My observation is the following :ANP on the ground by default, with all sensor position update selected to ON, is close to .06, which makes sense. Before the SP1, when I used to turn off the GPS update under the NAV OPTIONS section of the FMC menus, the sim frame rate would immensely suffer. This is now corrected.I am now turning the GPS update to OFF, and even after a 60 minutes flight, the ANP still lags around .06. Same result when I turn off in turn the VOR, DME and LOC update. The " IRS Nav only" messages then pops up in the scratch pad, which is normal. I was expecting the ANP value to start going up gradually as a result of an OPS software algoritthm that accounts for the IRS drifts and corrects the ANP accordingly.Is this feature simulated or is it designed as part of the sim product to keep the ANP value to a "Fictive" value?The 737-NGX is absolutely wonderful!Thanks,Marc Bergeron

Anybody knows if this feature is modelled?I tried it some more and also found out that if the radio update ( VOR and DME update is turned off ) is disabled, the ANP still stays at a pretty low value.Just curious,Marc Bergeron

It works (I love breaking stuff )

Mine use to jump all over the place like yours is before I installed the Service pack: it is not what you want it to do because you would have to replace your FMC (45000$) if it did that in real life !! After I installed the service pack, the position does not jump all over the place anymore but the ANP stays steady in the .05 range... also not expected? Any more ideas?Thanks a lot for your feedback and video!!Marc Bergeron

Edited February 16, 201214 yr by Seatrend45

Guys, the Inertial Reference System (the thing that takes forever to align when you start a flight) is extremely accurate, but because of errors in the system - over time - its accuracy degrades. With GPS updates, that accuracy is maintained to a high degree because the position can be updated and the error can be compensated. If you disable GPS updating, it's not like your systems are going to just immediately go off.The Concorde (among other older iron) still managed to make it across the entire Atlantic without GPS or even VOR updates (until closer to shore), with a good degree of precision.There are drift charts out there that will display how much drift is acceptable. You will find that the drift is actually pretty slow. 737 flights, being a lot shorter, tend not to show these effects much.

Edited February 16, 201214 yr by scandinavian13

I remember when flying to Level D 767, which simulates no GPS, across the Pacific to Hawai'i, when I finally got within VOR range for updates, the nav display showed me a bit off course, and the autopilot would then turn and intercept the intended route. One would just think that this is what would happen if it didn't have all the updating methods available to it, because the IRS does drift over time.I think what caught my attention in this thread was how the OP noticed no difference. If I get time, I will try a flight from SoCal to Hawai'i, same flight I have seen drift on other higher end payware aircraft and see how the NGX behaves. I will of course only be able to report back what I find. I do not have intimate details on the 737s IRS to determine if the results are within bounds. I will have to leave that up to other good people here on this board.

Thanks Scott!It would be nice when you have time if you could validate my findings. it will not change my appreciation of the NGX a bit but I would at least know it is not me that is missing something! :lol:See attached table that describes a typical expected ANP for the different sensor update inputs to the FMC.Marc Bergeron

Thanks Scott!It will be nice to find out what your Hawaii flight reveals next time you have time for it! As shown on the chart below, even with radio update (IRS/DME), the ANP should not be below 0.2. It is important to mention the ANP is not a direct measurement of the IRS drift. Whatever the outcome is, it will not change my appreciation of the NGX a bit! I will at least know what the limitations are so I don't commit a navigational error that could lead me to a mountain side, in palm spring for example, while doing an RNP SAAR approach ! :( .05 is an excellent ANP factor and you can fly with confidence tight approaches, but only if it is in a serviceable condition !Marc Bergeron

Edited February 17, 201214 yr by Seatrend45

Thanks Scott!It will be nice to find out what your Hawaii flight reveals next time you have time for it! As shown on the chart below, even with radio update (IRS/DME), the ANP should not be below 0.2. It is important to mention the ANP is not a direct measurement of the IRS drift. Whatever the outcome is, it will not change my appreciation of the NGX a bit! I will at least know what the limitations are so I don't commit a navigational error that could lead me to a mountain side, in palm spring for example, while doing an RNP SAAR approach ! :( .05 is an excellent ANP factor and you can fly with confidence tight approaches, but only if it is in a serviceable condition !Marc Bergeron

Edited February 17, 201214 yr by Seatrend45

Alright, took a bit of a delay but I am on my way to Hawai'i at the moment. I flew first from Chicago-O'Hare to LAX with all the updating on. As expected ANP stayed nice and low, for the most part at 0.05For the Hawaiian leg, I aligned the IRS, used the GPS for that, used the GPS coordinates for the present position, but once the alignment was complete, I disabled the GPS update and LOC update. Took off from LAX on rwy 25R, headed west on runway heading, with a turn to 265, then back to 250 mixed in ther from Radar Contact, and I see my ANP around .25 As time goes on I even see it go up to .29 at times.I just reached the point where I am now out of range of all VORs and VORDMEs. I get the alert on the CDU "IRS NAV ONLY". RNP now jumps up to 12.00 That makes good sense. ANP then crashes down from .29 to .05 where it has sat now for about 20 minutes. I have since turned off VOR and DME updating for the duration and will turn them on after being in VOR range for a time and will report what I see at that moment.But so far, it does seem GPS updating off does present a clear change in the ANP. Not sure if what I saw with the IRS NAV only and ANP going down is proper. Will report back in a few hours.

Got to within range of a few VORS, and the RNP and ANP remained the 12 RNP and .05 ANP the entire ocean crossing. Just turned on VOR and DME updating and the RNP drops to 2.0 and ANP is not 0.26, and the autoflight made a slight correction to get back on course. The IRS drift did indeed cause my plane to wander a bit off the path. Not nearly as severe as what I experience in the Level D 767, but I reckon that is due to the finer simulation of the whole Intertial System in the NGX. As I am shooting an RNP approach into PHOG that requires ANP of .30 or less, I may turn on GPS again to get that number down a bit so I can stay further from the out of bounds.

Thanks Scott, This appears to confirm a small bug with the ANP settling at .05 or so when GPS update and radio update is turned off, instead of increasing based on the chart above. Hope you had fun in Hawaii before returning ! Nothing wrong in having enhanced navigation compared to the real thing! thanks for the feedback.Marc Bergeron

I remember in one of the previews it was said that the ANP is real-time calculated and simulated. I guess Scott's findings confirmed it.