A320 FM Position

[MD83FO]

The wording on the FCOM keeps me confused.

What is the aircraft using for present position
at all times when gps primary:

actual position? (gps)
or
mix GPIRS? which is obviously drifting.

What I'm thinking is why would the a/c use a IRU drifting value
when it can use gps, then if gps pos is drifting from the 3 mix GPIRS's its can assume IRS pos.

cheers guys. gps

[kijangnim]

Greetings
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 you end up with :
IRS/GPS hybrid
GPS
and FMS position

Triple mix is more a technology used by HWL, they did want to use KAlman filtering saying that it was not proven, but today they use it in their latest ADIRU (only idiots dont change their mind) they are using the barycenter and the FMS is computed using a blending of that value and GPS or RADNAV...
I hope you are not more confuse now. :hmm:

[MD83FO]

is see.. i suppose that means the FMGS (actual position) is using a drifting value rather than a pure gps value.

i.e. to remain on the route centerline.

[kijangnim]

Greetings,
I see that you are located in Qatar so your aircrafts are equipped with the Thales FMS2, they should have given a pilots guide....:}
We have to understand why Airbus decided to Hybrid both signals, ie IRS, GPS.
Because for short term the IRS is extemely precise, whereas for long term the GPS is more precise. So hybridation gives the best compromise.
As far as drift is concerned, forget about it.
Basically, positions a compared along with the HIL FOM from the GPS reciever and these values enables tne FMS to elaborate what is the IRS drift and noise.
But as a pilot I dont think you need to go into that level of details.
:ok:

[TyroPicard]

Basically it uses GPS when GPS PRIMARY is displayed on the PROG page. This may help...

The FM position used to navigate is the most accurate of these three:
MIX IRS
RADIO
GPS

Imagine departing from an island in the ocean with no Navaids, and with GPS Deselected. Each FMGC will compute a MIX IRS position from the IR's and use that to navigate.

Now, as you near land you enter RAD NAV coverage, and each FMGC computes another position called the Radio Position. Provided there is a DME element the FMGC will use the Radio position to navigate. (1.22.20 p7). And it will now apply a bias to the MIX IRS position to make it approach the Radio position, thus it becomes more accurate (useful if you turn round and go back to the island).

Now enable GPS, and (1.22.20 p3) each IRS now computes a mixed IRS/GPS position called GPIRS. GPS is so accurate that to human pilots this is indistinguishable from the GPS position (subject to integrity criteria etc.). The FMGC will now use ONE of the onside/GPIRS3/offside GPIRS positions to navigate - not a mix. It will now apply a bias to the FM position to approach the GPIRS position it is using, so that it has a more accurate position available if GPS PRIMARY is lost.

And that is the important bit - you don't need to know all this, but you must know how to monitor navigation if you lose GPS PRIMARY......

[ampclamp]

when working gps gives excellent position.
when working that is.Nothing is perfect or guaranteed.In the vast majority of cases it is fantastic but...
aircraft must have some kind of dead reckoning / independent nav device.

Mixing provides cross checking and constant updating..If a solar flare took out some satellites or effected their ephemiral data or more likely a lightning strike could blow both GPS antennas.Seen that myself.

pre gps it was dme dme vor or similar with irs to fmc.
Checking cross checking.......To be sure to be sure...

[MD83FO]

thanks guys, so I can understand that due to redundancy a slight position error (bias) is allowed into the FM due to IRS drift, making the actual navigation performance greater than zero.

[Microburst2002]

Hi

In an Airbus, there are several different positions:

pure IRS 1, 2 and 3.
pure GPS 1 and GPS 2 (if installed)
GPIRS 1 (computed by ADIRU 1: IRS1+GPS1)
GPIRS 2 (computed by ADIRU 2: IRS2+GPS2)

Each FMGC computes different positions:
radio (DME/DME, VOR/DME or DME/DME/LOC)
MIX IRS (which I think is not just a mere average, but it also takes into account each IRS "historical accuracy")
and then, among all those positions, each FMGC makes a mathematical blend depending on circumstances (and I don't know if it uses those kalman filters which I 've never understood) and then comes up with its respective FMGC position, which is the one used for its associated displays.

This FMGC position, when there is no radio nor GPS, is not pure IRS nor MIX IRS alone, but the MIX IRS corrected by the bias, which is the difference between the MIX IRS and the radio or GPS position.
When there is GPS or good radio position available, the FMGC position is the "scotch blended" one.

Kijagnim please don't hesitate to correct anything which is not correct.

By the way, very interesting that they did not use Kalman filters in FMS 1, if I did understand correctly.
Can you explain what Kalman filters are and do, in simple terms, of for "dummies" or is it impossible? I mean, what is the problem they solve? and How did the FMS 1 solved the same problem without them?

Thankyou very much

[kijangnim]

Greetings
1.1.1 THE NAVIGATION FUNCTION
A glance at some Bits and pieces
1.1.1.1 The IRS:

The
IRS is the only navigation system which qualifies as a “SOLE
Mean of Navigation

” system, because it has availability, integrity,
and continuity.
Therefore the IRS will be the absolute reference for the FM, when
position and velocity has to be acquired.
1.1.1.2 IRS and GPS
If GPS has successfully passed all the tests, then the IRS position
and velocity data will be blended with the GPS position.
The result represents what we refer to as RAW data, this raw data is
acquired by the FM through the FG, in order to be used for position
computation.
If the GPS is not available, then the IRS will still provide position and
velocity raw data, however they are Inertial only data.
1.1.1.3 Kalman Filtering
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.
1.1.1.4 The Earth Model
The FM uses a simplified Earth model (WGS-84), and despite the
fact that it is accurate enough, it will still generate a small difference
in position with respect to where the FM “thinks” it is, versus where
the FM really is on Earth.
1.1.1.5 Trajectory representation
We have to remember that tracks, and courses are straight lines,
which is the way a great circle is represented on a Lambert conformal
chart.
1.1.2 COMPUTATION OF POSITION
1.1.2.1 The FM position computation:
1.1.2.1.1 Using IRS RAW DATA
As per Airbus philosophy based on system segregation, each
FM is using all three IRSs raw data inputs to obtain the
Inertial Position and Velocity, along with onside inputs from
Air Data.
The FM will apply the KALMAN FILTERING Technique to each
IRS in order to obtain a Corrected IRS Position and Velocity.
The FM will then compute the mean weighted average of the
three corrected IRS position and velocity to obtain a
corrected MIX IRS position and velocity.
The Corrected MIX IRS Position is then used by the FM as the
Aircraft position, this position also refers to the “FM position”
1.1.2.1.2 Using GPS Data
If GPS has successfully passed all the tests, then the FM will
use the GPS position to elaborate the error model, and feed
the Kalman filter with it.
1.1.2.1.3 Using Radio Navigation Data
If Radio Nav has successfully passed all the tests:
The same process described above is performed using
DME/ DME or VOR/DME, and the same results will
be obtained, however with a greater EEEssstttiiimaaattteeeddd
PPPooosssiiitttiiiooonnn UUUnnnccceeerrrttaaaiiinnntttyyy.
Further more, the FM through the navigation function
provides current aircraft state information consisting of
Present Position
Altitude
Wind
True Airspeed
Ground Speed
True and Magnetic Heading
True Track Angle.
1.1.2.2 SENSORS SPECIFICS:
The sensor selection is based on a precision criteria classified as
follows:
GPS
RHO/RHO, DME/DME
VOR/DME
MIXed IRS

and so on and so on So manyway to explain thing, there is no best one no worst one, the aim is to be understood

[MD83FO]

i couldn't ask for more except for one thing :)

i know the manual doesn't recommend to insert the gate coordinates when the irs drift is minimal, to prevent gross errors.
but will navigation be more precise if it's done?

[rudderrudderrat]

Hi MD83FO,

We "slew" the gate position using the up and down keys on the Lat & Long on Init page when doing PRNAV departures.

[kijangnim]

Greetings,
On the Thales FMS2 you dont need to slew, since GPS pos update with the Kalman filter will give a correct solution even if you are on the stand:ok:

[MD83FO]

then the kalman filter keeps the FM as accurate as possible, while taking note of the pos bias of the IR.

all great info! cheers.

[kijangnim]

Greetings
If you loose GPS, Kalman filter will maintain GPS accuracy for about 7 minutes or until the measured drift is greater than the Kalman filter value which ever occrus first, and then will use current accuracy, it will be reflected in the EPU which will be displayed with a higher value, as long as the EPU the less than RNP you will be in High accuracy, and do so on...:ok:

[Microburst2002]

Thanks Kigagnim

What documents is that? Can I find it in the web?

[kijangnim]

Greetings
It is a copy of the Thales FMS2 Pilots guide that was published just after the certification, as some pilots found it too complicated, they published another one.
The funy thing is that the Pilots whom rejected the first Guide came back later asking for the first edition:ouch: Too late....
It was written by a friend of mine I think he flies for Qatar