MC wrote:
wrote:
Ali Ghorashi wrote:

What does the following NOTAM mean:

GPS 02/001 GPS PRN 26 OTS WEF 0502082100-0502090900

I know it says "GPS Pseudo Random Noise 26 Out of Service" but what
are its implications? Does it mean that RAIM is not available
during the outage?

Maybe, maybe not

It's not at all intuitive what the effect is on RAIM. You can have
one
or more OTS and still have RAIM. Or you can have none OTS and still
lose RAIM (although with a fully healthy constellation, the
availability is damn near 100% these days, with 30 operational
birds up
there now).

I guess it depends on where you are.
In Oz, I often see in a TAF a few short-duration
holes in RAIM.
eg.

RAIM GPS RAIM PREDICTION 021401
YSBK
02030824 TIL 02030837
02040820 TIL 02040833
02050816 TIL 02050829
GPS RAIM UNAVBL FOR NPA


True enough, alth. Some notes to qualify things a little:


- With 1440 minutes in a day, 13 minutes still works out to around
greater than 99%. There's definitely some interesting geometry holes
down in that region though.

- TSO C-129A boxes were stood up prior to SA being turned off, and thus
have a larger sigma value (~33m) hard-coded in there for one of the
error components . This makes things more conservative than the reality
today.

- While the prediction can't account for every possible receiver
manufacturer implementation, the algorithm did follow and was validated
against the RTCA MOPS. Different manufs will get slightly different
availbility (start/end times differ by small margins, edge-of-coverage
cases will have one manuf flag where another may not). We went on the
conservative side there a little. It's a balancing act between
minimizing False Alerts and Missed Detections,

- Rumor has it that Airservices (or maybe CASA) might be experimenting
with a special 129 box which has the updated sigma value. I'd be
willing to bet you'd see most if not all of those short duration
outages disappear.

- The newer 145/146 boxes (whether they're in standalone mode or using
differential corrections) have the updated sigma value. My
understanding is that this was a primary driver for why the manufs
never built a 129 (129B?) box as they couldn't make a business case.
Might as well sell the newer boxes, right?


There are some interesting things are happening this year down under
with your Upper Airspace Program, e.g. ADS-B.


Regards,
Jon