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A320 RAIM Check

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A320 RAIM Check

Old 26th Apr 2021, 13:30
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A320 RAIM Check

Dear All,

This is the closest I could find to my question and somewhat helpful. A320 'GPS PRIMARY LOST'

Could anyone explain the difference of GPS Primary and RAIM, I am aware there is a difference but not able to imagine it clearly. In addition, how does one perform the RAIM check for an RNAV (or similar) approach? Is RAIM the Y Y Y Y seen in MCDU PROG page?

Thank you.
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Old 26th Apr 2021, 15:35
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To oversimplify things, RAIM tells you if you have a good signal. GPS primary tells you the bias used in computing the FMS position.

They both deal with GPS, but are otherwise unrelated concepts.
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Old 26th Apr 2021, 16:34
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Here’s my take:

Your GPS does RAIM continuously. If the monitoring fails you get GPS PRIMARY LOST.

Depending on the type GPS installed and operational configuration your onboard GPS can compute future RAIM. If you look on the predictive GPS page you will see 5 minute intervals of the destination or any pilot inputted waypoint . A “Y” means GPS will (should) be available for navigation.

What the box can’t know planned outages. Therefore we check NOTAMs at our destination, cross check them for time and approach planned to determine if GPS will be available to fly the approach.
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Old 27th Apr 2021, 03:43
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Here's my take:
The big problem is Airbus don't specifically use the term RAIM in their documents (not much anyway)

This is a quote from our FCOM;
"When in IRS/GPS made, the GPS PRIMARY status combines two different criteria:
- The accuracy criterion (HIGH / LOW accuracy)
- The integrity criterion: This is the capability to detect a failure and provide appropriate warning of it."

Additionally
"If the GPS data does not comply with an integrity criterion that is based on a Horizontal Integrity Limit (HIL) and on the automatic detection of failed satellites, the FMS rejects the GPS mode and uses the radio position update"

I have always interpreted the "detection of failed satellites" as a loss of RAIM.
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Old 27th Apr 2021, 09:12
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RAIM is a kind of selt-test made by avionics in order to predict the satellite coverage let’s say for a specific approach , or part of a route..

https://nbaa.org/aircraft-operations...ng-system-gps/
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Old 27th Apr 2021, 21:33
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The RAIM check is needed depending on your navigation. For horizontal and vertical, you need 4 sats. To have error checking, you need 5, You need 6 for it to error trap and null one out.
For navigation such as RNP-AR, the system will use the RAIM check to feed the HIL...(and the HAL alarm limit)
With RNP AR operations, you have to use the RAIM predictor prior to DEP....If the predictor shows insufficient coverage, you cannot use that procedure.
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Old 28th Apr 2021, 11:17
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I was actually thinking about that one, cause we have a requirement stating we're not allowed to give RNP approaches in general if we get a RAIM prediction message predicting outage. Doesn't matter what the pilots say on the matter.
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Old 28th Apr 2021, 14:21
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Does anybody’s SOP call for use of the “DESELECTED SATELLITES” function on the predictive GPS page? If so, can you provide some details?


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Old 30th Apr 2021, 17:47
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Cheers guys. So in a nutshell:

RAIM - Integrity of the satellites which make up GPS system.
GPS Primary - The mode used by the aircraft for navigation, ie: GPIRS.
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