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RAIM and RAIM outage?

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RAIM and RAIM outage?

Old 15th Jan 2021, 10:56
  #1 (permalink)  
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RAIM and RAIM outage?

A question from a controller to which I simply cannot wrap my head around, so help me a bit here.

RAIM, or Receiver Autonomous Integrety Monitoring, is a system built into a GNSS receiver in the aircraft, that is able to detect, FD, or detect and exclude, FDE, any faults in the signals received from GNSS satellites.

So far so good -> The aircraft GNSS reciever is itself able to monitor the integrety and alert pilots if any fault in the signal.

Now comes the real question:

Why, if the aircraft reciever is able to do so itself, do we issue "GPS RAIM prediction" -NOTAMS? And what does that mean?

Is it a safety layer on top of the RAIM onboard the aircraft, to advice that there in fact is faults on the signal within an area, and allow aircraft without built-in RAIM to operate? Or does it mean that the RAIM function in the aircraft simply does not work, which I don't really understand, since the function itself should always be working, it'll just show a warning (in the cockpit) if there is a fault?

And last but not least, if using SBAS (EGNOS/WAAS), can you then fly RNP approaches even if the RAIM is out of service? Considering that SBAS is a requirement for LPV minima (but RAIM not).
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Old 15th Jan 2021, 11:27
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This could be kept rather simple. Your assumptions are correct, RAIM is a self-evaluating algorithm internal to the on-board box, clue the name.

The receiver needs signals from 4 satellites to calculate its position in 3D space. If 5 were in view it is possible to calculate 5 separate, semi-independent positions, by excluding one satellite and using the remaining 4 for triangulation. Comparing those 5 positions one against another is what RAIM does and if a large enough discrepancy is found a fault is flagged.

GPS RAIM not available simply means there will not be 5 satellites in view. More rules to the game, but such is the layout of the course.

For the last sentence:
- you need RAIM to fly the approach
- if additional corrective signals (augmentation) are available, it is allowed to use lower minima on that RAIM-required approach since finer precision is assumed.

Approaches without RAIM do not exist, just like it is impossible to fly an ILS without somebody monitoring the signal and checking it on regular basis.

An indicated position without means to
- ascertain the signal is valid
- quantify the magnitude of possible location error
is useless and never used(*).

(it is on Air Crash investigators )
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Old 15th Jan 2021, 11:27
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For RAIM to work the receiver needs to be able to see a certain number of satellites (5,6?) so that it can calculate multiple positions, realise that there’s an error somewhere, work out which satellite is in error and discard its input.

If it’s known that satellite coverage in an area is going to be limited for a period of time so that RAIM won’t work then RAIM outage is advised.

I have no view on the SBAS thing. Intuitively it would seem logical that it would work because of the corrections put in by SBAS, but I really don’t know.

HtH
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Old 15th Jan 2021, 12:11
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So summing up the above and asking your questions. The notams are issued so pilots have the knowledge about posible raim outagest, and therefore be prepared for the unability to perform certain form of navigation / approach.
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Old 15th Jan 2021, 13:03
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WAAS and RNP would seem to go together, but they donít. You wonít have WAAS without RAIM either.

Iím sure some business jets have the capability, but Iíve never flown a plane that has the capability to do both LPV and RNP approaches.
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Old 15th Jan 2021, 15:23
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You are confusing on board real time monitoring vs 'prediction'. The on board RAIM works only in real time and gives you the current status of the accuracy and GPS functionality. The RAIM 'prediction' for a geographic location works out the expected satellite positions at any given time and works out when and if there could be a lack of satellites to satisfy navigation requirements.

Airbus now only requires that pilots check RAIM predictions when the destination has terrain around that may mask satellite reception or when flying an RNP-AR approach with legs that have RNP of below 0.3NM. This is because GPS coverage is now considered available 24 hours a day down to 0.3NM world-wide except if there is terrain around that masks signals.

Again, your receiver can check current satellite status but it isn't going to know whether in 4 hours when you get to Kathmandu and you need to do an RNP-AR approach due weather, if you'll have the required satellite coverage. You want to know that before you get there, not once you get there.
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Old 15th Jan 2021, 19:28
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Prediction is also used in flight planning. To be able to plan a certain RNP approach at destination (or alternate, Etops, ...) you have to know about an Raim outage and itís duration. So either you plan another approach type or you load more fuel.
Particularly useful in Africa.
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Old 15th Jan 2021, 21:03
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Originally Posted by Check Airman View Post
WAAS and RNP would seem to go together, but they donít. You wonít have WAAS without RAIM either.

Iím sure some business jets have the capability, but Iíve never flown a plane that has the capability to do both LPV and RNP approaches.
All of the Globals and Gulfstreams have both WAAS (LPV) and RNP (AR) approach capability. The rules on RNP(AR) are difficult for private operators to comply with, too many random routes, too many other options. LPV is good .
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Old 16th Jan 2021, 07:32
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Thanks everyone, really appreciate the answers.

I've been digging into the subject for quite some time now, but there was always a few things I really either couldn't find an answer on online, or brought up the question if my assumption was correct.

You confirmed my assumptions, so thanks.
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