EFIS Gyro Stability
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FPO, the 737 does not have internal IRU updating and is 0.10/GLS certified as standard equipment. The GPS receivers are part of the MMR, not part of the ADIRU. And one can see quite a bit of IRS run-off after a lengthy flight, just recently after a 6 hour sector had the IRS's around 2 to 3 NM off the FMC/GPS/radio position, and the ANP was still 0.02. There is no need for internal updating as the IRS is only used for its short term stability and the GPS provides long-term stability. Filtering based on that principle is done in the FMC.
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Denti:
Any 737s that can qualify for RNP AR IAPs of less than RNP 0.30 have to have FMS position updating to the MAP, so that in the event of a loss of GPS (sats, not airplane receivers) the FMS position derived from the IRUs has zero drift at the MAP.
None of this is required for long-range navigation.
When I flew the early generation 767, which had no updating, and no GPS, our IRU position error was typical around 1 mile for all three IRUs at the end of a 6 hour flight.
FPO, the 737 does not have internal IRU updating and is 0.10/GLS certified as standard equipment. The GPS receivers are part of the MMR, not part of the ADIRU. And one can see quite a bit of IRS run-off after a lengthy flight, just recently after a 6 hour sector had the IRS's around 2 to 3 NM off the FMC/GPS/radio position, and the ANP was still 0.02. There is no need for internal updating as the IRS is only used for its short term stability and the GPS provides long-term stability. Filtering based on that principle is done in the FMC.
None of this is required for long-range navigation.
When I flew the early generation 767, which had no updating, and no GPS, our IRU position error was typical around 1 mile for all three IRUs at the end of a 6 hour flight.
We do have all the latest wiz bangs in our CX a/c including GPS updating RNP etc...
You cannot update the basic raw Inertial Reference Laser Ring Gyro position in flight. Once the IR is aligned on the ground it's done and dusted.
You can however adjust that position after its left the IR unit using GPS, but the IRS still thinks it is where it is ..
Ahhhhhhhh it's all to complicated anyway, white mans magic !! ooh can I say that?
You cannot update the basic raw Inertial Reference Laser Ring Gyro position in flight. Once the IR is aligned on the ground it's done and dusted.
You can however adjust that position after its left the IR unit using GPS, but the IRS still thinks it is where it is ..
Ahhhhhhhh it's all to complicated anyway, white mans magic !! ooh can I say that?
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Unless I am very much mistaken, the original question was only about attitude and heading reference gyros, and nothing to do with position. So, notwithstanding the discussion around position that ensued, here's my 2c worth:
Many (most?) EFIS solid state gyros do NOT have GPS connections to aid them. They use conventional magnetic slaving and acceleration (gravity) based erecting.
They also have errors, some of which increase during a flight. These errors are often greater than those of an inertial navigation system, although the absolute errors are still quite small. The reason that even tiny errors in an inertial system are important is that they are integrated over a long period of time. For example, if your calculation of ground track is wrong by one quarter of a degree, then after 5 hours of oceanic flight, your position solution will be "out" by quite a bit. On the other hand, if your heading display on the EFIS is out by one quarter of a degree, you will never notice or care.
As a small digression, in the very low-end EFIS systems, such as those used in experimental aircraft, they use much simpler systems which primarily rely on accelerometers. These rely on integrating accelerations to determine attitude, and they also therefore accumulate errors quite quickly. They generally DO require a GPS input as a "hint" to continually constrain the error bound.
Many (most?) EFIS solid state gyros do NOT have GPS connections to aid them. They use conventional magnetic slaving and acceleration (gravity) based erecting.
They also have errors, some of which increase during a flight. These errors are often greater than those of an inertial navigation system, although the absolute errors are still quite small. The reason that even tiny errors in an inertial system are important is that they are integrated over a long period of time. For example, if your calculation of ground track is wrong by one quarter of a degree, then after 5 hours of oceanic flight, your position solution will be "out" by quite a bit. On the other hand, if your heading display on the EFIS is out by one quarter of a degree, you will never notice or care.
As a small digression, in the very low-end EFIS systems, such as those used in experimental aircraft, they use much simpler systems which primarily rely on accelerometers. These rely on integrating accelerations to determine attitude, and they also therefore accumulate errors quite quickly. They generally DO require a GPS input as a "hint" to continually constrain the error bound.
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As a small digression, in the very low-end EFIS systems, such as those used in experimental aircraft, they use much simpler systems which primarily rely on accelerometers. These rely on integrating accelerations to determine attitude
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OK465:
In airspace regulated by the FAA, no RNAV IAPs are permitted to use DME/DME updating. GPS is mandatory, whether conventional RNAV, RNP AR, or emerging advanced RNAV. DME/DME updating is permitted in Level 1 SIDs and STARS provided DME masking is demonstrated to be sufficent. In the case of RNP AR most, if not all, ICAO states have elected to use FAA's RNP AR criteria, as set forth in FAA Order 8260.52.
Indeed, a loss of GPS is unlikely but it is nonethess the assumption made for either minimums predicated on RNP of less than 0.30 and/or a missed approach that requires RNP of less than 1.0.
Agree, and thus a loss of GPS on a dual-string RNP AR IAP will likely require aborting the IAP prior to the MAP.
But, the assumptions in criteria are worst case; loss of GPS at DA.
Which is they way most, but not all systems, function. The point is, the output of the IRUs (or IRU) is updated so the apparent IRU position has no significant errors in the event GPS is lost.
There are three types of missed approach procedures permitted by Order 8260.52:
RNP of less than 1.0, also known as "telescoping RNP," which requires at least one IRU, and because the worst case assumption is loss of GPS at the MAP, positive course guidance is then required from the IRU or IRUs, which position is updated in the FMS continuously until/if a loss of GPS occurs. The telescoping increasing size of the missed approach procedure is calculated upon agreed upon assumptions for IRU drift once GPS is lost.
Following are the two RNP missed approach containment areas from Order 8260.52. The first one requires at least one IRU and turns are permitted either with TF of RF legs (RF legs highly preferred.) The second illustrated RNP missed approach containment area does not require updated IRU position (unless the minimums are predicated on less than RNP 0.30, in which case at least one updated IRU position is required). In this second case no turns are permitted until the splay to Level 1 RNP is competed.
If the terrain is benign the third option is a conventional TERPs (or PANS-OPS) missed approach procedure to any nav aid or even DR. And, a turn is permitted (if necessary) as low as 400 feet AFE.
(The RNP 0.10 final approach segment in Figure 4-1B is an example of maximum application of criteria. the FAS could be as high as 0.30 and, if this type of missed approach is dictated by obstacles or airspace, then at least one IRU is required.)
For any RNP AR below RNP 0.3 GPS is required. Even some 0.3 procedures do not allow DME/DME only solutions.
Though highly unlikely to lose GPS due to 'loss' of sats, as long as ANP is below RNP you can continue. (You probably wouldn't notice a sat loss unless you were observing that page in the MCDU.)
And in the 738, the first indication of FMS GPS positional problems (sats or receivers) will probably be an amber TERRAIN POS message on the ND as the ANP builds followed eventually by the amber UNABLE REQD RNP. With GPS probs, the IRU's will not prevent this from occurring even if they are perfect and you can't continue to the MAP.
But, the assumptions in criteria are worst case; loss of GPS at DA.
In the '73 the IR pos is third in the hierarchy as far as FMS weighting and the IRU's themselves (att or pos) are not in anyway updated by the GPS.
There are three types of missed approach procedures permitted by Order 8260.52:
RNP of less than 1.0, also known as "telescoping RNP," which requires at least one IRU, and because the worst case assumption is loss of GPS at the MAP, positive course guidance is then required from the IRU or IRUs, which position is updated in the FMS continuously until/if a loss of GPS occurs. The telescoping increasing size of the missed approach procedure is calculated upon agreed upon assumptions for IRU drift once GPS is lost.
Following are the two RNP missed approach containment areas from Order 8260.52. The first one requires at least one IRU and turns are permitted either with TF of RF legs (RF legs highly preferred.) The second illustrated RNP missed approach containment area does not require updated IRU position (unless the minimums are predicated on less than RNP 0.30, in which case at least one updated IRU position is required). In this second case no turns are permitted until the splay to Level 1 RNP is competed.
If the terrain is benign the third option is a conventional TERPs (or PANS-OPS) missed approach procedure to any nav aid or even DR. And, a turn is permitted (if necessary) as low as 400 feet AFE.
(The RNP 0.10 final approach segment in Figure 4-1B is an example of maximum application of criteria. the FAS could be as high as 0.30 and, if this type of missed approach is dictated by obstacles or airspace, then at least one IRU is required.)
Last edited by aterpster; 14th Aug 2011 at 16:49. Reason: Changed "ICAO" to "PANS-OPS"
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FlightpathOBN, your manual says "for GP/IR position calculation". As far as I can see, the original question was about "attitude".
The laser gyros are susceptible to cumulative errors. The accelerometers in the IRUs/ADIRUs, however, are sensing a gravity component (not just aircraft acceleration) and this applies a long term correction factor to attitude.
Rgds
NSEU
The laser gyros are susceptible to cumulative errors. The accelerometers in the IRUs/ADIRUs, however, are sensing a gravity component (not just aircraft acceleration) and this applies a long term correction factor to attitude.
Rgds
NSEU
And the GPS input to the ADIRS actually comes from the MMRs, so it's a separate component.
We all use FMC position which is derived from several inputs, the IRSs included. What happens to those inputs to produce the position is obviously of some interest to us, (the Aibus FCOMs are very good on this) but of little relevance to our everyday operations. If the ANP is within the RNP, then all is well and generally we are happy. I've used four different types of FMC in my career and all derived their position in different ways. One used a component called an EGI (Embedded GPS IRS) as the main source.
We all use FMC position which is derived from several inputs, the IRSs included. What happens to those inputs to produce the position is obviously of some interest to us, (the Aibus FCOMs are very good on this) but of little relevance to our everyday operations. If the ANP is within the RNP, then all is well and generally we are happy. I've used four different types of FMC in my career and all derived their position in different ways. One used a component called an EGI (Embedded GPS IRS) as the main source.
