![]() Longitude however can not in any way be calculated, and so the present position co-ordinates must be manually input.Īs far as racking problems go, yes it is vital, repeat VITAL that the INS/IRS is fitted securely to the rack, but all such units have a central locking pin to ensure positive location with the rack, you should never be able to get this wrong. An INS (or strap down IRS) can calculate latitude during alignment, by measuring the degree of platform tilt (or in the case of a modern IRS, the total component of all of the three RLG outputs). As the aircraft moved along, the platform was Schuler tuned with aircraft acceleration in order to guarantee exact horizontality. Once alignment was complete and NAV was either pre-selected or manually selected when alignment was complete the platform was then aligned to perfect horizontal, the gyros being torqued to retain their true horizontal also. Due to the earth’s spin axis, the platform would always dip in the east, making the establishment of true north automatic, the cardinal points were now locked in, until the system was shut down after flight. During alignment the X & Y axis gyros would sense the error generated as the earth spin was detected and tilt the stabilised platform in order to keep their position in inertial space. If you go back to the good old Delco Carousel 4 INS, true north was established as follows. ![]() When in ATT, there is no navigation data so "steaming errors" are quite large, hence we have to update the direction frequently. ![]() Once it's Navigating and moving over the earth's surface, it needs to know ground speed and position to allow for sensed errors (used to be called steaming errors on Ships with Sperry's gyroscope). The IRS constantly measures the direction of plumb vertical using accelerometers and hence can sense again the direction of True North. ![]() (For a gyro aligned parallel with earth's surface, Apparent wander is zero at the equator and 360 degs in 24 hours at the poles.) We have to put in present position because it can't sense Longitude (and it's a cross check that we know where we think we are against the Lat it accepts.) It measure the rate of apparent wander and it's direction and then calculates it's Latitude (N or S) without any input from GPS or gate position. It can work out the direction of True North during the align process. See the appendix in Sperry Gyrocompass Mark 14Īn IRS Laser gyro does a similar job. It doesn't just "remember it" - it constantly finds it. The Sperry Gyrocompass will point at True N. ![]() I don't think you would fail your IR renewal on that.Īlmost 100 years ago, Sperry and others used the fact that the axis of a spinning gyro, with gravity causing precession, will align parallel with earth's spinning axis and hence they had something which will point to True North. Thus in simple terms, after 10 hours, the indicated bank angle would be out by 0.2618 degrees. Not a lot! I make that 0.02618 degrees per hour. The effect of a 0.1 degree error in true north would simply mean that all the electronic 'fudges' applied to the platform 'attitude' to compensate for earth rotation would be in error by 0.1 degrees - not a lot! Looking at a platform at the equator with the platform 'pointing' north, the electronically 'applied' platform roll rate, for example, instead of 15 degrees/hour would be in error by 15*sine 0.1degrees. You just would not notice 0.1 degrees UNLESS you look at the displayed heading data. We have moved on from gyros spun up with string sitting on top of a miniature Eiffel TowerĪlso bear in mind that a difference of 0.1 degrees would not have any noticeable effect on your displayed heading, attitude, nor on navigation, especially since position is constantly updated whenever in range of the required signals. If you ran a platform for several days without an alignment then the errors would obviously would accumulate. Guys - providing you are prepared to accept that a modern inertial platform has VERY FEW errors, please accept that True North is NOT 'updated' at any time (without an alignment) but merely 'maintained. ![]()
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