INS aligment
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Stubbies seemed to imply it went the other direction - for known true heading, find mag heading. Stevie posits instead that for known mag heading, the jet finds true heading.
Ah, yes.
Well, can’t bring much to the debate. Interesting, still.
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Stubbies seemed to imply it went the other direction - for known true heading, find mag heading. Stevie posits instead that for known mag heading, the jet finds true heading.
Precisely. You cannot measure true heading directly…I don’t care what you use.
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I can think of a few implicit ways of getting it actually, but I dont think they are directly used by the F-16 based on my lay knowledge of the topic, so I may have to concede that one.
If you have GPS, and you have a solid alignment, but a fuzzy initial heading value, GPS’ constant update of your position could be used to back calculate the original true heading with high accuracy, and no use of mag heading at any point.
With that said though it would require some movement and turning. Perhaps this is related to the EIA?
I can see how given mag heading and a table you could get true heading, and I can see how an EIA could generate accurate heading data without any reference to mag heading at all. Seems like either method could be used, and thinking about it, seeing as the INS initialises with the same position/heading values it had when it shut down, that would make the GPS assisted method viable.
Then again historically the old systems didn’t have GPS, so they logically must have had to use the mag heading and mag var tables to work out true heading. Cant really rule out either method for the new jets though.
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Naval SINS doesn’t require a MAD either…at least not the one on the jet. It’s either cabled in or RF input…depending on operations. In both cases the alignment data are produced by an off board source.
F-16s don’t require a MAD or any external reference to a magnetic reading. Just the true heading it gets itself during standard alignment and the tables cover most flying that you would see done with an F-16.
I wouldn’t be surprised if the USAF just plain didn’t know they have a problem with embedded systems masking failures because in general the GPS portions of the systems are pretty reliable…and guys use them, so there aren’t many gripes until something breaks in tandem. The GPS keeps the filters tight, and that’s the issue…for everyone. I don’t think the USN has come up with any ideas as of yet either…and probably won’t as long as GPS keep working as well as it does.
Not so per one of my posts above. INSM locations are checked every flight to keep the INS system honest.
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…I’m still working with the real thing now (just not Vipers…for 30 years) and I can believe it uses tables. You can’t measure true heading directly - it must be calculated.
So the only thing to be taken from this is whichever bird you are working uses a completely different INS as the one the F-16 uses only requires initial lat/long/SALT for the start of the alignment and it finds true heading by itself and does the math for magnetic heading based on current lat/long and gives a good indication for mag heading within 45 seconds of the start of alignment.
My guess is the navy INS is different because you guys aren’t sitting on a stable surface with the only movement to the INS during alignment being the earth’s rotation. You have the aircraft carriers to take into account while producing an INS alignment.
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Precisely. You cannot measure true heading directly…I don’t care what you use.
Heh well according to that then the F-16 is breaking the rules every time it spins up because it does all by itself.
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That is entirely too much math listed on that page.
This is a more apt description of the SINS instead of the RLG of course but the end result is the same.
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You have to know mag heading in order to get true heading…that’s why mag var is so critical.
Again, No. Not on my IRS Stevie. Mine is elarobating a true heading and do not need any mag inputs. And is the var database isn’t present in the database of the FMS, my IRS will continue to work (but only in “true” mode.)
You cannot measure true heading directly…
Maybe not with your navalised system. But I can tell you that mine is precisely doing this => “measuring” the true heading, then calculates a mag heading using var table.
This is how my IRS is working. And As far as I know, same for airliners.
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Some good stuff here.
http://www.vectornav.com/support/library/imu-and-ins
“Inertial sensor can also be used to determine the orientation as a function of time. Without an absolute reference an IMU can only be used to track changes in orientation from some initial point in time. For absolute orientation tracking you will need to combine an IMU with an absolute reference sensor such as magnetometer, camera, or GPS. Combining an IMU with a magnetic sensor creates what is known as an Absolute Heading Reference System (AHRS). For this case we will look at the error caused by the accelerometer that influence the orientation estimate. An AHRS determines orientation by making the assumption that the measured acceleration vector minus the known inertial acceleration is equal to the gravity vector. For cases where the actual inertial acceleration is unknown the AHRS assumes that the measured acceleration on the accelerometer is the actual gravity vector. Due to this assumption any errors in the accelerometer will translate into errors in the estimated direction of the downward direction. The gyros will dampen out much of the time based disturbances in the acceleration, however any constant errors in the accelerometer calibration parameters will directly propagate into orientation errors.”
“Aside from the custom inertial navigation systems used on Intercontinental ballistic missiles, the marine grade inertial systems is the highest grade sensors that are commercially available. They provide the best overall performance in terms of both determining position and orientation. These systems are typically used on ships, submarines, and some spacecraft. A high-end marine-grade INS can cost over 1 million dollars. These systems will typically provide un-aided navigation solution drifts that are less than 1.8 km per day. This means that if the device were left stationary for one day, due to slight errors in the sensors and imperfect sensor calibrations, after integrating the position solution, the calculated position after one day would be 1800 meters away from the sensors actual position. This represents the best that is possible with commercially available inertial sensing technology. Navigation grade systems have slightly lower performance than the marine systems and are typically used on commercial airliners and military aircraft worldwide. A navigation grade system will typically have less than 1.5km drift per hour. These systems cost around $100,000 and have a size of about 6in x 6in x 6in. A navigation grade system can be combined with GPS to create positioning systems that are accurate to within centimeters in real-time. An example of a navigation grade IMU is the HG9900 IMU made by Honeywell.”
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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.