12 posts in this topic

Hi,

I wonder if someone can help me. I have been using the 737NG and have just noticed something very strange. The RNP displayed is 2.00 but my ANP is always between 0.06 and 0.05. Does anyone know what would cause the to happen?

I am using P3D v4, the 737 is a clean install and has up to date Nav data from Navigraph. As far as I am aware, I have aligned the IRS's correctly but no matter what I do, I always get the same result.

Is this a bug, or is there something I could be doing wrong?

Many thanks

Phill Morgan

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Isn't it just a reference of how accurate the nav system is?  RNP being Required Navigation Performance (or something like that) and ANP being ACTUAL Navigation Performance.  If the ANP is higher than the RNP, then you have an issue, and your LNAV/VNAV and RNAV approaches might not be accurate.

 

Or I'm completely wrong lol.

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Thanks Fishermanlvan, you are correct, what I do not understand is why my Actual is so low, as far as I am aware, I have performed all of the alignment and startup procedures correctly, but I just cannot get the required performance from the 737.

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5 minutes ago, Phill Morgan said:

The RNP displayed is 2.00 but my ANP is always between 0.06 and 0.05. Does anyone know what would cause the to happen?

What would cause this> Aircraft nav working good! :happy:

 

5 minutes ago, Phill Morgan said:

Is this a bug, or is there something I could be doing wrong?

I bet you don't exactly understand what RNP and ANP are, do you?

RNP is Required Navigation Performance. The precision in nautical miles of your position on your track that is required to comply with. The RNP depends on where you are. A STAR RNAV usually has a RNP1 which means that your aircraft must be able to calculate her position in a square of 1nm of lateral dimension.

ANP is Actual Navigation Performance. The actual precision of your aircraft position. 

As long as the ANP is lower than the RNP, you're good. If for any reason your ANP becomes greater than the RNP of your track.

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1 minute ago, Phill Morgan said:

Thanks Fishermanlvan, you are correct, what I do not understand is why my Actual is so low, as far as I am aware, I have performed all of the alignment and startup procedures correctly, but I just cannot get the required performance from the 737.

Well, the lower the ANP, the more accurate it is.  So .08 is good.

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Posted (edited)

5 minutes ago, Budbud said:

What would cause this> Aircraft nav working good! :happy:

 

I bet you don't exactly understand what RNP and ANP are, do you?

RNP is Required Navigation Performance. The precision in nautical miles of your position on your track that is required to comply with. The RNP depends on where you are. A STAR RNAV usually has a RNP1 which means that your aircraft must be able to calculate her position in a square of 1nm of lateral dimension.

ANP is Actual Navigation Performance. The actual precision of your aircraft position. 

As long as the ANP is lower than the RNP, you're good. If for any reason your ANP becomes greater than the RNP of your track.

Thank you for the lesson, however I do understand what they are. What I do not understand is why the actual is so low. The question was, "Is this a bug, or is there something I could be doing wrong?" What would cause the issue?

Edit:

Sorry I miss-read what you were saying. Thank you!

Edited by Phill Morgan
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5 minutes ago, Phill Morgan said:

Thank you for the lesson, however I do understand what they are.

Sorry.

5 minutes ago, Phill Morgan said:

I do not understand is why the actual is so low

ANP is your actual precision in nautical miles. So the lower the better. 0.06 or 0.05 means that your FMC is actually calculating your position in a square of 0.05 or 0.06nm of lateral dimension. Which much better than the 2nm required.

If your ANP was higher that would mean that either your IRS would be wrongly aligned or the positions given by the GPS would be somehow erratic.

 

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2 minutes ago, Phill Morgan said:

Thank you for the lesson, however I do understand what they are. What I do not understand is why the actual is so low. The question was, "Is this a bug, or is there something I could be doing wrong?" What would cause the issue?

So wait are you saying it's a bug because it's too accurate? I'm confused how you could have made a mistake aligning the IRS if you're getting better ANP?

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23 minutes ago, Phill Morgan said:

Is this a bug, or is there something I could be doing wrong?

Please avoid firing the word 'bug' out into the ether before you properly understand what's going on. Just because you do not understand it, does not mean it's a bug. Your last post asserts that you know what is going on, but your confusion as to why ANP is so "low" means that you don't. I apologize for being blunt here, but when I see 'bug' fired off casually when something is clearly behaving properly, it rubs me the wrong way.

The aircraft is behaving entirely normally.

 

What is RNP?

RNP is Required Navigation Performance. It is the required value that the aircraft cannot exceed. So, if RNP is 2.0, the aircraft must be able to determine that its calculated position within 2.0nm of its actual position.

What is ANP?

ANP is Actual Navigation Performance. It is the difference between the aircraft's calculated position versus its actual position. So, if ANP is 0.06, the aircraft is theoretically somewhere within 0.06nm of its actual position.

What is "proper behavior?"

Proper behavior is determined as ANP<RNP, where lower is better. If you could theoretically achieve 0.00, this would be perfect performance. This is not possible, however, as simply "as low as possible" is as ideal as you're going to get.

So...what's the issue here?

There is none. The low value is due to the aircraft having high confidence in its navigational capabilities. When you mix IRS, GPS, and radio navigation correlation, you're going to have high confidence, and a lower ANP value. Lower is better.

 

What is your understanding as to why this is a problem, or a bug?

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3 hours ago, scandinavian13 said:

What is ANP?

ANP is Actual Navigation Performance. It is the difference between the aircraft's calculated position versus its actual position. So, if ANP is 0.06, the aircraft is theoretically somewhere within 0.06nm of its actual position.

 

I always hated the term "Actual Navigation Performance" because it tends to lead itself to definitions such as the actual position of the aircraft. 

ANP is not the actual position of the aircraft, but the confidence of the Flight Management Computer or System of where it is at. Weighted values are assigned to different navigation data sources, and monitors, filters, and averaging terms are used to compute an ANP value. (Boeing, 2001) It is sort of like asking the FMS how confident are you with a 0.00 meaning I am 95% confident I am where I say I am. With GNSS the FMS has a high level of confidence and can predict it is within the elliptical error radius of the GNSS system (0.05nm or 30'.) Of course this is a perfect situation with azimuth to the SVs, signal strength, space weather, and atmospheric scintillation of radio waves all applying an impact. The important fact to know here is under RNP airspace requirements the aircraft may actually be up to twice the RNP distance before the system issues an alert to the flightcrew.

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7 hours ago, Phill Morgan said:

Hi,

I wonder if someone can help me. I have been using the 737NG and have just noticed something very strange. The RNP displayed is 2.00 but my ANP is always between 0.06 and 0.05. Does anyone know what would cause the to happen?

I am using P3D v4, the 737 is a clean install and has up to date Nav data from Navigraph. As far as I am aware, I have aligned the IRS's correctly but no matter what I do, I always get the same result.

Is this a bug, or is there something I could be doing wrong?

Many thanks

Phill Morgan

Not sure about the 737NG, but this ANP value is normal on the 777 with only GPS (GNSS) position updating which is certified to be able to do RNP AR approaches with a minimal RNP of just 0.11.

I would assume the same applies to 737NG if the FMC is certified to be able to do the same thing as the 777. 

 

For FMC with DME-DME updating only, the ANP can still be achieved below 1.0. 

 

I had a flight going from VHHH to RJAA a while ago when we lost both GPS on climb out of Hong Kong. So all we had to use the old DME-DME updating for the rest of the flight. 

And for the middle 1/3 of the flight which lasted about 1.5 hours we were relying solely on inertial. And during this period of time the ANP never gone above 4.0, which was perfectly fine on a route with RNP-5. 

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