Questions tagged [inertial-nav-system]

Questions about inertial navigation systems which sense an aircraft's motion to determine location as it travels.

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How do missiles navigate and determine their orientation without GPS?

When a missile is engaged in a GPS free tracking loop how is its internal orientation estimated with respect to ground? I'm assuming this orientation is needed because the force of gravity will be ...
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18 votes
1 answer
3k views

How do aircraft on the deck of a moving aircraft carrier align their INS/IRS?

I know that the F/A-18C is equiped with a rotary switch for inertial alignment. The switch has two positions, one for ground and the other for carrier operations. Leaving aside the GPS/GNSS, I wonder ...
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6 votes
2 answers
334 views

How does IRS estimate the alignment time?

In modern airliners, e.g. A320, once the IRS is starting the alignment process, there is an ECAM message that indicates the alignment time, e.g. ...
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3 votes
1 answer
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Can you fly with INS/ IRS at higher lattitudes?

IRS can't be aligned at higher lattitudes ( above 70 deg) , but if once aligned near equator, can we fly till higher lattitudes with IRS in that flight?
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1 vote
0 answers
73 views

How is the voting mechanism and FDI algorithm applied in transport aircraft?

How is the voting mechanism applied in modern transport airplanes for airdata/inertial reference sensors fault detection and isolation (FDI)? Are there any other FDI methods approved by the regulators ...
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10 votes
1 answer
692 views

Why do we have to perform a full ADIRS alignment between latitudes of 2 degrees North and South on the A320 family?

In the latest FCOM revision under the preliminary cockpit preperation normal procedures chapter ADIRS alignment has been added. However most of the conditions remain the same except for a new line ...
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7 votes
3 answers
428 views

Could airliners fetch data like AoA and speed from an INS?

If I understood what an inertial navigation system (INS) is, it should be able to calculate speed and orientation of the aircraft. If this is possible, why do airliners solely rely on sensors to ...
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0 votes
2 answers
537 views

What is the difference between AHRS and ADIRS?

Whats is the difference between Attitude and Heading Reference System and Air Data Inertial Reference System ?
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13 votes
1 answer
1k views

Why are true headings followed above 73N in the northern hemisphere, whereas in the southern hemisphere the change is made at 60S?

The question is in regards to the asymmetry of the changeover latitudes and NOT about why it needs to be changed.
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2 votes
1 answer
270 views

Why does the A320 IR2 MEL OPS procedure suggest to switch to ADR3?

The A320 34-10-05B IR2 MEL OPS procedure additionally calls for Air Data switching to FO3. When ADR 2 is working fine why replace its data on the FO PFD when only IR data is lost?
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1 vote
1 answer
146 views

Why do most commercial solid-state Inertial Measurement Units (IMUs) measure angular rate and not angular acceleration?

The vast majority of MEMS-based IMUs are formed by an accelerometer and a gyroscope, providing linear acceleration and angular rate measurements respectively. However, having available the angular ...
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2 votes
3 answers
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Why is there a drift value between the IRS position and the FMGC position present even after performing a full IRS alignment on the A320?

I have noticed this many times across my company's fleet of A320's that is even after a full IRS alignment has been completed many a times there is still a drift value present between the FMGC ...
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11 votes
2 answers
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Why did the Boeing 787 switch back to separate IRS systems?

The Boeing 737 uses two independent IRS systems (see also Why are there two IRS switches on the 737NG?): (source: 737 NG FCOMv2 (11.10.21 - Flight Management, Navigation - Controls and Indicators)) ...
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0 votes
1 answer
166 views

Can we use ECI coordinates for an airplane?

It is taught for navigation systems (like an airplane) on Earth we us ECEF (earth-centered, earth-fixed) and for space systems (like a satellite) we use ECI (earth-centered inertial). But why is ECEF ...
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6 votes
2 answers
814 views

Can you guide me about the purpose of fine alignment in inertial navigation systems?

I am new in inertial navigation field. I have some confusion about fine alignment process of INS. I want to ask that what is purpose of fine alignment? I know by process of alignment we find the ...
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1 vote
3 answers
2k views

How did KAL 007 get so far off course?

Regarding Flight 007 that got shot down in the Soviet Union, this was reported in the New York Times: Study Says Korean Airliner Was on Its Intended Course When Downed in '83 (nytimes.com) Excerpt: ...
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2 votes
3 answers
359 views

What does "inertial" refer to in "inertial reference"?

An ADIRU makes use of air data reference and inertial reference. Inertial reference calculates the heading, position, ground speed and attitude. I was wondering why is it called inertial. Is it ...
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4 votes
1 answer
602 views

How is the drift corrected in INS-driven attitude/heading indicators?

(wikimedia.org) EFIS instruments during a test flight. How is the drift (accumulated error) nullified in INS-driven attitude/heading indicators? The related post 'How are attitude indicators kept ...
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1 vote
3 answers
1k views

How many Inertial Reference Systems are on a commercial aircraft?

Nowadays all new airplanes have GPS on board so the use of IRS may lose importance, still they must be fitted. How many of them are usually on a commercial aircraft? Does the regulation impose the ...
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10 votes
2 answers
494 views

What makes inertial guidance unsuitable for auto-land?

Above shows the performance of a Lockheed Martin civilian landing system developed in the mid-90s. It works by using the weather radar in SAR mode to update the INS, and the final 100 ft are flown by ...
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12 votes
3 answers
4k views

What's the difference between orientation and position?

In INS (Inertial navigation system) sensors, is there a distinction between the following? Orientation (heading, pitch and roll) Position (latitude, longitude and altitude) Is the orientation ...
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8 votes
1 answer
577 views

How does an air-data computer convert ram air pressure from the pitot tube into airspeed?

I know how the analog airspeed indicator shows the airspeed from the ram air pressure from the pitot tube. But I'd like to know how an air-data computer converts the raw pressure data to airspeed that ...
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17 votes
7 answers
7k views

Is it possible to accurately measure airspeed without pitot tube?

This is a follow-up to my previous question: How does this IMU work and how to convert its output into meaningful information? for which many people asserted that I need a pitot tube and a static ...
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1 vote
1 answer
753 views

How does this IMU work and how to convert its output into meaningful information?

To understand more about IMUs and IRS, I have chosen the Honeywell 6DF-1N6-C2-HWL Inertial Measurement Unit as an example: I don't see any mention of the output of the IMU in the linked datasheet - ...
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2 votes
0 answers
52 views

How are IRUs and AHRSs different in terms of their functions? [duplicate]

By my understanding, both systems use gyros and accelerometers to provide aircraft's attitude and heading information. Given that the information they produce are more or less the same, why do the ...
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23 votes
4 answers
9k views

Do today's aircraft still have INS (inertial navigation system)?

Do today's new aircraft like Boeing 787 still have INS (inertial navigation system)?
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1 vote
1 answer
82 views

Can cellular towers be used to correct INS drift?

At a high enough altitude a line-of-sight can be established with multiple cellular towers. Can such information be used to accurately correct INS drift (assuming GNSS is not available)?
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  • 321
5 votes
3 answers
5k views

How does Kalman filtering work in inertial navigation systems?

As far as I understood it correctly most INS-systems use a Kalman filter to compensate measurement errors. I would like to know: Does the Kalman filter compensate the errors from all the onboard ...
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4 votes
1 answer
1k views

How does the attitude and heading reference (AHRS) work on Boeing 737 aircraft?

I would like to know how the AHRS work on Boeing 737 aircraft. If this aircraft (like every the other ones I guess) has an Inertial Reference System (IRS), composed of 3 accelerometers and 3 ...
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5 votes
2 answers
2k views

Why does Boeing 737 use 2 Inertial Reference Systems (IRS) and GPS?

According to this website the Boeing 737 aircraft use 2 IRS and 2 GPS: IRS L & IRS R. Why? Apparently they are totally independent of each other. Could somebody explain a little bit why they need ...
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5 votes
2 answers
5k views

What is the difference between IMU (Inertial Measurement Unit) and AHRS (Attitude and Heading Reference System)?

According to online sources like wikipedia the difference belies on the addition of an on-board processing system (I assume it means a microcontroller) which outputs already processed data and not ...
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  • 383
5 votes
1 answer
518 views

Are the IMU velocities equivalent to the GPS ground velocity, except for the coordinate reference system?

If the velocities measured by the IMU in the body-fixed reference frame are converted into earth fixed frame, are they equivalent to the ground velocity measured using the GPS?
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0 votes
0 answers
289 views

In an Inertial Measurement Unit, how are the body-axis velocities calculated?

What are body-axis velocity components (u,v,w)? Are they calculated from IMU acceleration components (Ax,Ay,Az)? How can this be related with the airspeed measured from the pitot tube?
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28 votes
2 answers
14k views

Why does IRS alignment take so much time?

Home GPS systems (like your car navigator) is able to locate you in like a few seconds, but 737NG needs to align itself for 6-8 minutes. Why do navigation systems in aircraft take so long to align ...
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1 vote
1 answer
142 views

Does the Zenith have anything to do with the read-out of an IRU accelerometer?

Let's say that I have an accelerometer as part of an inertial reference system. Will the position of the Zenith with-respect-to the current position (in terms of rotation) of my accelerometer, have ...
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10 votes
1 answer
718 views

How does an FMS shift after losing all external navaids?

It's not uncommon for polar flights to lose GPS signal (ionosphere issues due to the high latitudes). But it is not a concern because the typical IRS drift is 650 meters for every hour of flight, and ...
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1 vote
1 answer
424 views

How to make use of linear acceleration and gyroscope data? [closed]

This is strictly not aviation related so please feel free to move it to the correct section. I have a 9 axis IMU, which gives me yaw, pitch, roll, magnetics, gyros, accels etc. I currently make use ...
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7 votes
2 answers
1k views

What's the precision of a modern Inertial Measurement Unit, and how is it obtained?

This recent comment reports that: the IMU on new (plane) would localize the aircraft to within 3 feet after a cross-country flight, without any GPS input other than the starting location. I ...
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19 votes
2 answers
33k views

What is an Inertial Navigation System? How does it work?

In this question, the answer notes that some ADS-B systems are connected to Inertial Navigation Systems (INS), and says they are less accurate than GPS. What exactly is an INS? A couple sub ...
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