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HAWK21M
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Radio Altimeter Indication On Ground

Mon Jan 18, 2016 9:11 am

Anyone aware of the Altitude recorded on the LRRA when the Wheels of the Aircraft are touching the runway surface.
I believe it should be zero Altitude....so in effect the NLG height is negated during said calculation .
Is this common for all types.
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flyvabb
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RE: Radio Altimeter Indication On Ground

Mon Jan 18, 2016 11:29 am

It reads Zero with the MLG struts extended.
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atlamt
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RE: Radio Altimeter Indication On Ground

Mon Jan 18, 2016 1:47 pm

The 737NG's read -4 at the gate. At touchdown with the nose pitched up it reads 0. As the plane rotates back to a ground attitude and the gear compresses you see the negative altitude.
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Francoflier
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RE: Radio Altimeter Indication On Ground

Mon Jan 18, 2016 1:59 pm

Quoting atlamt (Reply 2):

The 737NG's read -4 at the gate. At touchdown with the nose pitched up it reads 0. As the plane rotates back to a ground attitude and the gear compresses you see the negative altitude.

Ditto the 777.
I believe those things are calibrated to show 0 at landing attitude.
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BoeingGuy
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RE: Radio Altimeter Indication On Ground

Mon Jan 18, 2016 3:31 pm

Quoting francoflier (Reply 3):
Ditto the 777.
I believe those things are calibrated to show 0 at landing attitude.

Correct. It's calibrated to show 0 at touchdown in a flare. Most Boeing airplanes that I'm seen show somewhere between -4 and -6 when at rest on the ground.
 
rendezvous
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RE: Radio Altimeter Indication On Ground

Mon Jan 18, 2016 7:09 pm

The Dash 8 Q300 shows -5 on the ground as well.
 
rwessel
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RE: Radio Altimeter Indication On Ground

Mon Feb 01, 2016 11:03 am

This was a topic years ago, and I ended up drawing a diagram to help explain it.

Basically the radar altimeter antenna is usually mounted between the nose gear and the mains, far enough away from either so that the radar doesn't get any false returns off them. So basically the radar altimeter distance to the ground depends on the attitude of the aircraft, and is typically calibrated so that it reads zero when the mains touch the ground in the standard landing attitude.

 
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HAWK21M
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RE: Radio Altimeter Indication On Ground

Mon Feb 29, 2016 6:34 am

Quoting flyvabb (Reply 1):
It reads Zero with the MLG struts extended.

I guess you mean MLG on ground, as MLG strut extended could be at any altitude above ground.

Quoting rwessel (Reply 6):

True real bad radio waves but the diagram made up for it..thanks  
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RCnoob
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RE: Radio Altimeter Indication On Ground

Mon Mar 21, 2016 1:33 am

This raises a question: how is the RA compensated for changes in pitch (or roll)?

Quoting rwessel (Reply 6):
So basically the radar altimeter distance to the ground depends on the attitude of the aircraft,

Without some mechanism, as the aircraft attitude changes the distance from the RA to the ground will appear to shift. Cosine (20 degrees) = .94 so at moderate levels of pitch or roll the error won't be large, but it would seem disconcerting to see the altitude "jitter" as the aircraft responds to wind gusts. And in military aircraft the errors could be very large.

I can think of several ways to compensate, but none that is really robust, nor that would have worked 50 years ago before accurate attitude sensors (gyroscopes) were routine.
Does anyone have a reference or know how this is done/ how well the compensation works?

Thanks.
 
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Florianopolis
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RE: Radio Altimeter Indication On Ground

Mon Mar 21, 2016 3:00 am

Quoting RCnoob (Reply 8):
This raises a question: how is the RA compensated for changes in pitch (or roll)?

I don't think the Radio Altimeter is quite as directional as you think. It doesn't shoot straight down a narrow straw orthogonally from the bottom of the airplane like a laser beam...

A signal radiates from the transmitting antenna, and then the receive antenna listens for the first reflected signal, which will always be from the surface nearest to the airplane, which is the ground directly below the airplane, and not a slant distance. Regardless of pitch and roll, the shortest distance from the transmit antenna to the receive antenna is a triangle straight down to the ground and back.

**[edit]** I may have misread your post. Were you asking about the RA altitude deviation from the "main-gear-height-at-touchdown-pitch-angle height" as the airplane moves around? In other words, so that even as the airplane sat on the ground, instead of reading a negative number, why not make the airplane realize its orientation compared to the main gear, and have it display a corrected "zero" altitude? If so, that might be getting too precise for the tool. I think they only claim 2% accuracy, and it's exacerbated by ground irregularities. Not to mention the fact that as you get close to the ground, the triangle path from transmit-ground-receive becomes non-vertical, and is another correction to make.

[Edited 2016-03-20 20:14:25]
 
nomadd22
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RE: Radio Altimeter Indication On Ground

Mon Mar 21, 2016 3:15 am

GPS position finders, AISs and sonars also have offsets programed in when you install them. Few devices have their transponders or transducers where you want the position indicated.

[Edited 2016-03-20 20:18:02]
Anon
 
RCnoob
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RE: Radio Altimeter Indication On Ground

Tue Mar 22, 2016 2:23 am

Quoting Florianopolis (Reply 9):
A signal radiates from the transmitting antenna, and then the receive antenna listens for the first reflected signal, which will always be from the surface nearest to the airplane, which is the ground directly below the airplane, and not a slant distance. Regardless of pitch and roll, the shortest distance from the transmit antenna to the receive antenna is a triangle straight down to the ground and back.

Thanks, that answers my question. You are right, I was mistakenly thinking of a highly directional beam. Here is some additional information on this I found:

Quote:
Radar altimeter antennas have a fairly large main lobe of about 80° so that at bank angles up to about 40°, the radar detects the range from the aircraft to the ground (specifically to the nearest large reflecting object). This is because range is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20°.

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