GPS Primary on Airbus

[DutchOne]

Hey to all,

Last time me and my captain where timing for GPS primary to appear after initializing the ADIRSs. It took 7 minutes.

In the meanwhile we wanted to know all requirements for GPS Primary to appear.

After a few seconds we saw that the GPS was in NAV/10 (on the GPS position page). 10 sats are enough for RAIM, but no GPS Primary. We inserted our position in the MCDU INIT page, but didn´t align the IRSs on purpose. We consulted the GPS perdiction page, but no prediction found.

After 7 minutes the GPS Primary appeared and the IRSs aligned automatically.

Can anyone tell me more about this?

Thanks a lot
D1

[rudderrudderrat]

Hi D1,
Can anyone tell me more about this?
It's all explained in FCOM 4.5.10.

"If the GPS is available, initialization is automatic, using the GPS position. Pilot intervention is not necessary.

However, automatic initialization may be manually overriden by pilot entry, at any moment during the alignment phase. In this case, perform the following procedure as soon as possible, to prevent delays if an alignment error occurs : etc."

We consulted the GPS perdiction page, but no prediction found.
If you don't tell the IRS its position initially, how can it predict what satelites will be visible?

[DutchOne]

Thanks rudderrat

Im aware of FCOM4.5.10, but that does not explain my problem. What do we need for the GPS to become the primary means of navigation (= GPS Primary)?

Like i said.. the gps was functioning and both receivers found 10 satellites (NAV/10).

Thanks again for any further info
D1

[Iamneon]

Maybe this is the problem:

You need some ir drif before you get GPS primary.
the Idea is that every single GPS positions it at least the closest to 1 IRS position. if not you will not get a GPS primary. (you always loose gps primary when you press align irs)
this is to check the reliability of the GPS position.
maybe this is useful
ciao

[Microburst2002]

I'm not sure, but I think that if IRS and GPS positions differ in more than 12 miles, GPS cannot be primary.

Also, in GPS fitted 320s, FMGC position is not GPS position. It is GPIRS position. Therefore you need the IRS, too. Actually, GPIRS position is computed by the ADIRU, not by the FMGC.

[rudderrudderrat]

Hi D1,

What do we need for the GPS to become the primary means of navigation
GPS is not the prime means of Navigation -but it can be the primary means of updating the GPIRS position.
The IRS are the primary navigators. Once they are aligned, the GPIRS compares the GPS position with the sensed IRS LAT (and possibly the last remembered LONG), and if within tolerance, will be accepted as primary position updating.

If it fails the test - then runway position will update the FMS position on take off and DME/DME position updates will be accepted when airborne.

Edit. A few places have had the incorrect ILS coordinates in the data base. Once locked onto the ILS LOC with the DME, the MAP position was "updated" to somewhere else. :uhoh:

[DutchOne]

Thanks guys. i appreciate the input.
Just one comment. If you need IRS input, or irs drift, how come that GPS primary does appear without aligning the IRS.

ADIRUs have been activated. this activates the MMR what includes the GPS, but i never aligned the IRSs, so no position was known. Only the position calculated by the GPS. The moment GPS primary appeared, the position was updated, the IRSs new their position and from that moment the GPIRS position was calculated.

With other words, IRS did not need to be aligned to for the "GPS primary" to appear.

Did some research on the web, and I believe that GPS primary does appear when there are enough satellites signals received and RAIM is positive. Even with NAV/10 but no positive RAIM, GPS primary is lost.

Thanks again to all,
D1

[TyroPicard]

The IR does need to be aligned - but instead of the pilot sending the Lat/Long to the IR by selecting "ALIGN IRS" prompt, the GPS sends the position... as rudderrat explained a while ago.

[Iamneon]

HI DutchOne

the questions was
"Last time me and my captain where timing for GPS primary to appear after initializing the ADIRSs. It took 7 minutes."
From the Honeywell FMGS manual
"
GPS Position
On the aircraft, the GPS position is not actually a pure GPS position, but is a combination of GPS and IRS position (also known as GPIRS or hybrid GPS position).
Using a combination of the highly accurate GPS position and smooth and reliable IRS position ensures the hybrid GPS position that is sent to the FMS is very accurate and stable. If the aircraft is configured for GPS and the FMS determines the hybrid GPS position has the smallest EPE, this GPS position is combined with the mixed IRS position and used for navigation (IRS/GPS). However, the FMS must first validate the hybrid GPS data.
The FMS only uses the hybrid GPS position if the GPS data has integrity and the GPS bias (the difference between the mixed IRS position and the GPS position) is within limits. The GPS bias is compared to a threshold limit that accounts for any differences between the GPS position and mixed IRS position because of the IRS drift since alignment. The integrity of the GPS data is provided by RAIM when using Honeywell ADIRUs and AIME (autonomous integrity monitored extrapolation) when using Litton ADIRUs. When integrity is no longer available because the horizontal integrity limit exceeds the threshold limit (defined by RNP and update integrity limit), and no satellite failure is detected, the FMS uses alternate integrity monitoring (AIM) to extend the use of GPS position.
If GPS position is unavailable, the FMS also applies the GPS bias to the mixed IRS position to increase the navigation accuracy. The FMS continues to use the bias in the position calculation until GPS position is again available or the EPE for the another sensor combination is smaller than the EPE for GPS position.
At any time, the pilot can deselect GPS for navigational use by pushing 5L (DESELECT GPS) on the SELECTED NAVAIDS page.
"