Vertical ANP (not lateral ANP)

[Hardy Heinlin]

Moin,

we know the lateral Actual Navigation Precision (ANP) depends on the radio update status, GPS, IRS drift etc.

Does anybody know how the vertical ANP is determined? Is there a fixed standard ANP value for the case when a ground based GPS is used for geometric altitude sensing, and maybe another fixed standard ANP value when just barometric sources are available? How could one determine a specific precision value of the static ports and the ADCs -- if not using just two states: operative and inoperative? Plus a third status: Ground based GPS in use. But the 744 doesn't even have the avionics for this.


Regards,

|-|ardy

[double-alpha]

I have this below for the 787 :

Vertical Actual Navigation Performance

Vertical ANP is the FMC estimate of the quality of its altitude determination. It displays on RNP PROGRESS page 4/4. Vertical ANP is the estimated maximum altitude error. The FMC computes the vertical path and the certainty that the actual altitude is within a vertical range equal to plus or minus the displayed vertical ANP value.

Note: Vertical ANP is calculated from the baro corrected altitude provided by the Air Data Reference System. The pilot must set the baro setting reported by ATIS or by the setting given in the approach clearance for the display to be valid.

Vertical Required Navigation Performance

Vertical RNP is the vertical navigation accuracy required for operation within a defined airspace. It is expressed in feet. The FMC supplies a default vertical RNP value for enroute, oceanic/remote, terminal, and approach phases of flight. The FMC alerts the flight crew if vertical ANP exceeds vertical RNP. When required, vertical RNP values may be manually entered or displayed on RNP PROGRESS page 4/4. Manual entry of a vertical RNP value greater than that required for a phase of flight results in the CDU help window message VERIFY VERT RNP PROG 4/X to display.

[Mariano]

Hardy,

I'm on the way to the airport, but I very quickly scanned this advisory circular and there might be a lot of good data in it. Looks like beginning on page 55, you might find answers.

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC120-29A.pdf

Best regards,

Mariano

[Hardy Heinlin]

Thanks, guys.


|-|ardy

[Hardy Heinlin]

Looks like the vertical RNP is always linked with the lateral RNP, e.g. the typical lateral RNP of 0.30 (which is often coded in database route legs) always corresponds to a vertical RNP of 125 ft.

Still searching for how to calculate the vertical ANP of the onboard avionics status ...

Maybe the ILS radio is relevant here as well ...


|-|

[fft82]

My (non expert) understanding is that procedures only rely on barometric vertical navigation, not GPS based altitude. Procedures are designed accounting for temperature related deviations, and thus are approved for a specific OAT range.

Quoting
https://www.faa.gov/documentlibrary/media/advisory_circular/ac_90-101a_chg_1.pdf

(1) The RNP AR approach chart will identify outside air temperature limits applicable to
operators using barometric vertical navigation (baro-VNAV). Cold temperatures reduce the
effective glidepath angle while high temperatures increase the effective glidepath angle without
cockpit indication of the variation. Figures 1 and 2 provide examples of temperature limits.

NOTE: Temperature affects the aircraft's altitude indications. The effect is
similar to high and low pressure changes, although not as significant. When
temperature is higher than the International Standard Atmosphere (ISA),
the aircraft will be higher than the indicated altitude. When temperature is
lower than standard, the aircraft will be lower than indicated on the
altimeter. For additional information, refer to Aeronautical Information
Manual (AIM) Chapter 7, paragraph 7-2-3, Altimeter Errors.

Quoting
https://www.faa.gov/documentlibrary/media/advisory_circular/ac_90-105a.pdf

Operating Procedures (General). For baro-VNAV approach operations, the flightcrew
should be familiar with the following operating procedures.

Temperature Limitation. Because of the pronounced effect of nonstandard temperature on
baro-VNAV systems, RNAV (GPS) approaches with an LNAV/VNAV line of minima
should contain a low and high temperature limitation. Uncompensated baro-VNAV
systems may not operate to the LNAV/VNAV DA when the actual temperature is below
or above the temperature limitations. The temperature limitation will be shown as a note
on the procedure. If the aircraft contains a temperature compensation capability,
manufacturer instructions should be followed for use of the baro-VNAV function

[Hardy Heinlin]

Thank you very much. That's all clear. I just wonder how the vertical ANP is determined. The lateral ANP, for example, is reduced when the GPS fails. I see no similar possibilities re vertical ANP: The source is -- probably -- always barometric and it's never compensated for non-ISA OAT. That means the vertical ANP is probably always constant for a certain aircraft type. The barometric system is either on or off. When it's on, the ANP is permanently, say, 45 ft or something -- on the 744. And maybe 75 ft on the 737. -- I don't know ...


|-|ardy

[tango4]

Actually vertical guidance is based on BARO VNAV for RNAV approaches using LNAV/VNAV minima.


But when using LPV minima (stands for Localizer performance with vertical guidance if I recall correctly) it uses
a SBAS signal for vertical guidance (EGNOS/WAAS) and the aircraft must be tuned to a specific channel for the procedure and is more precise.


So perhaps in that case, if the system detects of loss a EGNOS signal it will lose its vertical precision. There are regulations for that but I think prior to the FAP, the crew can revert to LNAV/VNAV if that happens.


Charles

[Hardy Heinlin]

Thanks, Charles.

I just found this text for the 747:

The VERT ANP accounts for altimeter splits, by computing the difference of the baro-corrected altitude inputs. If the difference between the Captain and the First Officer's baro-corrected altitude is greater than the normal operation VERT ANP, this difference is displayed as the VERT ANP and reflected in the allowable deviation limits on the PFD, when in VNAV path descent. The detection of the altitude split is primarily for blunder checks on baro-set and undetected air data failures.
For example at the operating speed of Mach 0.8, the VERT ANP is normally 145 ft, based upon the flight test derived Mach-based polynomial given in section 2.11.2.1. If the split between the two barometric altitude inputs is 170ft, then the FMC would display, use, and output 170 ft as the VERT ANP to reflect the altitude split.