IRS = LONG. but how?
IRS = LONG. but how?
A simple explanation for how an INS works out Latitude:
If I drew a quater circle on a piece of paper, then handed that paper to you with a pen, you would have no trouble completing the rest of the circle. Looking at this completed circle you could then tell me:[list=1][*]how big the circle is, and[*]where the centre of the circle is.[/list=a]That is all an INS does.
When you turn the INS on (ie place it in NAV mode) it starts tracking its movement thorough space. As it is stationary on the Earth's surface, the only movement it is tracking is the rotation of the Earth - so it starts drawing a circle. After around 10 minutes it has enough of an arc to mathematically complete the rest of the circle.
The size of that circle tells it the Latitude (how far North or South it is on the planet) while the centre of the circle is True North.
Once you tell it its Longitude it looks up a table of world wide magnetic variation to determine magnetic direction, which it then displays on the DG/Nav display.
INS aircraft don't have any magnetic sensors (like a flux gate) to sense the Earth's magnetic field, as they aren't required. The standby compass is a normal magnetic compass, however.
[This message has been edited by Checkboard (edited 18 May 2001).]
extra note : this is not how the INS/IRS actually works out latitude, but it is a nice easy way to understand the concept. See Dick's post below if you are looking at JAR exams or want a more correct technical answer!
If I drew a quater circle on a piece of paper, then handed that paper to you with a pen, you would have no trouble completing the rest of the circle. Looking at this completed circle you could then tell me:[list=1][*]how big the circle is, and[*]where the centre of the circle is.[/list=a]That is all an INS does.
When you turn the INS on (ie place it in NAV mode) it starts tracking its movement thorough space. As it is stationary on the Earth's surface, the only movement it is tracking is the rotation of the Earth - so it starts drawing a circle. After around 10 minutes it has enough of an arc to mathematically complete the rest of the circle.
The size of that circle tells it the Latitude (how far North or South it is on the planet) while the centre of the circle is True North.
Once you tell it its Longitude it looks up a table of world wide magnetic variation to determine magnetic direction, which it then displays on the DG/Nav display.
INS aircraft don't have any magnetic sensors (like a flux gate) to sense the Earth's magnetic field, as they aren't required. The standby compass is a normal magnetic compass, however.
[This message has been edited by Checkboard (edited 18 May 2001).]
extra note : this is not how the INS/IRS actually works out latitude, but it is a nice easy way to understand the concept. See Dick's post below if you are looking at JAR exams or want a more correct technical answer!
Last edited by Checkboard; 15th Apr 2003 at 07:41.
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.....and I guess that's why most systems are usually limited to about 70 degrees North /South for successful alignment:
The circle drawn during the 10 minute alignment process is too small ('short') to accurately calculate the latitude.
And a little note on the magnetic input:
The model is called the 'International Geomagnetic Reference Field and is updated every 5 years. There's good info on all things magnetic:
http://www.ngdc.noaa.gov/seg/potfld/faqgeom.html
The circle drawn during the 10 minute alignment process is too small ('short') to accurately calculate the latitude.
And a little note on the magnetic input:
The model is called the 'International Geomagnetic Reference Field and is updated every 5 years. There's good info on all things magnetic:
http://www.ngdc.noaa.gov/seg/potfld/faqgeom.html
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IMHO the IRS works out latitude by recording the X axis rate, which should be 15.04xCos Latitude (deg/hr). But this is only correct if the system X axis is pointing at true north. This is not necessarily so until azimuth alignment is complete, so the internal calculation of latitude is only approximate. The sysytem must have a very, very accurate latitude input, and this comes from the entered PPos data of ramp position. If this varies from the rough internal calculation by, usually, one degree, the system will call for a check, as it will if the entered position is different from the recorded shutdown position.
Azimuth alignment is done by taking the Y axis rate, which would be zero if the azimuth were spot on. If it is not, the Y rate will be 15.04xCosLatxSin of the error angle. As latitude gets nearer 90deg Cos Lat gets near to zero, and the error reading gets so small that it becomes unusable. That is why azimuth alignment - not the latitude calculation - does not work above 70deg latitude.
Dick W
Azimuth alignment is done by taking the Y axis rate, which would be zero if the azimuth were spot on. If it is not, the Y rate will be 15.04xCosLatxSin of the error angle. As latitude gets nearer 90deg Cos Lat gets near to zero, and the error reading gets so small that it becomes unusable. That is why azimuth alignment - not the latitude calculation - does not work above 70deg latitude.
Dick W
