receiver to make use of the WAAS data.

Do not confuse "WAAS Compatible" or "WAAS Enabled", the ability to receive WAAS data (easy), with "WAAS Capable", the ability to receive AND process the WAAS data (hard). 

Caveat Emptor

Regards
Brent Regan