reply to post by ownbestenemy
If I may, you also need to add the distinction between Magnetic North and True North.
"True North" is the actual, geographical "physical" axis of rotation. Of course, it's an "imaginary" line....but, the Earth is massive, and thus that
axis, relative to the overall mass of the planet, is fixed geographically.
Depending on your location, which line of longitude
you are at, when you "look" up North (it works same in the southern hemisphere, only with
the South Pole, of course) the "direction" from where you are, to the True North Pole, lies exactly along one line of longitude. To make it simple
here, imagine you have moved to be EXACTLY on the meridian of 82 degrees, 32 minutes West. (You can refer to the TPA Airport Diagram in my previous
post, since that line is drawn on that Chart).
Up near the top of that Chart is listed the "magnetic variation" figure...."4.8 West". That is the subtended angle, from that location, to the North
Magnetic Pole. These values are known, and accounted for by the software in the systems you mentioned, in your post. So, for pilots navigate, is
generally referenced to magnetic north. The units "know" their heading based on True North, and then add/subtract (as appropriate) the variation, to
display on the instruments the proper magnetic heading information.
Even before GPS, when we just had Inertial Navigation...it TOO was based on geographical-related True North....and it "knew" the local magnetic
variation (also called "declination") for every spot on Earth. It is the same today, the GPS is just far more accurate, as it provides precise
updating capability to the basic Inertial Reference mechanisms and computations. (Earlier versions used physical gyroscopes, spinning at high
rotational velocities...and the "precession" that they underwent with acceleration of movement. Today, they use non-moving parts...well, light
actually....it moves....called "Laser Ring Gyros". Same principal, the laser light beams bounce in mirrors, in a "circular" pattern, and the
deflection of the beam is precisely measured, as the unit is subjected to accelerations in different directions). Pretty clever, eh???
In case you're interested, typical Inertial Reference System control panel, for a triple installation (Boeing commonly):
It is in the "lights test" mode, there....but the digital numbers display segments are normal LEDs, and display according to what's chosen by that
rotary selector. (The other three selctors are, of course, the "On/Off" switches).
Up there (it's located on the overhead panel) it only displays in True....down on the forward instrument panel, display is magnetic. Although, you
flip a switch, to display TRUE....this is for those rare times when you are WAY North (or South)....beyond about N80 -- S80 degrees
latitude. (But depends on longitude location). Navigation criteria change, up(down) there, since magnetic references are unreliable.
5 January 2011 by weedwhacker because: (no reason given)