Inertial Heading Measurement

[L Peacock]

Can anyone point me to, or present good explanation of the heading measurement phase during alignment for a mechanical INS.

Thanks

LP

[dgordon42]

INS True Heading Measurment

The first step in the alignment process is to level the platform. The platform is "torqued" by electric motors until the X (fore and aft) and the Y (left and right) sensing accelerometers sense zero acceleration, and the Z (up and down) accelerometer senses a maximum acceleration. This acceleration is due to gravity. When the platform is level, the system uses gyros on the platform to measure any drift of level, and uses the torque motors to restore the platform to level. The clever bit is that the earth rotates, and as it dose so the platform, which is being held level in space, appears to drift towards the east. Once you know which direction true east lies in, you know the true heading of your aircraft.

[L Peacock]

That's the bit I've not completely bottomed. How are the three acceleration axes initially resolved to determine true east and hence true north.?

[Bullethead]

G'day Mr Peacock,

The inertial platform detects the rotation of the Earth and so can calculate what it is rotating around, which is the Eath's axis, which lies true north/south and equipped with global magnetic variation data can compute the local magnetic heading. The inertial platform needs an accurate Lat/Long position to get things started. As latitude increases the detection of the Earth's spin axis takes longer as the local vertical approaches the axis itself.

Regards,
BH.

[BOAC]

The final bit of your puzzle is that, knowing the latitude of alignment, the platform knows the 'earth rate' at that latitude and rotates the platform until the 'east west' axis is requiring that tilt rate to stay level. To 'de-Greek' that, if your 'east west' axis was intitially north south and you were at the equator, say, it would rapidly get out of level (at 15 deg/hr) as it was being corrected for an earth rate of 15 deg/hr when in fact it was not requiring any correction when aligned north/south.

Try using two sticks set in a cross, held horizontally, mark them with different coloured tapes and 'rotate' them as if they were earth aligned but with different initial alignment errors. Start at the equator and when you have that sorted, move to the poles.

That even used to work for Lightning pilots.........

Then you can progress to explaining 'strap-down' alignment

[L Peacock]

Thanks guys