Galaxy XV / PRN 135 WAAS signal unofficially healthy again ?

Since March 1st, PRN 135 WAAS ranging has changed from NM (Not
Monitored) to NPA (Non precision approach). From what I observed this
wouldn't be possible with MT0 (test mode) being broadcast by said GEO.
This is also typically the first step when a GEO is transitioned from
Test mode into regular operation, with follow-on improvements in UDRE
(ranging accuracy).

As I write this, PRN135 is crossing paths with PRN138, which causes
mutual interference between both GEO broadcasts, forcing their ranging
into DNU, that's another indication MT0 is removed from PRN 135, since
GEOs don't go into DNU if MT0 is being broadcast (they stay in NM
status).

However there is no official news of PRN 135 health information on the
FAA NSTB (WAAS status page) or the FAA WAAS news page.

So with the current position of PRN 135 and its resuming of normal
operations, redundant WAAS coverage to most of Alaska seems to be
restored, and as PRN 135 coasts into its 133 West slot, NW Alaska
coverage will progressively be restored.

Obviously don't use this information for flight planning purposes...
If LPV approaches into NW Alaska are NOTAMed, that's it. In a day or
so, the vpl real time charts will show if PRN 135 signal is actually
good for WAAS corrections.

Marcelo Pacheco

The DNU indication and TEST mode are independent.

DNU refers to the quality of the ranging signal. It means
the corrections don't fit in the message structure or a
safety monitor has tripped. The corrections data from
a WAAS GEO is still good even when the ranging is DNU.
For example WAAS frequenty sets GEOs to DNU while
it catches up with satellite manuevers that are bigger
than the WAAS design was tuned for.

TEST mode is an indication that aviation receivers are
not permitted to use the satellite for ranging or for the
corrections message data.

WAAS uses TEST mode in two different manners:

1) Message Type 0 (TEST Mode message) is the
only message sent and the contents of every message is
all zeros. This prevents all users from using the GEO.

2) Message Type 0 having the contents of Message
Type 2 (fast clock corrections for 1st 13 GPS) and
all messages broadcast normally except Message 0 is
substituted for Message Type 2.

PRN-135 is currently using TEST mode in the 2nd
manner. It is usable for non-aviation, but cannot be
used for aviation.

Thanks for the clarification.
Two points:

1 - http://www.nstb.tc.faa.gov/incoming/Waas_SV_Status.txt could be
updated to include a TEST flag. That would be useful.
2 - It looks like PRN 135 signal is now way, way better than it was in
the last week before it was shutdown a few months ago. Then why not
just set it healthy, instead of doing the usual GEO procrastination ??
After all, SBAS messages have a 24-bit CRC, the odds of a user
actually getting a corrupt SBAS message, with a CRC that passes check,
is like one in a billion anyways.

My main criticism is the year long GEO testing period. What a waste of
money. 2-3 months testing should be more than enough.
Same thing for requiring a multi month retesting a GEO that was
operational just a few months ago.
Stop wasting public money. Be efficient FAA !
Rhetorical point. Sorry. I wish they actually listened. They act like
lawyers that only worry about lawsuits.
My target isn't s_anode@comcast.net, it's the FAA in general.

Marcelo Pacheco

www.gpsworld.com informs that PRN135 is now back to full healthy,
available for aviation and other safety of life applications.
But there's no word to that end on http://www.nstb.tc.faa.gov/ as well
as
http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/waas/news/
yet.
At the same time WAAS nstb SV status page shows PRN135 UDRE now at 50
meters. At this point PRN135 begins to really useful as a ranging
source for non precision approaches. This suggests that PRN135 has MT0
really removed. And this should mean absent any signal quality issues
in less than a day or two ranging UDRE improves to 7.5 meters (the
best possible for WAAS GEOs). UDRE 7.5 meters is useful for precision
approaches.

Marcelo Pacheco

.

The VPL realtime charts show coverage to NW Alaska fully restored.
This confirms without doubt that PRN135 is fully healthy for WAAS
integrity/corrections.
PRN 135 still has 12 degrees west to go until it reaches its normal/
backup orbital slot.
Also its currently operating on a 50 meter UDRE for ranging, which is
useful for non-precision approaches, but is useless for LPVs. This is
probably due to PRN 135 still being repositioned. Ranging allows WAAS
GEOs to be used like a regular GPS satellite, except that they are
always less accurate than a regular GPS satellite (GPS satellites
reach 3 meter UDRE after they come in view of a lot of WAAS reference
stations, while WAAS GEOs are limited to 7.5 meter UDRE, even though
they are monitored by most reference stations 24x7).

Marcelo Pacheco