pilotnik

Hey,

Why is RAIM prediction only valid for a preflight planning? If there is a NOTAM stating that RAIM is not available at the specific airport for a specific time, how come you are allowed to perform the RNP approach anyway?

Thank You

OutsideCAS

It might be to do with the on-board almanac update (happens roughly every 5 days?) not matching the more reliable and comprehensive data available 'real world' and available in a NOTAM, that's updated much more frequently and accurately, and thus required at the panning stage legally.


Perform the approach despite the RAIM prediction (NOTAM'd) saying N/A - I'd suggest this wouldn't be allowed for a purely GNSS approach. Depends on ground-based NAVAID's and all sorts of other considerations (WAAS?) also to allow approach other than purely GNSS...maybe?.

RUMBEAR

RAIM not available at planning means you need to plan contingencies such as ensure other approaches available. In flight the ability to commence an RNAV/RNP approach depends on actual nav system status. If RAIM actually not available you have alternative options.

pilotnik

Let me tell you why I am asking this. I am an ATCO and there was an RAIM unavailability prediction over the airport for a short time. Nevertheless, during this very time, TWO aircraft requested RNP approach. Both were informed about the lack of RAIM and even though they insisted on the approach.

Check Airman

The preflight RAIM check is just a prediction. It's possible that they did a "live" check at the start of the approach, and found that RAIM was available. The avionics will also let you know real time if RAIM is unavailable.

Not saying it was right- just raising a few possibilities.

What was the wx at the time? If it was VMC, they may have been able to fly the procedure without RAIM.

Empty Cruise

Pilotnik - you in the EU, your status says... now, if SBAS coverage is ok, you are fine without RAIM, so unless you also had an EGNOS outage, or your RNAV approach does not have LPV minima promulgated, the crew may be perfectly legal to fly the approach during a RAIM outage....

latetonite

A RAIM prediction is just a prediction. What you fly on is ANP. If you could get a better ANP than the RNP, you are good for the approach.

FE Hoppy

Exactly this.

We are LPV capable, use SBAS and have no requirement to check RAIM because the integrity is done through the SBAS signal.

custardpsc

pilotnik - what the others said - if the augmentation is satisfactory ( EGNOS in eu, WAAS in USA) then no need to check RAIM.

But I didn't get Empty Cruises point about LPV minima. I know that LPV requires WAAS but if you have integrity and there are no LPV minima what stops one using LNAV/VNAV approach / minima ?

pilotnik

As far as I know there is no way to check RAIM availability on B737. The WX was IMC.

OK, I was thinking about that too, but consider this: If RAIM is not available then the computer is not able to detect a bad signal from satellite. So if a bad signal cannot be detected the ANP will not reflect the actual performance at all. RNP approaches are ONLY allowed based on GNSS sensors, so it seems to me that without RAIM, the ANP reading is useless. Isn't that right?

I think you guys have a wrong idea on the RNP approaches, BUT it may also be that it is me who is wrong From what I have read SBAS (or WAAS in USA) is only used for LPV approaches. It has nothing to do with LNAVs or LNAV/VNAVs. The only GNSS augmentation for the purpose of RNP APCH to the LNAV or LNAV/VNAV minimums is RAIM.

Skyjob

RAIM check can only be carried out by some operators preflight as aircraft are unable to detect it or access the (internet or ground) sources to retrieve it in flight.

If RAIM is therefore suitable preflight it can be used unless ATC advised to the contrary, they have access to last minute data.

In the OP follow up post he advises he is an ATCO and asked why aircraft asked to do such an approach when RAIM is not suitable.

Possibly the crew forgot to check preflight or perhaps their company manuals do not require them to do so (I doubt the latter).

If company does not include RAIM prediction for destination by default on the flight plan it usually requires additional crew actions preflight to access this data from another source, this can be time consuming and hinder an on time departure, or simply can be forgotten.

It would be recommended to advise operators to include this data for crew to read as standard during preflight, making them aware.

With RAIM outages far and few between with intervals not exceeding sometimes some minutes in EU, it is easy to miss an outage especially when they are printed in a graphical timeline of 3 days instead of numerical readout. Black and white printers producing the colour image also do not aid the crew in this. Just some random issues observed...

Empty Cruise

Custard - you are of course right, if you are under SBAS coverage and intend to fly an RNP-1 STAR - no RAIM check required.

The point about promulgated LPV minima still stands (kinda) - because, if you are in an areas that should be covered by SBAS - why would you chose to not promulgate LPV minima? Obstacle clearance survey is basically the same as for LNAV/VNAV, so there is no money to save - which would make me think that something else is preventing a successful LPV implementation.

We operate in and out of SBAS coverage areas on a daily basis - so we have just decided to be done with it and always perform a PRAIM-check before departure, unless both crew agree that - yes, this is SBAS all the way.

In theory - SBAS ‘only’ saves one of the required 5 sats in visibility and decent geometry for FDE - if there is no GPS signal, SBAS is useless. So I do get your point - have been too lazy to consider it myself...

underfire

pilotnik.

RAIM prediction is used preflight, and if the RAIM prediction shows that RNP is not available enroute (depending on the procedure) or on final destination, legally, the flight cannot be planned to use the RNP procedure. (especially AR)

That is the simple fact of the flight plan, if RAIM prediction shows not available, you cannot plan to use the procedure.
We have been trying to change this, if conditions change enroute, but the criteria prevails.

Conversely, if RAIM prediction shows that it is avail, and during flight RAIM goes downhill on you, you cannot use the procedure...

I hope you are kidding.

In general, reading through this thread on RAIM, the explanations and reasonings provided are sad, very, very sad.

aterpster

With TSO C145/146 (WAAS) avionics, if WAAS (SBS) is not available, FDE negates the need for a RAIM check, at least in FAA-dom.

latetonite

So, anybody starting an approach with: 'RAIM prediction from this morning checked, ANP checked'? And the actual RAIM, where you read it?
I would not mind starting an LNAV/VNAV approach, provided it was within my RNP, even if it was obtained with a sextant, so to speak.

FE Hoppy

undefire

Think you better take this up with BBD and the certifying authorities because FCOM 1 16-03-04 says the following:

"The SBAS performs integrity monitoring when the aircraft is operating in
SBAS coverage areas and the GNSS is operating in SBAS mode. When
the SBAS position is available, the predictive RAIM check is not required."

So no, I'm not kidding and the info I added could hardly be describes as sad. Accurate yes, sad not so much.

A Squared

No, it is not right. RAIM stands for Receiver *Autonomous* Integrity Monitoring. In plain English, that means the GPS receiver's ability to determine *by itself* the integrity of the GPS signals. The operative concept is *by itself* (that's what "autonomous" in the title means.) There are other means of verifying integrity and accuracy of GPS signals whcih are not "Receiver Autonomous". Systems of augmentation such as SBAS, WAAS, ground based augmentation, etc provide alternate means of determining the integrity of the GPS signals, so ensuring position integrity is not dependent on "RAIM". A NOTAM for lack of RAIM coverage does not make it illegal for all operators to fly an RNP approach. RNP only means "Required Navigation Performance" and there are different ways to achieve the required navigation performance for that approach. Without you having a detailed knowledge of the specific navigation equipment installed in the aircraft in question, and a knowledge of what was displayed on their cockpit navigation displays at the time, you have no way of knowing whether they were legal to fly the RNP approach. However, chances are pretty good that the were legal to fly the approach.

oggers

Very simply, the avionics will have indicated if they were able to carry out the procedure.

They may have been equipped with baro aiding which substitutes for one satellite or they may have been equipped with SBAS in which case RAIM is irrelevant. Even if they lacked either it is still quite feasible that the approach and the flight planning were totally legit The single piece of evidence “there was a predictive RAIM unavailability over the airport for a short time” is not in itself evidence of non compliance.

pilotnik

OK I think most of the answers are quite reasonable. A friend of mine just sent me some pics of A320 CDU and it indeed has live RAIM. I wasn't aware of that as on B737 there is no way to see anything more than ANP. I was trying to google what kind of GPS receiver do 737NGs have but I failed. Maybe there is an algorithm equivalent to RAIM inside but I can't find any info on it.

Another thing is that I really don't understand how could SBAS help doing RNP LNAV or LNAV/VNAV approach? Or why would baro aiding allow for one satellite less in case of LNAV/VNAV? For LNAV, baro aid is obsolete. For LNAV/VNAV baro aid is a must and if it allowed for a satellite number reduction it would mean that reduced number of satellites is in fact a standard number... And for LPV? This is the only, fully RNP GNSS approach and baro aiding here is useless.

I think some of you guys confuse ABAS, SBAS and even GBAS. I've read through dozens of documents on PBN and interchangeable use of augmentation types is nowhere to be found. You can't just use GBAS or SBAS to compensate for lack of satellites in order to perform LNAV/VNAV. Am I crazy or am I missing something important here?

A Squared

Well, as far as the RAIM and baro-aiding, Here's how RAIM works on a conceptual, mathematical level. A GPS receiver computes a position by solving for 4 unknowns: Coordinates in 3 Dimensions (X,Y,Z) and receiver time offset from the GPS satellite system. The latitude and Longitude you see displayed on the receiver are translated from the GPS systems native geocentric (earth centered) coordinate system. As you may remember from algebra, to solve a system of equations for 4 unknowns, you have to have 4 equations. Each measurement to a different satellite adds one equation to the system, so if you have measurements to 4 satellites, the receiver can solve for X,Y,Z and dT. However, with only 4 satellites you have no way of checking that the 4 signals are all correct. However if you have measurements to 5 satellites you have a redundant, or over-determined solution, and with a redundant solution you can tell if the measurement from one satellite doesn't fit the solution. But you have no way of telling which measurement is bad, just that the 5 measurements don't all fit the solution within a certain tolerance. If you have measurements to 6 satellites, that gives you a tie breaker, which allows you to determine which satellite signal is bad, and drop it from the solution.

Now, how does baro-aiding fit into that? Baro-aiding can be used to add one more observation to the solution. If you have baro-aiding, you know what your altitude is, and that essentially reduces the number of unknowns that you have.

If it helps, you can think of it as the receiver solving for lat, long, altitude, and time offset, requiring 4 satellites for a solution, but with baroaiding, altitude is known, so you can solve for lat, long, and time offset with measurements from 3 satellites. That's not quite accurate, like I said the receiver is calculating in Geocentric Cartesian coordinates, but conceptually it's close enough that it's not incorrect, exactly. This was common in the early days of GPS, especially for marine applications, when the GPS satellite system was not yet complete and coverage was spotty. GPS receivers had an option of the user entering the elevation if known (sea level plus antenna height for a boat on the ocean ) which allowed navigation on only 3 GPS satellites. Baro-aiding does the same thing, it automatically enters the altitude derived from the barometric altimeter to the GPS solution and reduces the number of satellites required by 1; 3 satellites fro a minimum position solution, 4 satellites for an over determined position that checks for a good solution, and 5 satellites for the minimum satellites to catch an error, and identify the satellite signal in error.

So, that's the basic concept. I would expect that the TSOs and other standards under whcih the systems are designed and approved would require a higher level of redundancy, so it's likely that "RAIM Availability" from a regulatory standpoint requires more satellites than in my description, but at the algebra/geometry level, that's what's required.