Dear Michael,
Great answer! Thank you for your detailed research.
Cheers,
Pat Naughtin
Author of the ebook, Metrication Leaders Guide, that you can obtain
from http://metricationmatters.com/MetricationLeadersGuideInfo.html
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has
helped thousands of people and hundreds of companies upgrade to the
modern metric system smoothly, quickly, and so economically that they
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On 2010/04/22, at 00:21 , Michael Payne wrote:
All large modern aircraft built since around 1975 have Inertial
Reference Systems (IRS) for Navigation. Nowadays IRS is a Laser Ring
Gyro with no moving parts. Since around 1995 GPS came on the scene
and the IRS get its initial position from a GPS or pilot input
source, once aligned, the IRS measures motion and acceleration in 3
axis (plural?) and will determine its own position. This position
information is fed into 3 Flight Management Computers (FMS) that
store the route of flight as Latitude/Longitude coordinates. The
flight management computers receive input from 3 IRS and 2 GPS
receivers and 2 VHF Distance Measuring Equipment receivers (DME),
updating using the distance from 2 or more known DME sources on the
ground to triangulate position using lateral and altitude
information. The FMS will give preference to GPS position, then FMS
position then DME position. Additionally each FMS compares its
position to that of the other two, if one goes beyond a certain
distance from the other two it's voted out of the computation and
the other 2 FMS are used only. The theory is that if GPS and DME are
lost over the oceans the aircraft position would be known to the FMS
based only on IRS information which has a known drift rate. Before
GPS came along IRS was the only source of position over the oceans
until ground stations could be picked up and the position updated
based on DME/DME. It's a very sophisticated and expensive piece(s)
of equipment.
You could say that nowadays GPS is the primary source of navigation
data with IRS as a backup. Large aircraft also use the IRS as the
sole source of attitude information for the pilots (there are 3 of
them), there is no separate attitude or directional gyro, there is
no longer a magnetic compass either, the FMS computes position and
required track based on True North, then adds (or subtracts) the
local magnetic variation to give a magnetic track, all of this from
the database used for navigation.
It is possible to look up the GPS altitude but it's not used for any
purpose. We still use the same pressure level converted to a Flight
Level (1013 hPa) or an altitude MSL based on a local altimeter
setting (QNH). The change from Flight Level to Altitude varies
worldwide by country, in the North America it's 18000 ft, Mexico
20000 ft. Europe 3-5000 ft above the airport, Australia 15000 ft,
Africa 3-5000 ft above the airport.
It's been my experience that GPS is normally accurate to around 5 m
laterally, there are times when there are insufficient satellites to
use for a GPS approach and the pilot is warned. The navigation is so
accurate that ICAO recommends aircraft offset to the right of track
when out of radar contact in case someone is coming the other way on
the same track at the same altitude. This happened in Brazil about 4
years ago when 2 aircraft coming in opposite
directions collided, the winglet of one apparently sliced the wing
of the other aircraft which crashed, one landed OK.
Mike Payne
----- Original Message ----- From: <mech...@illinois.edu>
To: "U.S. Metric Association" <usma@colostate.edu>
Sent: Monday, 19 April 2010 03:32
Subject: [USMA:47172] Re: Air flight altitudes in meters
What is the contribution of GPS data to the navigation of large
aircraft?
Do GPS data dominate barometric data?
---- Original message ----
Date: Sun, 18 Apr 2010 18:10:10 -0700 (PDT)
From: "John M. Steele" <jmsteele9...@sbcglobal.net>
Subject: [USMA:47170] Re: Air flight altitudes in meters
To: "U.S. Metric Association" <usma@colostate.edu>
Cc: "U.S. Metric Association" <usma@colostate.edu>
Actually, it is nominally based on height above
sea level. Sitting on the runway, with altimeter
correction dialed in, it will read the published
height of the runway above sea level.
At cruise levels, no altimeter correction is used
and reading is called flight level. It is the
height above sea level IF sea level were at 15 °C,
101.325 kPa, and a lapse rate of -6.5 K/km to the
stratosphere (11 km), and zero lapse rate above that
to 20 km. Further it it uses a height variable
called geopotential height, the height that would be
true if gravity were constant with height. There is
a transformation between that and geometric height
in the standard.
------------------------------------------------
From: James R. Frysinger <j...@metricmethods.com>
To: U.S. Metric Association <usma@colostate.edu>
Cc: U.S. Metric Association <usma@colostate.edu>
Sent: Sun, April 18, 2010 8:47:07 PM
Subject: [USMA:47168] Re: Air flight altitudes in
meters
Altimeters work off of atmospheric pressure
readings, Pat. But the readout is in terms of height
above terrain. So assignments and reports are always
in length units. No human pressure to altitude
correlation procedures are used.
Jim
