Found this in the pprune.org-archive from user kijangnim on 26.JAN-2010 at 13.58h in Tech Log, A320 FM Position...
Denis
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
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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