DME/DME correction of INS & France?
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I think the general principle in all cases is that a gyro gives you an acceleration-related signal, which you integrate once to get velocity, and integrate again to get distance i.e. position.
The implementation is normally complex due to the need to make small corrections for all kinds of things (e.g. temperature related drift) and then you have to do some more maths (old well established stuff though) to work out where you are in terms of lat/long on the earth's surface.
The current holy grail is to make something useful with solid state gyros but they aren't good enough by orders of magnitude. You might get 10 minutes of navigation out of them
The implementation is normally complex due to the need to make small corrections for all kinds of things (e.g. temperature related drift) and then you have to do some more maths (old well established stuff though) to work out where you are in terms of lat/long on the earth's surface.
The current holy grail is to make something useful with solid state gyros but they aren't good enough by orders of magnitude. You might get 10 minutes of navigation out of them
Cunning Artificer
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BOAC is correct. The laser gyros only measure the aircraft displacement rates about the x, y and z axes. The displacement rates are integrated to obtain angular displacement from the starting attitude - when the aircraft was hopefully parked right side up somewhere upon the surface of the earth. When the operator enters the geographic coordinates into the system the IRS computes the acceleration rates and refines the raw aircraft attitude into a true hoizontal starting point. Once the system has an accurate starting point and horizontal reference, all motion along the x, y and z axes is resolved by mathematical integration of accelerometer outputs into velocity, direction and distance. The tricky bit with "strap down" laser gyro systems is that the three accelerometers are fixed to the aircraft axes, so the mathematical calculations become quite complex. In old "steam driven" systems the accelerometers were attached to a gyro stabilised level platform.
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Please correct me if I am wrong but the term strap down actually refers to the old INS where there was a set of gyros on a platform that would basically remain gyroscopically erect while the aircraft rolled around this platform. An update to this system was the strap down platform as you mentioned. This system had the gyros attached to a platform that was attached to the aircraft hence it was only the individual gyros that moved. Using the term strap down for the modern ring laser gyro is sort of a misnomer as they are attached to the aircraft regardless. So the term strapdown is a bit old hat really. Again please correct me if I'm wrong.
Cunning Artificer
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Older (civilian) systems had gyro stabilised platforms with accelerometers mounted on a stable platform - that is, they remained in a horizontal plane. The popular Carousel system did complicate matters somewhat, by rotating the platform at 1 r.p.m. to enable any platform leveling inaccuracy to be isolated as a one cycle per minute error signal in the accelerometer outputs. However, all gyro stabilised systems isolated the accelerometers from the aircraft roll, pitch and yaw movements and held them in a level plane.
In a ring laser gyro system, accelerometers are fixed to the aircraft, aligned with the aircraft's roll, pitch and yaw axes. The laser "gyros" are used to accurately measure the roll pitch and yaw movements of the aircraft to enable the spacial alignment of the accelerometers to be determined by the processor. The resolution of the flight path is performed by computing the various vectors obtained from each accelerometer - each of which will simultaneously be measuring an acceleration in each of the three planes. The IRS computer resolves the complex pattern of accelerometer outputs into horizontal and vertical components by reference to the measured attitude of the aircraft and thus of the accelerometers themselves.
This is what I intended to convey by the term "strap-down" - that the accelerometers are not isolated from the aircraft attitude by a stabilised platform, but that the entire reference unit - including both the accelerometers and the ring lasers - is firmly attached to the aircraft with no moving parts.
The IRS measures the movement of the aircraft away from its starting point and in general cannot be realigned in flight. Indeed, for most systems in use today, the aircraft cannot be moved during IRS alignment on the ground and even aircraft loading can disrupt the alignment process. Position updates are performed by adding an offset to the computed position, the IRS continues to use its own computed position, then adds the offset to its output before displaying position to the crew and other aircraft systems.
You can find the value of this correction at the end of any flight by perfoming a terminal error check as per the AMM.
In a ring laser gyro system, accelerometers are fixed to the aircraft, aligned with the aircraft's roll, pitch and yaw axes. The laser "gyros" are used to accurately measure the roll pitch and yaw movements of the aircraft to enable the spacial alignment of the accelerometers to be determined by the processor. The resolution of the flight path is performed by computing the various vectors obtained from each accelerometer - each of which will simultaneously be measuring an acceleration in each of the three planes. The IRS computer resolves the complex pattern of accelerometer outputs into horizontal and vertical components by reference to the measured attitude of the aircraft and thus of the accelerometers themselves.
This is what I intended to convey by the term "strap-down" - that the accelerometers are not isolated from the aircraft attitude by a stabilised platform, but that the entire reference unit - including both the accelerometers and the ring lasers - is firmly attached to the aircraft with no moving parts.
The IRS measures the movement of the aircraft away from its starting point and in general cannot be realigned in flight. Indeed, for most systems in use today, the aircraft cannot be moved during IRS alignment on the ground and even aircraft loading can disrupt the alignment process. Position updates are performed by adding an offset to the computed position, the IRS continues to use its own computed position, then adds the offset to its output before displaying position to the crew and other aircraft systems.
You can find the value of this correction at the end of any flight by perfoming a terminal error check as per the AMM.
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If anyone wants a pretty thorough run-down on IRS/INS I recommend http://en.wikipedia.org/wiki/Inertial_guidance_system
There is a link in this site to Honeywell and 'Align-in-motion' too.
There is a link in this site to Honeywell and 'Align-in-motion' too.
