RAIM Check

[GregP]

What's the gen on checking for RAIM using the aircraft's equipment .. is it mandatory even if one gets timely Notam advice about possible outages for specific locations?

If there's a mandatory requirement to get RAIM prediction onboard (in flight or pre-departure) even though one has a clear Notam in the hand, where is this stipulation to be found?

From what i've read, Notam advice regarding RAIM availability is possibly a superior source of the info' than what the onboard machine might say(?!).

[wishiwasupthere]

My understanding is that the RAIM check within the GPS uses it's onboard almanac to predict if a suitable number of satellites will be available at the time and place specified, whereas the NAIPS RAIM check incorporates other real time information such as satellite health and serviceability to provide a RAIM prediction.

[Old Akro]

At least for C129a GPS it's mandatory to check RAIM prior to an instrument approach. It is an almanac look up process.

The C146a GPS units have fault detection, so they may automatically warn of loss of RAIM. But, I have C129a, so I'm not sure.

It's rare not to have RAIM, but it does happen. GPS can be jammed also.

[27/09]

At least for C129a GPS it's mandatory to check RAIM prior to an instrument approach. It is an almanac look up process.

The C146a GPS units have fault detection, so they may automatically warn of loss of RAIM. But, I have C129a, so I'm not sure.


If you have a RAIM prediction from an authorised source like Air Services or Airways NZ there is no need to do a RAIM check prior to the approach at least In NZ anyway.

A RAIM prediction from Airways is more up to date/accurate than one done in the aircraft.

TSO C129 units will give a RAIM warning.

[Oktas8]

TSO C129 and 145/146 certified receivers must have an automatic RAIM prediction service prior to passing the FAF.

They must not enter Approach mode if the prediction fails, or if in any event RAIM is not available at the FAF.

If a receiver doesn't do those things without pilot input, it's not suitable for IFR use.

This information is found in the FAA's TSO archive.

As wishiwasupthere says, a paper NOTAM prediction is superior.

Apparently there are some ATOs who mandate a manual RAIM prediction in flight. It's good airmanship to know how to do this, but in most cases there is no legal requirement to do so.

[2bigmellons]

From the NAIPS GPS RAIM Australia page:

"The Airservices Australia Receiver Autonomous Integrity Monitoring (RAIM) Prediction Service (RPS) provides predictions of RAIM availability for flight planning purposes only.
Pilots-in-command are reminded that the predictions are based on information received from the GPS Operating Authority.
In flight, pilots-in-command should use the RAIM prediction facility of their GPS equipment for RAIM availability predictions.
Presence of RAIM should be continuously monitored whenever GPS is used for navigation."

[iPahlot]

As wishiwasupthere says, a paper NOTAM prediction is superior

I don't think the RAIM prediction is that smart that it will look in to the future and predict that a satellite will develop a fault while you are enroute.

So even if there isn't a legal requirement for it, I think "It's good airmanship" should apply.

[Oktas8]

Hmm. It seems there is a lack of traction on the issue of automatic versus manual RAIM events.

2bigmelons,In flight, pilots-in-command should use the RAIM prediction facility of their GPS equipment for RAIM availability predictions. All pilots use the RAIM prediction facility of their TSO'd receiver. It's automatic, prior to all GPS approaches.Presence of RAIM should be continuously monitored whenever GPS is used for navigation. And indeed it is; continuous and automatic, for any TSO'd GPS receiver in NAV mode.

The issue is, will the pilot notice the little flashing light or little blinking message that tells you RAIM isn't or won't be available. That's what needs monitoring, and that is at the heart of the NAIPS & CASA advice.

iPahlot,
I don't think the RAIM prediction is that smart that it will look in to the future and predict that a satellite will develop a fault while you are enroute.

You are right, of course. But here is the problem: if you do a manual prediction 30mins before ETA, I can ask you to do another one at 29mins. Or 28, or 27. At any stage, a hitherto unpredicted fault might develop. Where does it end? The law requires a prediction before flight (NOTAM) and a prediction before the approach begins (automatic). Where do the extra checks end? How can you justify a manual check at time T, but not at time T+1?

I mentioned good airmanship. Here's when I might do a manual prediction:

Proceeding to destination, then having to divert to a destination,
a) for which I will need a GPS approach and
b) for which I don't have a NOTAM prediction and
c) knowing I will have limited fuel on arrival for go arounds and holding.

That I think would require a manual receiver prediction as soon as possible.

Regards all,
O8

[glekichi]

I don't think the RAIM prediction is that smart that it will look in to the future and predict that a satellite will develop a fault while you are enroute.

So even if there isn't a legal requirement for it, I think "It's good airmanship" should apply.

The RAIM prediction in the box comes from an almanac - it won't detect the failure of a satellite that occurs after that almanac is published.
From memory the almanac in the box is downloaded once a week, at least on the type I fly.

While the RAIM prediction data from Airservices might be a couple of hours old, the data on your GPS/FMS is more likely several days old.

A RAIM prediction is totally different to a warning or failure.

[MakeItHappenCaptain]

The receiver will not sequence onto appch (or LNAV) mode if RAIM won't be available, so the idea of doing the manual check is so that you can select a different approach before you even begin, rather than waiting until it's about to transition and being unable to continue.:ok:


The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.

[Bladeangle]

The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.

Its not irrelevant if your relying on using a 145/6 unit as one of your 2 approaches required (CHTR/RPT) or as your sole approach (PVT/AWK) its required. But as someone has already mentioned here, the outages are usually only for 6 minute or so intervals (historically).

I understand the NAIPS FD/FDE prediction is more accurate as it can have info on scheduled outages/maintenance which the units almanac will/may not have.

The receiver will not sequence onto appch (or LNAV) mode if RAIM won't be available, so the idea of doing the manual check is so that you can select a different approach before you even begin, rather than waiting until it's about to transition and being unable to continue.

Exactly! Why wouldn't you do one in flight? Finding out on a 7nm final in IMC is prob not "best practice".

From memory the almanac in the box is downloaded once a week, at least on the type I fly.

Possibly if the unit is only turned on once a week. Im sure its updated a little more often that that.

[27/09]

MakeitHappenCaptain:
The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.

I don't believe this statement is correct.

Whether or not the unit scales to approach mode/approach active has nothing to do with whats in the almanac. If the GPS has the required satellite coverage for RAIM, it will scale, if it doesn't it won't scale, simple as that. It doesn't matter what the almanac says.

I've seen units fail to go to approach mode/approach active even though RAIM was available. Poor satellite geometry relative to the ships GPS aerial might cause this.

All the almanac does, is provide data to predict if RAIM will be available. Nothing more nothing less.

Baldeangel:
The receiver will not sequence onto appch (or LNAV) mode if RAIM won't be available, so the idea of doing the manual check is so that you can select a different approach before you even begin, rather than waiting until it's about to transition and being unable to continue.
Exactly! Why wouldn't you do one in flight? Finding out on a 7nm final in IMC is prob not "best practice".
Why would you bother if you had the RAIM prediction from ASA or Airways. The RAIM prediction from the ships GPS unit isn't going to tell you anything that you don't know. As has been already said the ASA or Airways prediction is going to be more accurate anyway.

[Bladeangle]

If the GPS has the required satellite coverage for RAIM, it will scale, if it doesn't it won't scale, simple as that. It doesn't matter what the almanac says.

How does the receiver predict RAIM will be available for 5 minutes after passing the FAF without knowing where the satellites will be in the constellation?
I thought it would use the almanac?

I've seen units fail to go to approach mode/approach active even though RAIM was available. Poor satellite geometry relative to the ships GPS aerial might cause this.

How do you know RAIM was available at the time?

Why would you bother if you had the RAIM prediction from ASA or Airways. The RAIM prediction from the ships GPS unit isn't going to tell you anything that you don't know. As has been already said the ASA or Airways prediction is going to be more accurate anyway.

I think you've answered your own question.

[glekichi]

So the problem arises when a satellite is scheduled to be offline for whatever reason, but the status is changed to be online, and this happens between the retrieval of NOTAMS and the almanac getting to the aircraft (once again, pretty sure from training material this happens once a week only on the particular Universal FMS I use).

Airservices updates their information to correctly reflect this, NOTAMS say all ok, but the box doesn't know, so refuses to go into approach mode because it thinks it won't have RAIM but in fact it will. Thats assuming the satellite that it thinks is offline will make the number available less than that required for RAIM.

VERY rare scenario, but I guess you could say it is a reason to do the RAIM prediction in flight.

As for the flight test / instrument renewal form item - check RAIM availability - is there a RAIM warning or failure flag? If not then you have RAIM and checking for the absence of a flag IS checking its availability.

[Bladeangle]

Having a read of the Universal user manual for the unit I use interestingly says:

During enroute and approach operations, the FMS provides pilot- requested predictions of RAIM availability at the selected ARRIVE waypoint for a period of ETA ±15 minutes in five minute increments.

NOTE: The FMS does not provide automatic RAIM prediction at approach arming and activation.

Hense why it would be prudent to conduct one in flight prior to approach.

[glekichi]

If its got the new software that doesn't do the old prediction and lock out the approach if it doesn't predict RAIM, why do one?

Are you going to not shoot the approach if it says no RAIM but the more up to date NOTAM says you will have RAIM?

[27/09]

How does the receiver predict RAIM will be available for 5 minutes after passing the FAF without knowing where the satellites will be in the constellation?
I thought it would use the almanac?

I don't think this is the way it works. The GPS doesn't go to approach mode/approach active based on a RAIM prediction and doesn't concern it self as to whether there is going to be RAIM 5 minutes after passing the FAF.

My understanding is RAIM warnings are inhibited after the FAF and the GPS will provide a DR nav solution for 5 minutes after the loss of RAIM after passing the FAF, meaning an approach can be completed and if necessary missed approach can be carried out in DR mode.



How do you know RAIM was available at the time?

The RAIM prediction from Airways said RAIM would be available. The other box didn't have a RAIM warning.

I still don't see any point in doing a RAIM prediction just prior to the approach so long as you have one from ASA or Airways. You are not going to learn anything that you don't already know.

Remember we are talking about a prediction based on an almanac, what happens in real life can be and is different.

While there may be enough satellites to ensure RAIM there can be other factors that will cause a GPS unit to not have RAIM available.

[Bladeangle]

Not sure if the units have the new or old software, I know its 1000 series software and are 146 units. Have done approaches in day VMC without the
in-flight RAIM prediction, and there was no lock-out of APPR mode.

Are you going to not shoot the approach if it says no RAIM but the more up to date NOTAM says you will have RAIM?

No I wouldn't.

[glekichi]

AFAIK older universal systems locked you out of approach mode if the internal almanac said you weren't going to have RAIM at the ETA, based on an automatic RAIM prediction when an approach was armed. Later revisions removed this restriction because it was outdated and from a time when the more accurate/up to date database was not readily available..... I could be totally wrong on that, but it is my understanding.

Are you going to not shoot the approach if it says no RAIM but the more up to date NOTAM says you will have RAIM?
No I wouldn't.

So, an out of date RAIM prediction says no, a more current one says yes, and there has not been a RAIM failure or warning, but you're going to turn around and go home if thats the only approach?
Why?
You've still got enough satellites to navigate and to tell if one of those is giving erroneous information - if you didn't you'd have a warning.

[Bladeangle]

So, an out of date RAIM prediction says no, a more current one says yes, and there has not been a RAIM failure or warning, but you're going to turn around and go home if thats the only approach?
Why?

Is it an out of date prediction?

Company SOP's. Regs say RAIM must be available prior to descending below LSAT/MSA.

Its a bit like not tuning and testing a ground navaid just because it is not NOTAM'd as being unserviceable.

[glekichi]

The below is from Universal themselves.

PROBLEMS WITH RAIM PREDICTION
The GPS almanac is released every seven days and is automatically down loaded by the GPS in
the FMS. During the power up of the GPS the almanac date is compared to that transmitted by the
GPS satellites. If the residing almanac is out of date a new one is down loaded.
The problem occurs in that real world satellite changes may not be reflected in almanac data
currently in the receiver. In other words, when on day five a RAIM prediction is made the satellites
used may not reflect the real world.


Hey, I do a RAIM prediction as well, because SOPs say so; the point is that its pointless - the NOTAM is more up to date than the almanac in the FMS.

If your SOPs just say that RAIM must be available, same as the flight test requirement, like I said earlier, it IS available provided you don't have a warning, INTEG light, etc.

[Oktas8]

Bladeangle, after reading your & Glekichi's remarks about late model GPS receivers, I did some reading. I learned something. :ok: (I use a C129 Universal receiver myself.)

In SBAS-enabled units, which includes Universal SCN 1000-series software, RAIM predictions are not automatic prior to an approach. I guess you know that you are much less likely to get a RAIM outage with an SBAS-enabled receiver, because it has a better GPS decoding algorithm (less likely to suffer from rain fade etc) and because the Japanese SBAS geostationary satellite is permanently in view, giving you an extra GPS satellite in view at all times in Australia. (Can't use it for SBAS, can use it as an extra GPS transmitter.)

So if your SOP requires you to have an in-flight RAIM prediction, and you have an SBAS-enabled receiver, you will need to do it manually. My operator does not require it for C129 or for 145/146 receivers, as the NAIPS notam system meets all practical requirements. But SOPs differ, as we all know!

If your SOPs just say that RAIM must be available, same as the flight test requirement, like I said earlier, it IS available provided you don't have a warning, INTEG light, etc. Correct, for all receivers. Do CASA regs require an in-flight prediction? I don't think so, although goodness knows I'm no expert on this particular country's regulations. They require a prediction before flight, and actual RAIM availability on the approach. AFAIK.

I've had RAIM warnings on approach even though both NAIPS and the in-flight prediction were ok.
I've seen a receiver give the ok even though NAIPS said there would be a RAIM hole (approach not flown in that case.)
I've had NAIPS say ok, only to have the receiver (briefly) predict no RAIM at the FAF, then flown the approach successfully.
You'll have to excuse my belief that manual receiver predictions are not worth the time it takes to perform them!

One error above, regarding loss of RAIM on the approach in the so-called 5 minute "coasting" segment inside the FAF. It's a bit off the topic. RAIM warnings are generated if a) RAIM is available and b) integrity limits are greater than required. These will always be presented to the pilot immediately, requiring a missed approach. Where a "loss of RAIM" occurs due to seeing only 4 satellites, the LOI alert may be inhibited for 5 minutes. Depending on the statistical integrity of the fix in other circumstances, "Loss of RAIM" may or may not be presented to the pilot in this 5 minute segment.

[Bladeangle]

Oktas8 thanks for the post! Interesting.

Unfortunately we are required to do both the preflight raim briefing on the ground for planning, and the in-flight check as well. Possibly to keep standardisation across both units. Fleet includes a mix of both the Universal 129 and 146 receivers (1k & 1lw). :8


PROBLEMS WITH RAIM PREDICTION
The GPS almanac is released every seven days and is automatically down loaded by the GPS in
the FMS. During the power up of the GPS the almanac date is compared to that transmitted by the
GPS satellites. If the residing almanac is out of date a new one is down loaded.
The problem occurs in that real world satellite changes may not be reflected in almanac data
currently in the receiver. In other words, when on day five a RAIM prediction is made the satellites
used may not reflect the real world.


glekichi, I do apologise for doubting you. I wasn't aware that it was only every 7 days, seems too long a period. Makes me wonder why there even is a facility to do a RAIM predict in flight if the one in-flight is of no purpose?

Unless...you were not on day 5 of the almanac cycle, but day 1, and it was more current than the one you obtained from airservices on the ground an hour before departure of say, a 4 hour flight?

Hense why when you posed the question:

Are you going to not shoot the approach if it says no RAIM but the more up to date NOTAM says you will have RAIM?

I said I wouldn't do the approach. (not sure I'd convince the other crew member either) Not because it wouldn't be safe, I know there would be an INTEG or RAIM warning if it wasn't right, but id rather do an alternate approach or alter the arrival time by minutes, rather than finding out half way down that it wasn't going to work.

Cheers.

[MakeItHappenCaptain]

Exactly! Why wouldn't you do one in flight? Finding out on a 7nm final in IMC is prob not "best practice".

That's exactly what I am saying...:rolleyes:

Form 645, Item 53, RNAV (GNSS) RAIM availability checked.

This is not just an ATO personal quirk, and notice how all of these requirements are chronologically ordered for each approach type? Notice how item 53 comes before the entry and holding pattern?
Same thing for DGA. RAIM availability checked again cometh before descent.

My understanding is RAIM warnings are inhibited after the FAF and the GPS will provide a DR nav solution for 5 minutes after the loss of RAIM after passing the FAF, meaning an approach can be completed and if necessary missed approach can be carried out in DR mode.

So you want to be tooling around at less than 800' AGL in IMC in DR mode when there was an indication RAIM wasn't going to be there? Really?:mad::ugh:

So, an out of date RAIM prediction says no, a more current one says yes, and there has not been a RAIM failure or warning, but you're going to turn around and go home if thats the only approach?

As before, you want to take that risk?

[27/09]

MakeItHappenCaptain:
This is not just an ATO personal quirk, and notice how all of these requirements are chronologically ordered for each approach type? Notice how item 53 comes before the entry and holding pattern?
Same thing for DGA. RAIM availability checked again cometh before descent.
If you already have current RAIM prediction from an approved source like ASA or Airways surely that would cover the requirement you mention. When you already have such a prediction doing a prediction using the aircraft's GPS is a pointless waste of time as the aircraft's almanac will not be as up to date as the one used for the ASA or Airways prediction.

MakeItHappenCaptain:
So you want to be tooling around at less than 800' AGL in IMC in DR mode when there was an indication RAIM wasn't going to be there? Really

No need for swearing and head banging

Firstly I used the wrong terminology, a GPS doesn't go to DR mode with just a loss of RAIM, it will navigate the same accuracy as before, but now it cannot ensure it's accuracy/integrity.

I never ever said I would commence/continue an approach if the RAIM prediction said RAIM would not be available. Show me where I said that.

What I did say is the RAIM prediction is just a prediction. There are times when the prediction shows RAIM will be available yet a RAIM warning will appear. You could do a prediction and find all was good and few minutes later have a RAIM warning.

What I also said it some units can inhibit a RAIM warning for 5 minutes after the FAF to enable the approach/missed approach to be completed.

[MakeItHappenCaptain]

27/09
If I was swearing at you I would have used the word fcuk, fcuking, fcuked or something similar. In any case any statement of willingness to be below LSALT/MSA on DR (as you have admitted was an incorrect statement) most certainly does require head shaped holes in the wall!!!

I am happy to admit when I'm wrong. Don't defend an incorrect statement.
The emoticons are my frustration that people are willing to forego a twist of the knob and pressing a couple of buttons to go that little bit further to make sure they will be able to complete the approach rather than having to execute the missed approach halfway through.

From the CASA GNSS course;
for most phases of flight the GPS receiver will provide an immediate annunciation of a loss of RAIM capability. The exception is five minutes following passage over the final approach waypoint of a non-precision approach. During this time, a loss of RAIM annunciation will be inhibited. GPS navigation may still be possible during this RAIM outage.
The GPS receiver automatically performs an approach RAIM prediction just before passing the final approach waypoint and will only enter approch mode if RAIM at the 0.3nm horizontal integrity limit is predicted to be available from the final approach waypoint to the missed approach waypoint.

Still don't believe that is correct?

Again...
Agreed, the NAIPs prediction is more accurate, however where do you think the GPS receiver is going to predict RAIM availabilty from? Would it not be safer to know that the receiver is not going to sequence onto APPCH mode before arriving at the final point?

The "requirement I mention" is a CASA requirement. It is on the test form as a requirement. Regardless of my personal opinion, it ain't my sandpit, I just have to play in it.

I never ever said I would commence/continue an approach if the RAIM prediction said RAIM would not be available. Show me where I said that.
But aren't you saying you don't need to check RAIM before commencing an approach?

You have said the receiver will continue to navigate while it inhibits the RAIM warning,
meaning an approach can be completed and if necessary missed approach can be carried out in DR mode.
And then,
Firstly I used the wrong terminology, a GPS doesn't go to DR mode with just a loss of RAIM, it will navigate the same accuracy as before, but now it cannot ensure it's accuracy/integrity.

Want to try again?

The prediction checks for RAIM at the APPCH mode tolerance (0.3nm) with a 15 minute window either side of the predicted time of arrival at the missed approach point. When you commence the missed approach, the receiver reverts to TERMINAL mode (1.0nm RAIM limit) to enable you to safely complete the missed approach. You will have been in TERM mode from 30nm out from the final destination of your GNSS flight plan.
If you want to keep going during an approach when there is anything less than the required integrity available is suicide. Any warning of RAIM requires an immediate missed approach.

The only thing I can't work out is why the approach inhibits an actual loss of RAIM? Trying not to distract the pilot? Anyone?:confused:

[27/09]

MakeItHappenCaptain

From the CASA GNSS course;
:
for most phases of flight the GPS receiver will provide an immediate annunciation of a loss of RAIM capability. The exception is five minutes following passage over the final approach waypoint of a non-precision approach. During this time, a loss of RAIM annunciation will be inhibited. GPS navigation may still be possible during this RAIM outage.
The GPS receiver automatically performs an approach RAIM prediction just before passing the final approach waypoint and will only enter approch mode if RAIM at the 0.3nm horizontal integrity limit is predicted to be available from the final approach waypoint to the missed approach waypoint.
Still don't believe that is correct?

Absolutely that is correct, I've never said otherwise.

Again...
Agreed, the NAIPs prediction is more accurate, however where do you think the GPS receiver is going to predict RAIM availabilty from? Would it not be safer to know that the receiver is not going to sequence onto APPCH mode before arriving at the final point?

Where does the GPS predict RAIM availability from?

It get it from the on board almanac, which may not be as up to date as the ASA or Airways prediction, in other words may not incorporate any outages since the almanac was updated. It's likely to show RAIM is available when the pre flight prediction from ASA will say RAIM not available.

It is my contention and that of others who have posted on this thread that an on board RAIM prediction won't tell you anything more than the pre flight prediction from ASA or Airways.

Remember also a prediction with no RAIM outages is no guarantee there will be no RAIM warnings.


But aren't you saying you don't need to check RAIM before commencing an approach? Yes, provided you have a prediction from an approved source like ASA or Airways. See above. An in flight RAIM prediction isn't required on this side of the Tasman, PROVIDED, you have a pre flight RAIM prediction from Airways.

Want to try again?

The prediction checks for RAIM at the APPCH mode tolerance (0.3nm) with a 15 minute window either side of the predicted time of arrival at the missed approach point. When you commence the missed approach, the receiver reverts to TERMINAL mode (1.0nm RAIM limit) to enable you to safely complete the missed approach. You will have been in TERM mode from 30nm out from the final destination of your GNSS flight plan.




I'm not sure what you're trying to say here, but I get the impression you think that once RAIM is lost the unit can no longer navigate. Not true.

Your comments about RAIM protection levels are correct, however you are talking about integrity monitoring not not navigation ability.

4 satellites are required for a 3 D nav solution, to have FD RAIM (TSO 129)available 5 satellites are needed and for FDE RAIM (TSO146) 6 satellites. With baro aiding the number of satellites required is reduced by one.

The loss of RAIM (going from 5 to 4 satellites in the case of TSO 129) doesn't mean the GPS can no longer navigate, it means it can no longer monitor it's integrity and warn if the navigation solution isn't within limits.

In that case of loss of RAIM after the FF the unit will still be scaled to 0.3 and navigating as it was before the loss of RAIM.

If there was a loss of signal reducing the number of satellites below 4 the GPS would flag and you would know about it.

If you want to keep going during an approach when there is anything less than the required integrity available is suicide. Any warning of RAIM requires an immediate missed approach. In the case of loss of RAIM after the FF, you would likely never know there was a loss of RAIM as there would probably be no RAIM warning. If it happens prior to the FF you will either get a warning or the unit will fail to go into Approach Mode and you would carry out the missed approach.

[Oktas8]

I have learned a couple of useful things from this thread. I remain keen however to put to bed the idea that Australian law requires an in-flight RAIM prediction. I've never bothered (unless I didn't have a GPS NOTAM), and my current employer, with high capacity RPT AOC, doesn't require it either.

It's not required in NZ or the UK either, by the way.

It's interesting that the flight test form mentions a RAIM prediction in the context of commencing a GPS hold or approach.

However. That this means that the prediction must be done at that point in time though, is just an interpretation. It is possible to take a different interpretation: that the ATO must confirm at that point that the candidate possesses a valid RAIM prediction, whenever that prediction may have been obtained.

I did some research. The rest of this boring-ly long post deals with what CASA says.

CAO 40.2.1 Instrument Rating: Appendix IV Section 2, GNSS NPA: Methods of RAIM prediction:
Syllabus requirements: Criteria to be satisfied by applicant: be able to predict RAIM availability at destination and ETA using aircraft GNSS receiver; and if available, an external RAIM prediction service.
Competence to be shown: Applicant to demonstrate that he or she can accurately predict, within 1 hour before departure, the availability of approach RAIM at the destination or alternate aerodrome within ± 15 minutes of ETA [and] that he or she knows any limitations which apply to the prediction.

I see this as a cognitive vs. a behavioural assessment, respectively. The candidate must be taught how to generate a receiver RAIM prediction, but it is not a competence to be shown.

CAAP 179 GNSS Procedures
Pg 44: All TSO-C129 and some TSO-C146a receivers have a built- in Approach RAIM prediction function available to the pilot. These are useful tools for in-flight use. However, these systems are ... unsuitable for flight planning purposes.
Pg 45: The Airservices Australia RAIM Prediction Service (RPS) uses NANU and the current almanac to provide GPS NOTAMs for flight planning purposes.

An in-flight prediction is a useful tool. There is no suggestion that it might be a requirement.

GNSS Overview (the green book, CASA courseware)
Pg 24: Before planning to use GNSS for approaches, get a RAIM prediction from the Airservices Australia briefing website.
Pg 25: You may use [receiver RAIM prediction] before commencing an RNAV GNSS approach.

I note a distinction here. There is an instruction for flight planning ("get a prediction..."), but not for in-flight predictions ("you may use...")

And so we are left with the flight test form for the IR, or the private IR. Must the candidate obtain a new prediction before the approach, or is it that the candidate must check that a valid prediction exists, prior to the approach? One of the difficulties of being an ATO is being able to take the broadest possible interpretation of the rules. It's important never to say "that's illegal" unless there is a black and white statement in a legislative instrument supporting the claim.

Would it not be safer to know that the receiver is not going to sequence onto APPCH mode before arriving at the final point?No, it would be convenient. Safety is assured by the behaviour of the receiver in real time, following loss of RAIM or a RAIM warning.

As an aside... The reason that navigation may continue following a loss of RAIM inside the FAF is because, statistically, it is extremely likely that there is a satellite masking problem (due to terrain) at this stage, rather than a navigation problem. Loss of RAIM is a great deal less serious than a RAIM warning, so it is treated differently.

The "requirement I mention" is a CASA requirement. It is on the test form as a requirement.The requirement exists only if it is in a legislative instrument, CAO, Regulation, AIP etc. I hope you might acknowledge the possibility of an interpretation problem, rather than a problem of legal facts.

It is sometimes necessary to obtain an in-flight prediction; I won't deny that. In my experience the receiver RAIM prediction has low reliability: false positives and false negatives are very common. I don't generally waste my time with them.

[MakeItHappenCaptain]

27/09
Post #13

MakeitHappenCaptain:
The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.
27/09
I don't believe this statement is correct.
Post #28
Absolutely that is correct, I've never said otherwise.

Where does the GPS predict RAIM availability from?
It get (sic) it from the on board almanac, which may not be as up to date as the ASA or Airways prediction, in other words may not incorporate any outages since the almanac was updated. It's likely to show RAIM is available when the pre flight prediction from ASA will say RAIM not available.

You really need to review your statements. I could say you think Helen Clark was sexy and although you'll deny it, you'd be agreeing six posts later!:} (And before you get in, I think Juliar Gillard looks like the south end of a northbound orangutan.)

See Bladeangle's post #24 ref the receiver's prediction being differrent, not more accurate, than the ASA/Airways/NAIPS one.

I'll concede to Oktas that there is no legislative requirement to do a RAIM check, but I would be concerned if people fronted their Initial CIR (in Aus anyway, don't know what the kiwi requirements are, that's 27/09's playground) and they refused to do a RAIM check based on pprune advice.

[27/09]

Quote:
MakeitHappenCaptain:
The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.
27/09
I don't believe this statement is correct.

The reason I said I didn't believe this to be correct is that even with a valid RAIM prediction if RAIM isn't available at the FAF the unit will not go into approach mode. So the factor that determines if it will sequence onwards isn't solely the GPS almanac.

I've said several times now that having a good RAIM prediction doesn't mean there will be RAIM available. Prediction and reality are different things.

Nothing incongruent with what I've said.

I'll concede to Oktas that there is no legislative requirement to do a RAIM check, but I would be concerned if people fronted their Initial CIR (in Aus anyway, don't know what the kiwi requirements are, that's 27/09's playground) and they refused to do a RAIM check based on PPRuNe advice.

I'd hope they'd do what they were taught during their training.

[Bomber ARIS]

AIP GEN 1.5

8.5.5.3

RNAV(GNSS) non precision approach (or RNP APCH) capability using a TSO-C129, C129a, C145a, C146a or C196 receiver and the valid prediction of approach availability from the Airservices Australia RAIM prediction service may be used to satisfy the requirements set out in ENR 1.1 para 58.3 and ENR 1.10 para 1.5.1. CASA may approve other receivers and prediction tools for these applications on the basis of equivalent safety performance.

[MakeItHappenCaptain]

Orright, how about this one,

Yeah, Australian Rules, of course.

AIP ENR 1.5 para 12.2.2
The following specific restrictions apply to the conduct of a GPS Arrival:
a. The database medium (card, chip, etc) must be current and of a kind endorsed by the receiver manufacturer.
b. The coordinates of the destination VOR or NDB, to which the descent procedure relates, must not be capable of modification by the operator or crew.
c. GPS integrity (eg, RAIM) must be available before descending below the LSALT/MSA.
d. The nominated azimuth aid (VOR or NDB) must be used to provide track guidance during the arrival procedure.
e. In the event of a significant disparity between the NDB or VOR track, and the GPS track indication, the pilot must discontinue the arrival procedure.

[Oktas8]

Well, yes. If RAIM wasn't available, you'd be in DR mode. Nowt to do with RAIM predictions however!

This whole topic can get confusing at times (not suggesting anyone here is; just a general observation. :) )

I try to remember that the whole system pretty well guarantees that any RAIM predictions will a) be prominently displayed and b) take away any right to use the equipment for navigation purposes. Minor exceptions apply, but only between ATC & the pilot, never for pilot-interpreted navigation.

If you take the view that receiver RAIM predictions are a dark art for the non-NOTAM equipped, never "required", it helps to clear the mind!

[Bladeangle]

The RAIM check requirement aside for a moment...

Now I understand that FD integrity monitoring is a process of the receiver sampling pseudo ranges a combination of 4 satellites out of a group of at least 5 (most likely many more) and comparing each solution with the HIL of the particular phase of flight, i.e. 2.0, 1.0 & 0.3. If it finds 1 satellite (out of only 5) to be providing a range that gives a solution with a radius greater than the limit for that phase of flight, the unit will flag, INTEG, RAIM what ever...

If you are in approach mode and this occurs, you press/select "enter missed approach" button, the unit then goes back from 0.3 HIL to terminal mode 1.0 HIL, and integrity monitoring continues and if the solution is within 1.0 HIL you can use the receiver for missed approach navigation. Have I got it right so far?

What I don't understand is beyond the FAF, if you lose FD RAIM, i.e. from 5 satellites down to 4, (and I mean only 4 satellites in sight of the receiver), and the RAIM warning is "inhibited" for 5 mins, how does the unit monitor integrity at all?

27/09:
If there was a loss of signal reducing the number of satellites below 4 the GPS would flag and you would know about it.


But what if 1 of those 4 didn't drop out, but the pseudo range from it gave a slightly larger solution?

Does the receiver still compare the actual calculated solution to the HIL even without the minimum 5 satellites (4 with baro)?

[Oktas8]

Ok. Some misunderstanding there bladeangle.

What you said about HIL is correct. A radius of probable position is created by comparing fixes generated from different combinations of satellites. You are almost certainly within the circle. (Never mind what the probability actually is.)

If the radius is greater than the requirement for that phase of flight, the unit cannot offer the required accuracy. You might be too close to the hills, and not know it.
If the circle is too big, a RAIM warning is generated.
It is as if you are using a VOR, and the "fail" flag has appeared.

It will be apparent that a RAIM warning requires RAIM to be present in the first place, and is a guarantee that the position fix is wrong. RAIM warnings are never inhibited.

If there are only four usable satellites in view, you cannot compare fixes generated by different combinations of satellites. You have a probable position, but no radius. The circle might be big or small, but you cannot calculate it.
If there is no circle, or you don't know the size of the circle, a "loss of RAIM" occurs.
It is as if you are using a VOR, and you discover the "fail" flag is jammed up out of sight.

It will also be apparent that a loss of RAIM may or may not imply a loss of navigation accuracy. If you've just descended into a valley and lost sight of a couple of satellites, a loss of RAIM is not surprising. It doesn't necessarily imply a loss of navigation accuracy. Loss of RAIM may be inhibited inside the FAF as long as at least four satellites are usable and their geometry is suitable for a quality navigation fix. Losses of RAIM may not be reported on approach. But they may be, sometimes.

From my FMS manual:

For activation of the GNSS/GPS approach, RAIM must be available. If RAIM integrity is available until crossing the FAF, then the aircraft enters the five minute coast segment. During this segment if RAIM integrity is down graded to a four satellite non RAIM position, GPS navigation will be allowed to continue for up to five minutes. The assumption is that loss of RAIM will not immediately degrade the GPS accuracy to a level that prohibits continued GPS navigation for a short period and that period is five minutes. The five minute count begins with the loss of RAIM and could occur at any point between the FAF and MAP. The absolute minimum required to continue an approach during the five minute coast segment is four satellites with an HDOP of < 4.

Hope that clears it up, a little.

[Bladeangle]

Thanks Oktas8, that does make a little more sense now.

I did use the incorrect terminology, I meant the unit will inhibit a RAIM loss. Of course an integrity warning will never be inhibited.

So you can have a RAIM loss due to insufficient satellites, it can/may be inhibited during the final approach course, but the receiver will continue monitoring of the navigation solution.

So really, the receiver never stops monitoring, even will a loss of RAIM?

Thanks for the extract of your FMS manual, thats what I was after. Cheers :ok:

[underfire]

Wow, interesting thread.

RAIM Prediction Tools (http://operationsbasednavigation.com/operations-based-navigation/463-2/)

There is a significant amount of confusion between RAIM prediction and real time RAIM monitoring.

When you file a flight plan that requires GPS, and especially RNP AR, you must check the RAIM prediction to see if it is available for approach. Note, this is NOT the RAIM onboard, one must check the approved State Source. If the prediction says no, then you cannot use that flight plan.

Because RAIM operates autonomously, it requires redundant pseudorange measurements. To obtain a 3D position solution, at least four measurements are required. To detect a fault, at least 5 measurements are required, and to isolate and exclude a fault, at least six measurements are required, however often more measurements are needed depending on the satellite geometry.

This is what the unit on the aircraft is doing to provide the HIL/HPL/HAL.
The horizontal integrity limit (HIL) or horizontal protection limit (HPL) is a figure which represents the radius of a circle which is centered on the GPS position solution and is guaranteed to contain the true position of the receiver to within the specifications of the RAIM scheme. (Note: this is NOT prediction, but real time RAIM measurement).
The HPL is calculated as a function of the RAIM threshold and the satellite geometry at the time of the measurements.
The HPL is compared with the horizontal alarm limit (HAL) to determine if RAIM is available.
Always remember, due to latency in the GPS signal (a single GPS transmission from each sat lasts about 3 seconds) and latency in the unit, the GPS system tells you where the aircraft WAS, not where it IS. The Kalman filter estimates where the aircraft IS. The unit will alarm if HIL is beyond HAL for your selected level. This is real-time.

Even though the RAIM prediction site may have told you that there was sufficient coverage, the terrain where you are going may affect the number of sats that your unit can see. As an example, many procedures in China involve flying down "valleys". If you note that the unit must see at least 6 sats for fault protection, terrain may mask enough where the unit will start degrading HIL.

That is why RAIM is monitored real-time by the unit. Note the Terminal/Approach check. This is usually more accurate, and will given outages on a time schedule, which is important to review before departure, to determine if RAIM is available/unavailable during certain time periods.

RNP AR operators must always check RAIM avail prior to departure for the RNP level on final. As noted in China, there are many RNP 0.1 finals that are not available due to RAIM.

Of course, if the HIL/HPL on your unit go to HAL, you have to use another method of navigation.

What I don't understand is beyond the FAF, if you lose FD RAIM, i.e. from 5 satellites down to 4, (and I mean only 4 satellites in sight of the receiver), and the RAIM warning is "inhibited" for 5 mins, how does the unit monitor integrity at all?


Units on commercial aircraft have the GPS connected thru the IRU. The inertial unit, with its predictions thru the filter, provide a certain 'pseudo RAIM' should RAIM drop for a brief period of time (usually about 15mins)

EDIT: Given the latentcy in the system, 5 mins is used as an acceptable drift rate for the approach to stay within the defined level.

http://i62.tinypic.com/1z2knx4.jpg

EDIT: Hey, I do a RAIM prediction as well, because SOPs say so; the point is that its pointless - the NOTAM is more up to date than the almanac in the FMS.

RAIM prediction by almanac prior to flight plan would not be acceptable. The State source should detail RAIM prediction for the route, and RAIM prediction for individual approach (as noted on the website above). The boys in the back room should have checked all this prior to sending the flight plan. If approach was not available, then the flight plan cannot be filed.

Curious about a RAIM NOTAM...I havent ever written one, so not sure what that would be about

[glekichi]

c. GPS integrity (eg, RAIM) must be available before descending below the LSALT/MSA.

This has zero to do with a RAIM prediction; it is about RAIM monitoring.

Curious about a RAIM NOTAM...I havent ever written one, so not sure what that would be about

My bad - strictly speaking its not a NOTAM. The RAIM predictions covering the NPAs for each location requested are delivered as a part of the SPFIB when you request WX/NOTAMS here in Australia. They are all that is legally required.

[underfire]

no worries.
RAIM predictions from the State Source are based on a 24 hour timeframe, and just means you are good to go with that flight plan.

Real time RAIM, and the reported HIL/HPL are a much different matter. Understanding those, in combination with HAL...that is what is really important when navigating.

It is virtually impossible to determine what HAL is (part of the FMS secret sauce)

I will say this (as an example) with HW, the 'circle' when set to 0.3RNP will alarm over 0.51/0.60. Sometimes it is useful to understand just how far off one is on alarm.

EDIT: I just love the AUS NOTAM's, based on ARP.....

[Oktas8]

GPS NOTAM:

In Australia & NZ, it means a statement of approach RAIM availability at certain airfields, based on US Notice Advisories to Navstar Users (NANU) and the published almanac.

In the UK, it means a statement of known GPS non-availability due (usually) to military exercises or other known disturbances.

In other countries it might mean yet other things!

O8

[scavenger]

Sorry if someone has posted this, I couldn't be bothered reading all the other opinions, but the NAIPS GPS RAIM prediction page says:

The Airservices Australia Receiver Autonomous Integrity Monitoring (RAIM) Prediction Service (RPS) provides predictions of RAIM availability for flight planning purposes only.
Pilots-in-command are reminded that the predictions are based on information received from the GPS Operating Authority.
In flight, pilots-in-command should use the RAIM prediction facility of their GPS equipment for RAIM availability predictions.
Presence of RAIM should be continuously monitored whenever GPS is used for navigation.

So if the people generating the NOTAMS think you should use the in-flight equipment for a prediction in addition to the NOTAM, that's good enough for me.

It also specifically says the RPS is for flight planning purposes only.

[underfire]

So if the people generating the NOTAMS think you should use the in-flight equipment for a prediction in addition to the NOTAM, that's good enough for me.

No, you are missing the point. Please read my previous post. Mixing up RAIM prediction with real time monitoring.
BTW, It is NOT a NOTAM, even in AUS, perhaps bundled, but certainly not NOTAM format. (semantics)

In flight, pilots-in-command should use the RAIM prediction facility of their GPS equipment for RAIM availability predictions.

That statement is either wrong, or not worded correctly. I will look into this with AsA and CASA.

They also said

Presence of RAIM should be continuously monitored whenever GPS is used for navigation.

Ahh yes, finally. Of course, if one cannot monitor RAIM real-time, how does one determine HIL/HPL/ or HAL?

In reality, the RAIM prediction from the almanac is a useless waste of memory on the unit. The RAIM prediction is checked PRIOR to submitting the flight plan and departure. That is not done by the aircraft, but by fltops. The RAIM check must be done with the State approved source, which is NOT the unit prediction.

Predictions are based o a 24 hour timeframe, IF and only IF, your flight plan crosses the timeframe, do you need to re-evaluate RAIM prediction enroute. Again, this CANNOT be done from the ac, it must be done back at fltops. (unless you decide to violate the FAA rules and use you tablet to access the internet source for the RAIM prediction for final) hmmm.

I have fielded calls, in reality, many, many calls while an ac is enroute on an RNP procedure, and RAIM drops, (HAL). The frequently happens when the ac enters mountainous terrain. It should be noted that some RNP coded procedures are 400nm long in China. (ie you really, really cannot disco the procedure) in S America, you almost always have HAL on procedure.

[27/09]

underfire:

No, you are missing the point. Please read my previous post. Mixing up RAIM prediction with real time monitoring.

I think more than one person on here has been getting RAIM prediction and RAIM monitoring mixed up, leading to misunderstandings about what is actually being said.

[ForkTailedDrKiller]

I wonder how many "General Aviation" aircraft do RNP approaches?

Dr :8

[glekichi]

Lots of good information here:
http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCoQFjAA&url=http%3A%2F%2Fwww.casa.gov.au%2Fscripts%2F26-30.pdf&ei=1kGHU6K6D8S-kAWmo4HgBw&usg=AFQjCNGGZ0xsKCp9NYfg_d47MjiGPBXBew&sig2=RmPmoDvCaXt7uWDzyTIYog&bvm=bv.67720277,d.dGI


RPS AVAILABLE
The Airservices Australia RPS is available through the NAIPS access. This
service provides three types of prediction – TSO C129 FD; TSO C145/6
FD; and TSO C145/6 FDE. These predictions are valid for 72 hours and are
available for aerodromes that have published RNAV (GNSS) approaches.
This system (based in the Brisbane ATS Centre) uses inputs from both an
inbuilt GPS receiver and also the US Government GPS status messages
advising when GPS satellites are planned to be out of service. The RPS
software is re-run each time a NANU (GPS NOTAM) is received. This
means that these predictions can be more accurate than just using your
GPS receiver prediction, as the receiver will not know that satellite ‘X’ will
be out of service at some time in the future.
The CASA CAAP 179A provides guidance on how these various RPS
outputs should be used for IFR GNSS fl ight planning. Essentially, the only
time RPS is operationally required is for primary means and RNP fl ights,
but common sense dictates it should be used for other RNAV (GNSS)
approach operations.


THE BOTTOM LINE
In summary, RAIM and its associated prediction services are a fundamental
part of adapting the GPS system for use in IFR civil operations. An
understanding of its role is essential to the safe use of GNSS.
The bottom line is that regardless of the RPS notification, if you have not
received a ‘RAIM not available’ or ‘RAIM Warning’, or equivalent message
(such as ‘Loss of Integrity”) from your GPS during IFR operations you
can continue GNSS operations. Conversely, if you do get such messages,
you must discontinue any approach operation using GNSS or, if enroute
in controlled airspace, advise ATC after five minutes.

[underfire]

That is probably the best explanation of the AUS service, what it actually does, and who really needs it....

Other Countries use some different parameters, so it is always good to check.

Here is the FAA Site RAIM Prediction-FAA (http://www.raimprediction.net/) which includes the route check

I wonder how many "General Aviation" aircraft do RNP approaches?

RNP APCH or RNAV (GPS) is well with reach of GA IFR

[ForkTailedDrKiller]

RNP APCH or RNAV (GPS) is well with reach of GA IFR

RNAV yes, but RNP? Not that I know of - hence my question!

Dr :8

[underfire]

Dr.

For ICAO (and AUS) RNP APCH and RNAV (GPS) are the same.

RNP APCH is the general ICAO designator for PBN approach procedures that are not Authorization Required operations.
As GNSS fulfils the basic requirement of RNP for on-board performance and monitoring, both RNAV (GNSS) and SBAS LPV procedures are types of RNP APCH operations.
RNP APCH procedures will be identified as:
• RNP APCH – LNAV
• RNP APCH – LNAV/VNAV (where a vertical guidance system is used)
• RNP APCH – LPV (Localiser Performance with Vertical Guidance)
• RNP APCH – LP (SBAS approach where vertical guidance is not available)

[Oktas8]

More info on Performance Based Nav (RNP etc) in the Australian context may be found here (http://www.casa.gov.au/wcmswr/_assets/main/lib100178/pbn-booklet.pdf).

The way I read it, it will be business as usual for GPS-equipped IFR aircraft. However, we will have to absorb a new terminology from the Dept for Name Changes. As noted by underfire...

[underfire]

Oktas,

Agree, as noted, the terminology, or rather, the assimilation of the terminology to a lowest common denominator is rather irritating.
Having designed many of the tailored RNP-AR procedures in AUS, I am really not sure why ....

Stuff like RNP APCH LNAV just kills me for some reason.... :mad:

[Oktas8]

Stuff like RNP APCH LNAV just kills me for some reason

It's the Americans. Gotta have acronyms, but can't do sensible ones. Shouldn't "localiser performance with vertical guidance" be "LV"? And shouldn't "LNAV" be suppressed altogether, because, let's face it, all navigation systems offer some form of lateral navigation?

But, they invented it, they developed it, they paid for (much of) it. So I can't complain about it. Except here, of course...

A question for you underfire.

Earlier you said that RAIM predictions are done no more than 24 hours in advance, and, for flights crossing the 24-hr mark, a new one is generated by Flt Ops. However, NANU are generated with 72 hour validity, and the almanac is valid for quite a long time, up to a week. Why is there a 24 hour limit on your predictions?

[27/09]

For ICAO (and AUS) RNP APCH and RNAV (GPS) are the same.

The difference is RNAV does not require on board monitoring whereas RNP does.

[underfire]

OKtas,

NANU from Navstar is typically for 72 hours...these cover anticipated outages of Sats. These give you an idea of sat health and availability. You need other tools for flight planning.

As an example, the FAA RAIM prediction is based on a 24 hour daily cycle. It is also updated with 15 mins of an unexpected outage.
These sites are what you need for the route and approach RAIM availability. The route tools not only use the number of sats and strength, but will include the mask angle along the route and the individual approach.

The mask angle is very important. In mountainous terrain, its pretty easy to start losing them and/or the signal to become sporadic, which throws off the error trapping of the GPS unit.

[Oktas8]

Thanks underfire. Mask angle can be predicted a long time in advance, I think.

But I didn't know that FAA tools include unpredictable outages on an hour to hour basis. Thanks.

[underfire]

Mask angle is difficult in terrain.

While the sats are in orbit, angles from each sat are always changing. While 5 degrees is the standard...terrain is the wildcard, especially when flying..

The RAIM prediction is just not set up for that level of detail. Its a guess, but as we open up areas of the World to Aviation, the holes begin to open up...

As you may be aware, I take nothing for granted.

Fly this, and lose RAIM and go to HAL..options? (notice the elevation)

http://i58.tinypic.com/wrgtp3.jpg