INS aligment
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Do not confuse IRS and AHRS. I have both in my a/c and they are not working the same way. AHRS needs MAD to provide heading, IRS does not.
For polar navigation (very rare), instead of IRS + AHRS, we can ask for replacing the AHRS by another IRS so we have two IRS (safer because in polar nav, MAD do not work any-more.)
EDIT: BTW …
INS/IRS provides attitude, true heading, speed, position in 3D, inertial vectors …
AHRS provides attitude & mag/grw heading. But no speed, no position. -
For that paper I linked it goes into detail about what they mean by IMU and AHRS.
“In the context of this writing we will use the term IMU in accordance with its classical meaning to describe the combination of only an 3-axis accelerometer combined with a 3-axis gyro. A onboard processor, memory, and temperature sensor may be included to provide a digital interface, unit conversion and to apply a sensor calibration model. The IMU by itself does not provide any kind of navigation solution (position, velocity, attitude). It only actuates as a sensor, in opposition to the INS (Inertial Navigation System), which integrate the measurements of its internal IMU to provide a navigation solution. For instance an Inertial Navigation System (INS) uses an IMU to form a self-contained navigation system which uses measurements provided by the IMU to track the position, velocity, and orientation of a object relative to a starting point, orientation, and velocity.”
It sounds like when you combine a magnetic sensor to an IMU you get a AHRS. That would make sense as to why IRS + AHRS is rare for polar nav.
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If a plane had three GPS antennas, one on the nose and two in the wing roots for example, creating a sort of triangle shape, couldn’t it then figure out its true heading based just on the orientation of the three combined GPS antennas?
No idea how the F-16 does it but that’s just an idea my simple brain came up with.
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If a plane had three GPS antennas, one on the nose and two in the wing roots for example, creating a sort of triangle shape, couldn’t it then figure out its true heading based just on the orientation of the three combined GPS antennas?
No idea how the F-16 does it but that’s just an idea my simple brain came up with.
Find another idea (GPS has a 10 to 100m precision).
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Assuming all three receivers got a very high accuracy, yes. Seems like it would be more prone to error than alignment based on earths rotation though.
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Find another idea (GPS has a 10 to 100m precision).
Then how does a jdam impact within 3 meters?
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Then how does a jdam impact within 3 meters?
Because of their specific receivers.
I am talking about the average precision on L1 & L2 (with Y code) with common airborne military GPS (not necessarily talking about the very last versions made for specific use like the one for JDAMs) … and precision depends also on a/c’s coordinates system itself (some are using xx°xx,xxX’, some other xx°xx,xx’ … etc …) Depends on their HIL vs MaxHIL, DOP, actual height over (or below) geoide … etc …
But even with a one meter precision your idea do not work to elaborate a true heading with at 1° precision (At least, it is not how it is done on aircraft).
GPS can only give a track (ground track) when it is moving. Then, with the wind vector provided by IRS, it can calculate an heading. But alone, a non moving GPS can only give a position. -
Its all a case of leverage. The aircraft is what, about 13 meters long? Even if your three receivers, 15 meters apart, have a CEP of 1m, that gives an uncertainty much greater than 1 degree.
If the receivers were perhaps more like 400m apart, you could start getting degree resolution.
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The f-16 has a provision to set true heading while in flight. I guess it’s for heading drift correction or if normal startup alignment fails and pilot needs takeoff in a hurry.
The pilot uses backup magnetic compass and hsi compass rose calibration nob (left of hsi). INS must be attitude mode. Then hud will display the new true heading.
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The f-16 has a provision to set true heading while in flight. I guess it’s for heading drift correction or if normal startup alignment fails and pilot needs takeoff in a hurry.
The pilot uses backup magnetic compass and hsi compass rose calibration nob (left of hsi). INS must be attitude mode. Then hud will display the new true heading.
Something like that is an absolute last resort and is only going to provode basic pitch, roll, and yaw information but all steering information is gone. You would never use ATT mode for drift correction as an in flight alignment (if done correctly) will give you a much better setup than ATT mode.
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Something like that is an absolute last resort and is only going to provode basic pitch, roll, and yaw information but all steering information is gone. You would never use ATT mode for drift correction as an in flight alignment (if done correctly) will give you a much better setup than ATT mode.
I was just talking about heading drift. Position drift is corrected by gps. Attitude drift is nulled by gyro and accelerometers using normal acceleration for refferance. Wouldn’t hurt a thing imo.
I think you can go back to ins normal mode after correcting heading.
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The f-16 has a provision to set true heading while in flight. I guess it’s for heading drift correction or if normal startup alignment fails and pilot needs takeoff in a hurry.
The pilot uses backup magnetic compass and hsi compass rose calibration nob (left of hsi). INS must be attitude mode. Then hud will display the new true heading.
Think that option isn’t in EGI equipped jets anymore.
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Think that option isn’t in EGI equipped jets anymore.
Then the hsi would not be standalone device. …right?
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…didn’t think it was? I thought the EGI equipped jets had the EHSI? Doesnt that link with the MNF?
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EGI jets have en EHSI like the one in the BMS MLU as far as I’m aware of.
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JDAM also gets its accuracy from not being a GPS weapon but an INS weapon with GPS filtered updates. There is software to predict the best satellite conditions over a particular target if you want to schedule the optimized RAIN conditions. I haven’t seen JDAM accuracy quoted at better than 13m CEP with exception of additional optical or radar sensors. A 3m standard JDAM impact is a bit on the lucky side.
Light travels about 30cm in 1ns which is pretty tight timing for most devices (1 GHz).
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I was just talking about heading drift. Position drift is corrected by gps. Attitude drift is nulled by gyro and accelerometers using normal acceleration for refferance. Wouldn’t hurt a thing imo.
I think you can go back to ins normal mode after correcting heading.
Once again the ONLY reason you would ever want to put the INS in ATT mode is for a complete failure of the INS system so you could get back basic pitch, roll, and yaw signals. No you do not go from ATT mode back to NAV. The only reason you are adjusting the HSI to match the whiskey compass at that point is because the INS is so far gone it has no idea which way it is pointed.
If, for whatever reason, some pilot was that desperate to get airborne that he took an aircraft without an INS alignment up he will go for an in flight alignment if the INS is still good. If the INS is bad he can go ATT mode and have P, R, & Y inofrmation but no steering and a rough idea for a heading based on the input from the whiskey compass.
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Then the hsi would not be standalone device. …right?
The HSI never was a stand alone device. It is driven from signals from the INS.
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Once again the ONLY reason you would ever want to put the INS in ATT mode is for a complete failure of the INS system so you could get back basic pitch, roll, and yaw signals. No you do not go from ATT mode back to NAV. The only reason you are adjusting the HSI to match the whiskey compass at that point is because the INS is so far gone it has no idea which way it is pointed.
If, for whatever reason, some pilot was that desperate to get airborne that he took an aircraft without an INS alignment up he will go for an in flight alignment if the INS is still good. If the INS is bad he can go ATT mode and have P, R, & Y inofrmation but no steering and a rough idea for a heading based on the input from the whiskey compass.
FLCS has its own gyros and accellerometers. All ATT mode does is turn off INS for position navigation, only gps is used. INS still drives ADI, HSI etc. I would believe ATT is used if a faulty INS is degrading position data. In this case there is a backup ADI if needed. I guess you don’t get a hsi.