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
