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Loadhaul

IRS L/R and GPS L/R position data disagree

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Hi

During alignment of the IRS the GPS on the ground from the default PMDG cold and dark panel state there seems to be changing values of the GPS data whilst the plane is not moving, additionally once the IRS's have aligned there is a disagree between IRS L and IRS R.
FCOM VOL 2 (11.20.5) illustrates the procedure of this process.
Once sufficient DC power is supplied I switch both IRS gyros from off to nav.
I verify that  white align indication is displayed and not flashing.
I then monitor the time taken for them to align which is on the realistic setting, as stated in VOL 2 (11.20.5) I enter the reference airport in the FMC and go to POS REF page 2.
On page 2 both GPS indications are displayed; in my case they disagree, but even more odd to me they change, though the aircraft is not moving during the IRS alignment process.
Once I select GPS L for example and insert it into the prompts on the previous page, GPS L on page 2 changes coordinates from the ones I entered. Not to mention GPS L disagrees with GPS R, this is before the IRS has completed alignment.
After the IRS alignment is complete IRS L and IRS R disagree on the IRS display selector knob and on POS page 2. 

From my understanding these positions will update ONLY when the aircraft is in motion and not stationary.
During flight I have large navigation inaccuracies related to what ever I'm doing wrong here.
I tested using a pre-aligned panel state and noticed there is no disagree/deviation between either IRS.

I'm certainly no new user of the 737, been flying this aircraft for about 2 years now and have obviously done this correctly then, but I must be missing something here since I haven't flown it for a while, or i'm just noticing something that is normal...

Here's an example of how these coordinates change over approximately a 30 second interval, keep note the aircraft is not moving.

GPS L - S27 23.5 E153 07.3
GPS R - S27 23.6 E153 07.4

After about 30 seconds - 1 min

GPS L - S27 23.6 E153 07.4
GPS R - S27 23.7 E153 07.4

GPS L - S27 23.6 E153 07.3
GPS R - S27 23.6 E153 07.4

Is it normal to see these coordinates change and disagree whilst the aircraft is not moving? I understand there might be some disagree under some circumstances but during flight the IRS disagree is enough to cause navigation errors.

Thanks, Mitchell.

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Posted (edited)
10 hours ago, Loadhaul said:

Is it normal to see these coordinates change and disagree whilst the aircraft is not moving?

Yes.

The GPS satellite constellation is in MEO, and not GEO. More specifically, they are about 15,000km short of remaining in the same exact position above you at all times, and therefore have an orbital period of about 12 hours. Because of this, the constellation changes above you, even if you are stationary on the globe. While you aren't moving with reference to the globe, you are with reference to the satellites by which you are determining your position. As such, changes in their position will result in variation in the triangulation calculations, and the number and ability to see various satellites in that constellation.

10 hours ago, Loadhaul said:

I understand there might be some disagree under some circumstances but during flight the IRS disagree is enough to cause navigation errors.

IRS, by its very nature, is subject to a degradation of position determination because of the way it calculates moving. Inertial reference uses your inertia to determine where you are based on where you've come from. In other words, in order to determine where "here" is, it looks at its last position reference, and runs a calculation on all of the motion vectors.

To use an alternate example, take a 1-foot ruler, place it on the ground on what you calculate as East, and then plot out 20 feet of travel in that Easterly direction. Over those 20 feet, you will actually end up introducing a little bit of error into the end result because of the imprecision of moving the ruler each time. This is similar to how calculating IRS positions over time actually naturally introduces error.

 

 

...so if IRS is still so error-prone, why do we use it?

The answer is over on that last IRS detent that we rarely use. The IRS is based off of sensitive instruments that detect inertia. Ignoring the part of the IRS that people think that it is for - the aforementioned summation of the vectors for position data - the part that determines each vector frame is incredibly good at giving us data for accel/decel, turns, and artificial horizon info. Note that the ATT position basically removes the lat/lon position function of the unit, and drops it into giving us an attitude indicator. Even if the position data degrades so much that it is useless to provide it, it can still give us that attitude data in basically real time.

GPS, on the other hand, is very poor at providing us this data because of the polling rate, and the fact that, in order to give us any sort of flight information, we need to run a calculation on P2 vs P1. As such, GPS only really gives us historical data instead of real time data.

Putting this together, we use GPS for position because it remains the most accurate over a flight, with the IRS as a fallback (might as well as long as we have it, and the GPS can update its accuracy along the way, periodically). We use the IRS as a source of flight information (mainly the attitude indicator, but also the accel/decel pointers on the speed tape), with the GPS as a fallback (I think this is mainly relegated to informational CDU pages). They work well together, but alone, they have their own strengths and weaknesses.

Edited by scandinavian13

Kyle Rodgers

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Thanks for the in depth reply.
Just sorted it out the original issue by setting the GPS position in POS page 2 before the IRS's completed their alignment process and they all matched up!
After a flight for almost 4 hours there were some differences as expected, but were sorted out by doing a fast re-alignment when stationary.
IRS's > NAV > ALIGN > NAV

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Posted (edited)

Just completed a flight with some strange behavior, I believe it was unrelated to the IRS as it decided to fix it self later in the flight.
I was able to replicate this issue twice on two different occasions, both were however with the -700.
I departed PHNL 08R without a SID as it was an assigned heading departure and SIDS were not being used, In the CDU it looked 08R DCT to my first waypoint. 

PHNL DCT DATBE DCT RAGDE DCT WOSLU DCT XI PLCH

After being vectored out of PHNL I needed to fly direct to my first waypoint which was fine until the aircraft started weaving around and drifted left of the magenta line in which required me to use HDG intervention.
The plane got worse and worse and started to oscillate; after flying the active waypoint it was then fine.
I thought this was reported a while back that if you flew DCT to a waypoint after departure sometimes the aircraft would drift off track; thence if you flew a SID it was perfectly fine.
I fly mostly in Australia; our departure routings are very frequently routed by assigned heading, e.g. YBCS, YPPH, and YBTL just to name a few. This has never happened before during the countless times I have done this...
I got the POS REF page up on the FMS just for a reference in the screenshot. As I mentioned earlier the plane was perfect after reaching the first waypoint.

https://imgur.com/a/HGyqLAG

 

Edited by Loadhaul

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On 6/15/2019 at 8:40 AM, Loadhaul said:

The plane got worse and worse and started to oscillate; after flying the active waypoint it was then fine.
I thought this was reported a while back that if you flew DCT to a waypoint after departure sometimes the aircraft would drift off track; thence if you flew a SID it was perfectly fine.
I fly mostly in Australia; our departure routings are very frequently routed by assigned heading, e.g. YBCS, YPPH, and YBTL just to name a few. This has never happened before during the countless times I have done this...

I think this is a known issue for us.

Legs that have a particularly large distance between points causes the AP to freak out a bit. I don't think I've ever seen a flight plan from the real side of aviation that will cause it, though.


Kyle Rodgers

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