RAIM Check
The below is from Universal themselves.
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.
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 If the residing almanac is out of date a new one is down loaded.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.
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 If the residing almanac is out of date a new one is down loaded.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.
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.
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Bladeangle, after reading your & Glekichi's remarks about late model GPS receivers, I did some reading. I learned something. 
(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!
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.
(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.
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.
Last edited by Oktas8; 21st May 2014 at 12:44.
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).
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:
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.
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).
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.
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.
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?
Cheers.
Exactly! Why wouldn't you do one in flight? Finding out on a 7nm final in IMC is prob not "best practice".
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, 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?
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.
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.
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
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
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.
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;
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.
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,
And then,
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?
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.
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.
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.
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.
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.
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?
MakeItHappenCaptain
Absolutely that is correct, I've never said otherwise.
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.
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.
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.
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.
From the CASA GNSS course;
Still don't believe that is correct?
:
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.
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.
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?
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?
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?
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.
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.
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.
Last edited by 27/09; 25th May 2014 at 22:19.
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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.
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 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.
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?
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.
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.
Last edited by Oktas8; 25th May 2014 at 23:14.
27/09
Post #13
Post #28
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.
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.
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.
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.
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.
(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.
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.
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.
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.
More food for thought...
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.
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.
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.
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.
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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!
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!
Last edited by Oktas8; 27th May 2014 at 02:28.
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:
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)?
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.
Does the receiver still compare the actual calculated solution to the HIL even without the minimum 5 satellites (4 with baro)?
Last edited by Bladeangle; 27th May 2014 at 04:55.
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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:
Hope that clears it up, a little.
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.
Last edited by Oktas8; 27th May 2014 at 07:08.
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
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
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Wow, interesting thread.
RAIM Prediction Tools
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.
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.
EDIT:
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
RAIM Prediction Tools
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?
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.
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.
Curious about a RAIM NOTAM...I havent ever written one, so not sure what that would be about
Last edited by underfire; 27th May 2014 at 20:09.
c. GPS integrity (eg, RAIM) must be available before descending below the LSALT/MSA.
Curious about a RAIM NOTAM...I havent ever written one, so not sure what that would be about
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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.....
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.....
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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
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