[time-nuts] WAAS

[Charles Steinmetz]

Hal wrote:

>The WAAS satellites don't provide timing info.  They provide corrections to
>the timing a receiver gets from normal GPS satellites.  So if all you can
>hear is the WAAS satellites, you won't have any timing info to correct.

I'm confused every time I read about WAAS.  Some say the WAAS 
satellites transmit only correction data, some say they transmit both 
correction data and timing/ranging data.  One suggestion I have 
heard, which could explain the two descriptions, is that the original 
WAAS satellites broadcast only correction information, but that the 
newer satellites also broadcast timing and ranging information.  It 
would be very nice to know for sure.

For example, Wiki (sometimes a dubious source) says:

>The space segment consists of multiple geosynchronous communication 
>satellites which broadcast the correction messages generated by the 
>WAAS Master Stations for reception by the user segment. The 
>satellites also broadcast the same type of range information as 
>normal GPS satellites, effectively increasing the number of 
>satellites available for a position fix.

This suggests that the WAAS satellites also broadcast GPS positional 
data (which, presumably, would necessarily include timing information).

Is there anyone on the list who has extensive, hands-on experience 
with WAAS and can say with certainty whether WAAS satellites 
broadcast only correction data, or also provide timing/ranging data?

One advantage of using WAAS is that it reduces the positional errors 
from GPS -- the FAA found the reduction, in practice, was from 2.5m 
lateral/4.7m vertical (for GPS alone) to 0.9m lateral/1.3m vertical 
(GPS augmented by WAAS).  A WAAS receiver might, therefore, do a 
better survey and show smaller timing jumps when switching 
constellations.  Correcting for ionospheric distortions should also 
reduce the dispersion of the GPS timing signals.  So, even if the 
WAAS satellites do not send timing/ranging data, they could improve 
our recovery of the GPS timing signals.  (And, even if the WAAS 
satellites do send timing/ranging signals, relying on just WAAS 
satellites for a survey would not be ideal because their location in 
the GEO arc makes them too close to each other -- lacking in vector 
diversity as seen from the receiver -- for an optimum positional solution.)

Best regards,

Charles