IRS Align Mode - Carefully Measuring Earths Rotation @53C52

53C52 For several decades, long-range aircraft have used Inertial Navigation Systems/Inertial Reference Systems as sources for position information and as the primary sources for orientation information (pitch/bank/yaw/heading).

This video demonstrates the alignment process of a Honeywell IRS as it carefully measures the slow rotation of the earth to determine the aircraft's latitude and direction to the geographic north pole.

Key points to note are:
1. The IRS consistently determines the correct direction to geographic north, even when it is initialized to an incorrect location.

2. The IRS can be initialized at a grossly incorrect longitude because it is unable to use the rotation of the earth to determine longitude, and therefore is unable to complete a reasonableness check of the initial longitude being supplied to the device.

3. The IRS uses Earth rotation to calculate an accurate latitude which it will use to cross-check the initial position that is supplied during the align mode for reasonableness. This calculation of latitude using direct measurement of Earth's rotation makes it impossible to operate the IRS in navigation mode if the initial position sent to the IRS contains a grossly incorrect latitude.

Early Inertial Navigation Systems use 3 precision mechanical gyros. The gyros are mounted so that each spins on an axis that is 90 degrees to the other two along with 3 linear accelerometers mounted in a similar fashion.
Those components are mounted on a platform that is free to rotate on gimbals within the case. In order to use an INS, it first needs to go through an alignment process. During alignment, the aircraft needs to be stationary so that the gyros are subjected to movement caused only by the very slow rotation of the earth. This rotation is sensed as gyroscopic precession which is then used to drive servo motors to rotate the platform to align it in the direction of geographic north and parallel to local level.

The video linked below is an example of an INS coarse alignment. This rapid alignment is sufficient for the instrument to output attitude information, but the full alignment process to get the instrument ready for position tracking takes several minutes.
youtu.be/w4yaNn4Xmfg

Today's Inertial Reference Systems are an improvement on that old design and use ring laser gryos in place of the mechanic gyros. RLGs are not really gyros in the traditional sense of the word because they do not consist of a spinning mass, and do not exhibit the gyroscopic properties of precession and rigidity in space. They are, however, very good at measuring very small rates of rotation.

IRS does not use a floating platform as was used with the old mechanical gyros. The accelerometer and RLG orientations with respect to the airplane are fixed. This is why this type of unit is referred to a "strap-down" inertial. Like the older INS, IRS also needs to complete an initialization process called alignment, but in the IRS there is no physical alignment of a platform taking place.

updated 6 years ago

IRS Align Mode - Carefully Measuring Earths Rotation

53C52 2018-05-06 | For several decades, long-range aircraft have used Inertial Navigation Systems/Inertial Reference Systems as sources for position information and as the primary sources for orientation information (pitch/bank/yaw/heading).

This video demonstrates the alignment process of a Honeywell IRS as it carefully measures the slow rotation of the earth to determine the aircraft's latitude and direction to the geographic north pole.

Key points to note are:
1. The IRS consistently determines the correct direction to geographic north, even when it is initialized to an incorrect location.

2. The IRS can be initialized at a grossly incorrect longitude because it is unable to use the rotation of the earth to determine longitude, and therefore is unable to complete a reasonableness check of the initial longitude being supplied to the device.

3. The IRS uses Earth rotation to calculate an accurate latitude which it will use to cross-check the initial position that is supplied during the align mode for reasonableness. This calculation of latitude using direct measurement of Earth's rotation makes it impossible to operate the IRS in navigation mode if the initial position sent to the IRS contains a grossly incorrect latitude.

Early Inertial Navigation Systems use 3 precision mechanical gyros. The gyros are mounted so that each spins on an axis that is 90 degrees to the other two along with 3 linear accelerometers mounted in a similar fashion.
Those components are mounted on a platform that is free to rotate on gimbals within the case. In order to use an INS, it first needs to go through an alignment process. During alignment, the aircraft needs to be stationary so that the gyros are subjected to movement caused only by the very slow rotation of the earth. This rotation is sensed as gyroscopic precession which is then used to drive servo motors to rotate the platform to align it in the direction of geographic north and parallel to local level.

The video linked below is an example of an INS coarse alignment. This rapid alignment is sufficient for the instrument to output attitude information, but the full alignment process to get the instrument ready for position tracking takes several minutes.
youtu.be/w4yaNn4Xmfg

Today's Inertial Reference Systems are an improvement on that old design and use ring laser gryos in place of the mechanic gyros. RLGs are not really gyros in the traditional sense of the word because they do not consist of a spinning mass, and do not exhibit the gyroscopic properties of precession and rigidity in space. They are, however, very good at measuring very small rates of rotation.

IRS does not use a floating platform as was used with the old mechanical gyros. The accelerometer and RLG orientations with respect to the airplane are fixed. This is why this type of unit is referred to a "strap-down" inertial. Like the older INS, IRS also needs to complete an initialization process called alignment, but in the IRS there is no physical alignment of a platform taking place.

Anticrepuscular rays 2

53C52 2020-09-24 | Crepuscular and anticrepuscular rays converging towards opposite points in the sky just before sunrise.

Sectional Chart for Daniel Erdman - Phoenix

53C52 2019-07-24 | Daniel has doubled down on his claim that all lines are straight on U.S. Sectional charts, and has specifically pointed to the Phoenix Sectional as a perfect example.

Not surprisingly, just as with the Point Barrow youtu.be/_v22axzvs3w chart and the Miami chart youtu.be/A_MZxJEKj14 , the latitude lines are all clearly curved and the longitude lines converge towards the north.

Sectional Chart for Daniel Erdman - Miami

53C52 2019-07-23 | I'm guessing Daniel will complain that the latitude lines in the first video (youtu.be/_v22axzvs3w) are curved only because I used an Alaska Sectional.

So I've uploaded this video (initially unlisted) of an example of curved latitude lines on the Miami Sectional ahead of his complaint!

Sectional Chart for Daniel Erdman - Point Barrow

53C52 2019-07-23 | Daniel claims the lines on all FAA Sectional charts are straight.
This chart tells a different story.

I threw in a couple of bonus ONC charts from the southern hemisphere.

Helping 7th Day Truth Seeker with his Google Earth One More Orbit Track

53C52 2019-07-19 | 7th Day Truth Seeker had some difficulty plotting One More Orbit's circumpolar track using Google Earth. Here's where he went wrong...

Tracking One More Orbits QE11 (Difference between ground-based and sat-based data)

53C52 2019-07-18 | Here's the difference between ground-based flight tracking and satellite-based flight tracking.

Thanks to Florence Gomer for pointing out that although the Aireon ADS-B position reports weren't included in the logs for QE11's record-breaking circumpolar flight, they were shown on FlightAware's coverage map this week.

WheresWa11y's excellent demonstration of the difference between an aircraft's actual position and the estimated position shown by online flight trackers when a flight is beyond the coverage area of their ground-based receivers... youtu.be/zolmv5AGMCU

Here are a few photos taken by the crew of the cockpit navigation display...
Over Antarctica, roughly 1,500km from the south pole at 43,000 ft. Outside air temperature -73C
twitter.com/ActionAviation0/status/1149033326998708224

About 140km from the south pole at 43,000 ft. Outside air temperature has dropped to -80C
twitter.com/ActionAviation0/status/1149249767958036480

After crossing the south pole, outside air temperature has dropped to -83C.
As confirmed by the Aireon data, they're forced to descend to warmer air.
twitter.com/ActionAviation0/status/1149250424857403392

GS QFA63 1 - Loading Route in ForeFlight

53C52 2019-03-02 | Time and date in upper left is EST (UTC-5).
Loaded Jan29 QFA63 SYD-JNB flight plan route in ForeFlight app.
Added great circle route (magenta line) to show the relationship between the two routes.
Turned on winds aloft to show that the planned route takes a large detour from great circle route in order to avoid an area of very strong headwinds.

Paul On The Plane and his Flat Earth 29-Year Pilot

53C52 2018-12-09 | Paul On The Plane's interview with "Zac" the "professional pilot" who believes the Earth is flat was riddled with dubious claims.

Is "Zac" deceiving POTP about his credentials and/or beliefs, or is POTP intentionally deceiving his viewers with a script full of lies?

For information from a genuine pilot, visit Wolfie6020's channel.

Sydney to Santiago with Honeywell FMS - for That Guy

53C52 2018-07-17 | Planning Sydney (YSSY/SYD) direct to Santiago (SCEL/SCL) using Honeywell FMS.

Note the lines of longitude converging to meet at the south pole.

Sydney to Santiago with Aviation Charting Software - for That Guy

53C52 2018-07-17 | Planning Sydney (YSSY/SYD) to Santiago (SCEL/SCL) using two well known aviation charting and planning apps.
First up is JeppFD, the second is ForeFlight.
Note that both apps plot the route using the shortest path on a globe.

Anticrepuscular rays

53C52 2017-09-24 | Anticrepuscular rays seen from 38,000 feet.
Rays converge at a point ahead of the airplane while the sun is behind the airplane.
Still image: drive.google.com/open?id=0B4aJti-aC8gAMDlFcFJBQUo3aVU

Water level?

53C52 2017-08-14 | ...

HGS eye box

53C52 2017-08-12 | Demonstrating that moving eye position up and down from the HGS "eye box" has a dramatic effect on the amount of symbology visible in the HGS, but little effect on the location of the zero-degree pitch line in relation to the visible horizon.