The WAAS almanac data consists of a limited set of parameters to allow rough calculation of the satellites position. These are based on the assumption that the satellites are in geostationary orbit and so their positions relative to the the ECEF coordinate system do not change much.
Only 8 paramters are broadcast consisting of X, Y and Z values in ECEF coordinates and their rates of change, i.e. dX, dY, dZ and the Time of Day (TOD) to use as the starting point for the calculations.
x = X + dX*(tod - TOD)
WAAS satellites use the Time of Day in seconds based on GMT (Greenwich Mean Time) for the calculations instead of the Time of Week (TOW) used by the GPS Navstar system.
The corrections broadcast by the WAAS satellites are a mixture of Fast and Long term, to use the terminology in the interface spec.
For Fast corrections a mask is first received which shows which of the 52 GPS satellites the corrections apply to, followed by the Fast corrections themselves in groups of 13. Fast corrections are sent every 6 seconds and are corrections to the pseudorange measured by the receiver.
Now at this point you maybe saying but there aren't 51 satellites, but there is room for future expansion.
Slow or Long term corrections are broadcast less frequently and consist of changes to the GPS Navstar satellites calculated position and clock time. The values are a fixed term plus a rate of change that is multiplied by the difference in time from the time of applicabilty (toa) or reference time for the parameters.
The Ionospheric data is also sent as a Mask followed by the actual values for the Ionospheric corrections. The Mask first tells the reciever which one of the 11 bands it is supplying data for, and there are several bands where a partial set of grid point values are sent by the WAAS satellites.
Originally there were only 9 bands (0-8) but 2 more were added later to give better coverage at high polar latitudes and the structure of these 2 bands is different from the original 9 bands.
The 9 original bands each cover 40 degs of longitude starting at -180 degs West and consist of points every 5 degs in latitude between 55 degs South and 55 degs North, then points every 10 degs for latitudes 65 and 75 degs N/S, followed by one point at either 85 degs North or South for each band except band 8. These last points are every 90 degs apart with the North and South set being offset by 45 degs.
The 10th (North) and 11th (South) bands cover the full 360 degs of longitude but start at 60 degs of latitude with a spacing of 5 degs. Then for 65, 70 and 75 degs of latitude they are spaced 10 degs of longitude apart and finally at 85 degs of latitude they are spaced every 30 degs of longitude.