RAIM prediction valid only for a preflight phase
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I think some of you guys confuse ABAS, SBAS and even GBAS. I've read through dozens of documents on PBN and interchangeable use of augmentation types is nowhere to be found. You can't just use GBAS or SBAS to compensate for lack of satellites in order to perform LNAV/VNAV. Am I crazy or am I missing something important here?
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A Squared, I think you are correct.
Now one question. Let say you have an integrated navigation system (like on the airbus) that computes your position and accuracy (and also your Required Navigation Performance), by using your gps in priority but also Dme, vor, ndb and dead reck with the intertials if you are in big ****.
Now you loose your 2 gps. However you still receive dme and/or vor coverage and your navigation performance is still computed and (theoretically) within limits for approach. Would it mean you can still start your RNAV approach?
The next day, you loose not only your 2 gps but also all your antenas that include vor/dme/adf just approach, bad day...luckily your fms is still serviceable to calculate your actual and required navigation performance for an RNAV approach...now your 2-3 irs become your primary means of navigation, but because of short time they didn't drifted much and your nav perf shows "good to go"...so do you do your RNAV approach?
Thanks for your feedback, that's an interesting topic.
Now one question. Let say you have an integrated navigation system (like on the airbus) that computes your position and accuracy (and also your Required Navigation Performance), by using your gps in priority but also Dme, vor, ndb and dead reck with the intertials if you are in big ****.
Now you loose your 2 gps. However you still receive dme and/or vor coverage and your navigation performance is still computed and (theoretically) within limits for approach. Would it mean you can still start your RNAV approach?
The next day, you loose not only your 2 gps but also all your antenas that include vor/dme/adf just approach, bad day...luckily your fms is still serviceable to calculate your actual and required navigation performance for an RNAV approach...now your 2-3 irs become your primary means of navigation, but because of short time they didn't drifted much and your nav perf shows "good to go"...so do you do your RNAV approach?
Thanks for your feedback, that's an interesting topic.
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Tech knowledge
A Squared beat me with the provision of a pretty good explanation, but I'll post this anyway.
The question of the O.P. is a good one and in trying to answer it correctly we do see the limits of technical knowledge that is supplied to us, operators, via FCOM’s, Ops Manuals, etcetera.
We can only hope that the guys writing the books were correct in every detail.
Let’s look at GNSS basics – a receiver needs signals from 4 satellites in order to be able to compute a position. If the signal from a satellite is flawed, the computed position will be wrong.
If the receiver can track more than the minimum number of 4 satellites, it can use the multitude of acquired ranges to check whether all the different combinations of ranges yield the same position as a result. If all the position computations with the different sets of ranges give (nearly) identical position results, then the position has integrity, is reliable.
If the receiver can track 5 satellites, and one satellite signal is flawed, then the integrity check will fail.
If the receiver can track 6 satellites, it can detect a flawed position and it can determine which satellite signal is flawed and can exclude that signal from the process (FDE, Fault Detection and Exclusion).
If the receiver can track even more satellites, more redundancy is available.
If a GNSS receiver cannot supply a position, it is required to provide an alert to the pilot – what the pilot books do not properly explain, is exactly what conditions would trigger that alert.
Of course, if the GNSS set would be completely dead, due to receiving no electrical power, it is obvious that it would be declared faulty; however, whether it supplies a failure flag when, for instance, it cannot see enough satellites in order to perform the integrity check, that is not explained.
It would be logical if the GNSS position would be declared unusable if it was made up of only 4 satellite signals, but we, as pilots, just do not know whether the sets work that way.
In order to receive a usable signal, a receiver (antenna) needs line of sight to a satellite.
Since the satellites orbit relatively low above the earth, they are not geostationary, therefor the visibility of satellites to receivers changes continuously.
It is possible, from knowledge of the satellite orbits, coupled with announced outages of satellites, to compute whether at a certain position, at a certain time, there will be enough operational satellites visible to a receiver, for performance of RAIM. Such a computation, named RAIM prediction, can be made for an entire route of flight.
If a receiver only has RAIM, visibility of 5 satellites would be required. If the receiver is fancier (capable of FDE), 6 satellites would be required, and the prediction algorithm would be set for that requirement.
Regulations require that, for portions of a flight, where GNSS is required for RNP, during flight planning, a check is made of availability of enough satellites for RAIM. If gaps of more than 5 minutes are predicted, replanning is required, either via route adjustment, or by time adjustment of the flight.
Once in flight, there is no longer a requirement to pay attention to the RAIM availability predictions, only in case of inflight replanning, there could be a requirement that the operator’s dispatch unit recalculates a prediction (from a regulatory viewpoint, inflight re-planning is more or less equivalent to preflight planning). For the actual flight execution, the only thing important is that the performance of the NAV system of the aircraft keeps Actual Navigation Performance (ANP) within the bounds of Required Navigation Performance (RNP).
Our OM-A literally states: An in-flight RAIM availability check is not required prior to commencing the procedure. This should answer question of OP for my specific company and aircraft type.
As a pilot we can only assume, for lack of technical knowledge, that then the GPS receiver must annunciate a failure if it does not receive enough satellites to perform RAIM.
As a further note about the predictions: we are required to use the FDE algorithm for the prediction, and the mask angle is set at 5 degrees. So, the prediction tells us whether at least 6 operational satellites will be higher than 5 degrees above the horizon. As explained above however, already 5 satellites are enough to determine whether the derived position is reliable or not. Also, a satellite at 4.95 degrees above the horizon might provide a usable signal, but it would not have counted in the prediction. So the prediction is conservative and in real life GNSS might work just fine even though the prediction would have said NO.
All specific remarks about equipment and regulations will depend on equipment actually built into aircraft and ops manuals of company.
As a final note, just to be sure, RAIM is not a sort of special signal that has to be sent by satellites or has to be received in any way, it is a check, internally in the GNSS receiver, that can be made when more operational satellites can be received simultaneously then basically required for computation of position.
The receiver will track as many satellites as possible (up to the maximum capability of the receiver) and it should be simple to provide an alert if the number of tracked satellites is less than required for performance of RAIM to confirm the integrity of position.
The computation of ANP is another area where our documentation is very sparse with details – the NAV set computes and displays ANP and alerts the crew when ANP exceeds RNP, that’s as deep as it goes.
There are a couple of websites that provide technical principles, but detailed info about specific equipment in one’s aircraft is hard to get.
ESA (European Space Agency) Navipedia.net
US GPS site gps.gov
The question of the O.P. is a good one and in trying to answer it correctly we do see the limits of technical knowledge that is supplied to us, operators, via FCOM’s, Ops Manuals, etcetera.
We can only hope that the guys writing the books were correct in every detail.
Let’s look at GNSS basics – a receiver needs signals from 4 satellites in order to be able to compute a position. If the signal from a satellite is flawed, the computed position will be wrong.
If the receiver can track more than the minimum number of 4 satellites, it can use the multitude of acquired ranges to check whether all the different combinations of ranges yield the same position as a result. If all the position computations with the different sets of ranges give (nearly) identical position results, then the position has integrity, is reliable.
If the receiver can track 5 satellites, and one satellite signal is flawed, then the integrity check will fail.
If the receiver can track 6 satellites, it can detect a flawed position and it can determine which satellite signal is flawed and can exclude that signal from the process (FDE, Fault Detection and Exclusion).
If the receiver can track even more satellites, more redundancy is available.
If a GNSS receiver cannot supply a position, it is required to provide an alert to the pilot – what the pilot books do not properly explain, is exactly what conditions would trigger that alert.
Of course, if the GNSS set would be completely dead, due to receiving no electrical power, it is obvious that it would be declared faulty; however, whether it supplies a failure flag when, for instance, it cannot see enough satellites in order to perform the integrity check, that is not explained.
It would be logical if the GNSS position would be declared unusable if it was made up of only 4 satellite signals, but we, as pilots, just do not know whether the sets work that way.
In order to receive a usable signal, a receiver (antenna) needs line of sight to a satellite.
Since the satellites orbit relatively low above the earth, they are not geostationary, therefor the visibility of satellites to receivers changes continuously.
It is possible, from knowledge of the satellite orbits, coupled with announced outages of satellites, to compute whether at a certain position, at a certain time, there will be enough operational satellites visible to a receiver, for performance of RAIM. Such a computation, named RAIM prediction, can be made for an entire route of flight.
If a receiver only has RAIM, visibility of 5 satellites would be required. If the receiver is fancier (capable of FDE), 6 satellites would be required, and the prediction algorithm would be set for that requirement.
Regulations require that, for portions of a flight, where GNSS is required for RNP, during flight planning, a check is made of availability of enough satellites for RAIM. If gaps of more than 5 minutes are predicted, replanning is required, either via route adjustment, or by time adjustment of the flight.
Once in flight, there is no longer a requirement to pay attention to the RAIM availability predictions, only in case of inflight replanning, there could be a requirement that the operator’s dispatch unit recalculates a prediction (from a regulatory viewpoint, inflight re-planning is more or less equivalent to preflight planning). For the actual flight execution, the only thing important is that the performance of the NAV system of the aircraft keeps Actual Navigation Performance (ANP) within the bounds of Required Navigation Performance (RNP).
Our OM-A literally states: An in-flight RAIM availability check is not required prior to commencing the procedure. This should answer question of OP for my specific company and aircraft type.
As a pilot we can only assume, for lack of technical knowledge, that then the GPS receiver must annunciate a failure if it does not receive enough satellites to perform RAIM.
As a further note about the predictions: we are required to use the FDE algorithm for the prediction, and the mask angle is set at 5 degrees. So, the prediction tells us whether at least 6 operational satellites will be higher than 5 degrees above the horizon. As explained above however, already 5 satellites are enough to determine whether the derived position is reliable or not. Also, a satellite at 4.95 degrees above the horizon might provide a usable signal, but it would not have counted in the prediction. So the prediction is conservative and in real life GNSS might work just fine even though the prediction would have said NO.
All specific remarks about equipment and regulations will depend on equipment actually built into aircraft and ops manuals of company.
As a final note, just to be sure, RAIM is not a sort of special signal that has to be sent by satellites or has to be received in any way, it is a check, internally in the GNSS receiver, that can be made when more operational satellites can be received simultaneously then basically required for computation of position.
The receiver will track as many satellites as possible (up to the maximum capability of the receiver) and it should be simple to provide an alert if the number of tracked satellites is less than required for performance of RAIM to confirm the integrity of position.
The computation of ANP is another area where our documentation is very sparse with details – the NAV set computes and displays ANP and alerts the crew when ANP exceeds RNP, that’s as deep as it goes.
There are a couple of websites that provide technical principles, but detailed info about specific equipment in one’s aircraft is hard to get.
ESA (European Space Agency) Navipedia.net
US GPS site gps.gov
With TSO C145/146 (WAAS) avionics, if WAAS (SBS) is not available, FDE negates the need for a RAIM check, at least in FAA-dom.
Does this post from aterpster not really define the situation.
You must know the certified capabilities of what you have on board, and its operational limitations.
I can well imagine that some authorities have a pre-flight planning requirement related to RAIM predictions, but RAIM is not applicable to later generation GPS receivers. If you have C-129 based equipment RAIM is applicable in flight. This is so, whether or not you have WAAS/SBAS for a particular area, WITH c-145/146 BASED EQUIPMENT, all WAAS/SBAS availability does is change the minima.
