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FMGC Position

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FMGC Position

Old 17th Jan 2015, 05:22
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FMGC Position

Does FM use GPIRS, IRS MIX or FMS position?
What is FMS position? IRS+GPS+Kalman Filter? So what is the use of IRS MIX? When we read GPS PRIM on the ND, what does that mean? We are using FMS position?

When do we use:
-GPIRS position?
- FMS position?
- IRS MIX position?

Thanks

Last edited by AF330; 18th Jan 2015 at 21:42.
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Old 17th Jan 2015, 18:51
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Depends on the installation...

In the 744, the FMS uses the GPS position as primary. If the left FMC is Master (normal ops), the left GPS is used.

The IRS3 position is also calculated and used as backup (with current VOR/DME/ILS updates as applicable). If GPS fails, IRS3 will be used automatically.
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Old 17th Jan 2015, 18:56
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i asked this question several decades ago.
stil no reply
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Old 17th Jan 2015, 19:21
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Type? Mod state? What does the FCOM say?
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Old 18th Jan 2015, 21:42
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Thanks,

By IRS3 you mean IRS MIX? So why does the plane calculate the FMS position if it uses GPS?! When you say GPS, do you mean GPIRS?

Really confusing!
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Old 19th Jan 2015, 02:08
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By GPS I mean GPS.

By IRS3 I mean IRS3, but it might also be called IRS MIX in some installations.

Do you understand the concepts of "used as backup" and " If GPS fails"?
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Old 19th Jan 2015, 03:17
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This understanding is primarily for the Airbus. Please correct if incorrect.

GPS feeds to IRS. GPS->IRS=GPIRS. This is higher priority than radio updating.
GPIRS feeds to FM for position update.

3 IRS makes MIX IRS. [GPIRS/radio update]-[MIX IRS]=BIAS.

In case of GPIRS/radio update failure: MIX IRS+BIAS feeds to FM

Position is fed to FM for navigational use.
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Old 19th Jan 2015, 05:24
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Thanks,

1) Didn't get the BIAS thing! Could you please explain?

2) So why does the plane calculate the FMS position?? Or BIAS = FMS position?

Thanks!
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Old 19th Jan 2015, 07:43
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IRS drifts. This results in inaccuracy. This inaccuracy is assumed constant when GPS n radio updating goes offline, known as BIAS. This constant it applied back to IRS position to approximate present position. Without GPS n radio updating the FM reckons your position by using IRS applied with the bias. This means positioning is always by IRS updated via GPS, radio or BIAS.
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Old 19th Jan 2015, 21:18
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Thanks a lot!

1) By Radio position, do you mean that the plane also tries to calculate where it is with navaids?

2) So the Kalman Filter = BIAS?
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Old 21st Jan 2015, 04:49
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By radio position update I mean radio navaids updates IRS which in turn updates the FMC.

I do not know what is Kalman Filter. Could you explain how it works?

The 3 IRS has an algorithm which is a weighted computation to lessen the contribution of any IRS which drifts excessively. This algorithm results in a weighted average position of the IRS called the MIX IRS. Difference between this weighted centre n the GPIRS is the BIAS.

Therefore BIAS can be viewed as the difference between the IRS positions by itself n with updates.
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Old 21st Jan 2015, 17:10
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FMGS - What I have understood! :)

Well it's pretty confusing!

So here is what I have understood:

FMGS has got 3 different positions:
- GPIRS: calculated by the ADIRU. ADIRU get's GPS information with the MMR and 3 IRS/IRU/INS positions. ADIRU will then calculate a GPIRS hybrid position. It will basically make out one position.

It will then send the GPIRS position, the 3 different IRS/INS/IRU positions and the GPS position to the FMGC. (To the FG, I suppose)
We can see it here:



So, with this, the FMGS knows the GPS position and the GPIRS position. Now, the FMGS will calculate the MIX IRS position. But as IRS always drifts, it will use, if available:
a) GPS. GPS will correct IRS positions to calculate the IRS MIX POS. But If GPS fails, the plane will use:
b) Radios to correct IRS positions. So radios are basically navaids: VOR's (Airbus only autotunes VOR's as navaids).
But If the antenna's or the navaids fail, the plane will use:
c) BIAS to correct IRS position. BIAS is just a system which uses FMGS data to update/correct IRS MIX position.

The FG will send to the FM the GPIRS position (computed by the ADIRS).
If GPIRS fails, FG will send to FM the IRS MIX position.
If GPIRS and IRS MIX position fails, FM will use FM calculated position.

1) a) Do you agree with all this?
b) If IRS fails, will FG send to FM the GPS position or will it let FM calculate the FMGC position?

2) What does ADIRU use to correct IRS position to calculate GPIRS position? Only GPS? Or does it have BIAS too? Kalman filters? I don't think it get's radio update...

Now, I had found this post:
http://www.pprune.org/tech-log/40332...-position.html

So it says:

"Kalman filtering is a technique implemented in the FM, and applied
by the FM in order to compensate for IRS error.
As we know, IRS error grows with time.
It is interesting to note that as far as IRS versus GPS precision is
concerned, the IRS is less precise in long term but more precise in
short term.
The FM will compare the position obtained by the GPS (or computed
using Radio update), with the IRS raw position, the difference will be
translated into an error, and fed to the Kalman filter.
The Kalman filter will use the error and apply a correction based
either on the frequency or the statistical occurrence of that error."

3) Can we say that Kalman filtering is inside the FM and BIAS are inside the FG?

4) It also says:"On the Thales/Smith FMS position is a computed position, elaborated by the FMS, using Hybrid GPS/IRS position provided by the FG.
They use a Kalman filter, to cater for the drift and maintain a high accuracy."

So basically it uses the GPIRS position which it then corrects with the Kalman Filter??? But isn't it already corrected by the ADIRU while calculating GPIRS position? But he puts a "/" between GPS and IRS. Does he mean that the FM will make out another GPIRS hybrid position?
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Old 21st Jan 2015, 22:40
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Kalman Filters

A Kalman Filter in the context of a flight management system is a piece of software that allows the FMS to combine multiple different sensors to determine a very accurate position and velocity. It allows the system designers to combine inputs that have quite different error characteristics in a sensible manner. For example, modern FM systems will often have air data inputs (airspeed, altitude, rate of climb), heading inputs, the ability to tune and sense a VOR, the ability to tune and sense a DME, an input from whatever inertial platforms are on board (AHRS or whatever), and an input from one or more GPS sensors. Each input will be processed according to its' inherent error characteristics, and will generally be allocated a weighting - how strongly it influences the position and velocity solution. The cool thing about a Kalman filter is that it automatically and continuously updates these weightings based on current performance of each sensor. That's why it is a filter.

The previous post mentioned Kalman Filters for correcting IRS errors. That is one use, but they are more widely used in modern FM systems for multi-sensor blending.

Some FM systems have the ability to display what weightings are in force at any particular time. In our FMS, if all is well with the world, you can see that it is heavily oriented to the GPS input. If the GPS input is disrupted, it seamlessly drops the priority of the GPS and uses DME/DME scanning. On a really bad day it will eventually end up in dead reckoning mode, in which it is using airspeed and heading to estimate where you are.

But, even when the GPS is working perfectly our FMS never weights 100% to the GPS position source. It is always running all the other inputs, because that is how it can spot a GPS outage very quickly, plus the AHRS updates are quicker, so that in manoeuvring flight the FMS position is often a few metres different to the GPS position - and correctly so.
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Old 21st Jan 2015, 23:33
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Thanks,

So is the filter used only to calculate the FMS position? Because BIAS are used to calculate IRS MIX by FG?
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Old 24th Jan 2015, 05:00
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I am not sure but I think that Airbus FMS does not use Kalman filter in that way.

It chooses the most accurate and reliable navigation source depending on the sensors available, geometric dilution of precision, altitude, etc…

The MIX IRS position uses a weighed average of the three IRS positions, giving more weight to those IRS with historically better precision as compared with reliable radio or GPS position.

GPIRS position is an input coming from the ADIRU. It is not FM computed! Each ADIRU uses its associated GPS and gives a position with good accuracy (GPS) augmented with the ability of the IRS to sense acceleration and speed. (GPS is better if it remains still). It is even better than pure GPS position, basically. I don't know with mathematical method is used for that mix.

So when the FMS decides that GPS is reliable it will simply use GPIRS position.

If GPS is not reliable or available (or even installed), then the position will be either radio position (DME/DME or VOR/DME, or LOC DME DME) when this source is considered as the best one, or IRS if radio is not (low altitude just after take off, bad dilution of position, no navaids...). I think there is some blending of these two (radio and IRS) when there is a transition from one source or the other, this blend tending to one or the other. Perhaps it is carried out by those Kalman filters.

The BIAS vector is like the correction you make to your gun when you shoot at the target. If all your shoots go up and to the right by 5 inches, then shoot low and to the left by five inches and you will hit the target. If accurate and reliable radio position is 2 miles north and 1 mile east of IRS position and then you lose radio sensors, then the FMS will give an position which is 2 miles south and 1 mile west of the IRS position, instead of simply giving pure raw IRS position.

hope it helped
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Old 25th Jan 2015, 06:24
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Thanks a lot,

1) So when you read GPS PRIMARY, it's GPIRS? But we do not always see it....

2) So the ADIRU uses GPS position to correct IRS position? It then calculates GPIRS position.

3) So the FG corrects each and every IRS (before calculating the IRS MIX position) with GPS, radio position or BIAS, right?

4) FMGS will use GPIRS, IRS MIX position or FMGS position to navigate. But how de we know which one is being used?

5) So what is FMGS position???? What does it use?

Thanks
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