Hello,I have a little question regarding ANP RNP and LNAV.I noticed that sometimes the magenta "diamond" on the LNAV scale from the Primary Flight Display wasn't always centered.With a "normal" zoom (20- 80NM) on the Navigation Display, I can see "0.0" under the white triangle that represents the aircraft with RNP 1.0 and ARP 0.06. But when I zoom in (for example with 5 nm scale), sometimes I can see 0.02 or 0.04 left or then right then back to 0.00, 0.01 then again 0.02, 0.04 L or R etc... So the aircraft is slightly right or left from the magenta line.I've read that some people had troubles with their FSX config and LNAV. I don't think it's the case for my FSX and 737NGX. I don't have FSUIPC and I always have the default MTO config in FSX (wind 270/25). Same story with no wind.I would like to know if such little deviations from the magenta line are normal.Thanks in advance,Vincent.NB: I have the 737-800 NGX with service pack SP1b

Edited January 22, 201214 yr by Vincent737

sometimes I can see 0.02 or 0.04 left or then right then back to 0.00, 0.01 then again 0.02, 0.04 L or R etc...

I bet you would see similar deviations in real world. 0.02 is very little, it is on the order of 30-50 m, for enroute phase it is practically nothing, aircraft don't fly in still, uniform air hence such deviations are to be expected.

Edited January 22, 201214 yr by michal

You will see small deviations, RL. This isn't like the hunting that some have had problems with.

Ok, thanks for your replies guys :good:Vincent.

If I'm not mistaken ANP stands for Actual Navigational Performance. If you have an ANP of 0.06, that means that there is an error in your navigation of 0.06 nm. So if your position relative to the magenta line is jumping around by a distance of less than 0.06 nm (to either side of the line) then you are probably right on the line and just seeing the error (which is random).

I had 0.07 while sitting at the gate at PAKT before heading out to PAJN RNP Y8 last night. I think it stayed at that ANP the whole flight.

So the aircraft is slightly right or left from the magenta line.

The magenta line and the aircraft position are both relative to the FMC calculated FMC Position (FMC POS page 2), therefore ANP will never be visible relative to the magenta line. All things being equal, even if the ANP is 10.0 the aircraft will still be bang on the magenta. The only way to assess the actual position of the aircraft relative to the actual geographic world on the map screen, is to use raw data VOR/DME or a beacon crosscut by tuning the beacon(s) manually.FWIW, with GPS updating in the real world, we usually see ANP of around 0.03. If GPS updating is switch off, then this increases by an order of magnitude.Navigation is so accurate, that in RVSM on the airway, when an aircraft passes 1,000' below us (opposite direction of course), "1000" will flash up on the rad alt momentarily!

Edited January 25, 201214 yr by Vagabondo

ANP is NOT how far off the FMC course you are! (that's crosstrack error)ANP is the system's "self check" for the accuracy tolerances of the navigation system are. It's similar to your phone's GPS app saying your location is accurate to within 150 feet - it's a measure of the navigation system's internal accuracy. If you have an ANP of say 0.03 that means that the aircraft's position is guaranteed to be within .03 nm of where the plane "thinks" it is. Has nothing at all to do with crosstrack error for the magenta line. The plane could be right on the line and have a super high ANP - nothing's wrong as far as LNAV knows, but the whole map is shifted and you're not really where it thinks you are. That's the concept.

The plane could be right on the line and have a super high ANP - nothing's wrong as far as LNAV knows, but the whole map is shifted and you're not really where it thinks you are

Map shift is really evident, when the FMC position is not accurate and you are navigating on raw data, most usually when intercepting the localiser. In reality this not something one sees on the NG with GPS position updating, but was a regular feature on the Classic's which relied on IRS or beacon position updating (beacons are not really good down low with limited line of sight range as the IRS would tend to drift) where the EHSI in MAP mode would show the aircraft off the magenta line (cross track error), hence the generally accepted Classic piloting practice of using HDG SEL to intercept the localiser and not LNAV (as the FCTM suggests). Whereas on the NG with GPS, LNAV works perfectly every time :)