I've read in multiple online sources that two WAAS satellites, PRN 138 and 135, transmit both DGPS correction data and standard GPS navigation message, which means they can be used for a fix, in addition to the "normal" GPS satellites. PRN 133 on the other hand only transmits correction data, but not the GPS navigation message. My question is, is that correct? If so, how is that possible? AFAIK, commercial GEO communication satellites don't have atomic clocks on board. Neither is their position maintained to the same accuracy as GPS satellites, using multiple ground tracking stations, radars, lasers and what not.
My second question is, do SBAS systems like WAAS, EGNOS and MSAS transmit correction data for other GNSS constellations, like GLONASS and BeiDou?