On Mar 4, 3:32*am, macpacheco wrote:
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

I wish I had been right... (original reply only went to
sci.geo.satellite-nav), pasted he


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.