Originally Posted by Ross_M
My impression was ionospheric delay was entirely a DGPS correction. A ground station knows what its position is supposed to be and if the calculation yields another we know the propagation velocity error.
Not true. the process you describe is used in DGPS, but it doesn't yeild ionosphere delay. The error between the known position and the calculated position is the sum of all errors affecting GPS measurements, including but not limited to: Ephemeris error, timing errors, Ionosphere and Troposphere signal propagation errors. As noted by dcoded, the ionosphere delays can be estimated using ionosphere modelling, and this information is indeed sent to the satellites and broadcast in the signal.