ADF bank error

[Hardy Heinlin]

Some people say with the bank the relative bearing will decrease, some say it will increase.

I say it depends on the geometric situation. I wonder if anyone will disagree :-)

I say the relative bearing will be unchanged when it is 45 degrees in both vertical and lateral dimensions.

Imagine a cube. All sides are equally long.


Situation A:

Place the aircraft at one of the upper four corners. Place the NDB at the diagonally opposite corner at the bottom. Aircraft heading be any full quarter of 360, as you like, but so that the relative bearing is 045.

In this cubic geometry, the vertical and lateral distances are equal.

Keep wings level. Relative bearing is 045.

Bank 90 degrees. Relative bearing is 045.


Situation B:

Now take a hammer and hit the cube flat like a paper. The aircraft is now at the same height as the NDB. But the lateral positions are the same (diagonally edge to edge).

Keep wings level. Relative bearing is 045.

Bank 90 degrees. Relative bearing is 000.


Situation C:

Now make the opposite of the flat paper, take 5 million cubes and build a tower. The width of the tower is as big as one cube. Place the aircraft at one of the four corners on the roof of the tower. Place the NDB at the diagonally opposite corner at the ground floor of the tower. Aircraft heading be any full quarter of 360, as you like, but so that the relative bearing is 045 (with wings level).

The aircraft is now 5 million feet above the NDB. But the lateral positions are the same as in the above examples (diagonally edge to edge).

Keep wings level. Relative bearing is 045.

Bank 90 degrees. Relative bearing is 89.999999999.


Conclusion:

When the vertical difference is greater than the lateral difference, the banking ADF will increase the relative bearing.

When the vertical difference is smaller than the lateral difference, the banking ADF will decrease the relative bearing.

...

On a typical turn onto a final approach track with an NDB at the outer marker, the lateral distance to that NDB is usually greater than the aircraft height. So the relative bearing will decrease.

In situations like 1 NM to the NDB and 10000 feet AGL, e.g. when crossing an airway NDB, any banking will increase the relative bearing.


Any opposite opinions? :-)


Cheers,

|-|ardy

[Hardy Heinlin]

The following screenshots have been set using this altitude slider on the Instructor while the aircraft is frozen at a 90° bank angle.


Hong Kong's Cheung Chau NDB and VOR is at ca. 11 o'clock.

The extreme bank angle is used to demo how the vertical slant to the NDB replaces the horizontal bearing to the NDB. Any bank angle in between would work as well, but the deviation would be smaller and the demo less effective.

1. The aircraft is at 45000 ft, this is more than the horizontal ground distance to the NDB, thus the ADF tends to a 090° bearing:




2. The aircraft is at 16000 ft, this is about equal to the horizontal ground distance to the NDB, thus the ADF tends to a bearing between 000° and 090°:




3. The aircraft is nearly at MSL, this is less than the horizontal ground distance to the NDB, thus the ADF tends to a 000° bearing:



(The co-located green VOR is not affected for obvious reasons; the VOR is not a direction finder system.)


In other words, the greater the bank angle, the more the ADF becomes a "vertical path" indicator, similar to the turn error of the magnetic compass. A turn to the NDB must be stopped earlier or later depending on the altitude/distance ratio. In most cases, the distance is the greater variable, so the ADF will tend to a 000° bearing when the aircraft banks, and a turn must be stopped a few degrees later accordingly.


Cheers,

|-|ardy


(Please ignore the groundspeed indication, it isn't updated with the changing wind because the motion is frozen.)

[John H Watson]

I wonder what effect will be caused by the polarisation of the groundstation signal (at these attitudes)? At 90 degrees, will the antennae even work at all?

On the other hand, since the aircraft's (combined) sense/loop antennae are on top of the fuselage, the signal strength will be greater in some respects, especially when the aircraft is closer to the station (but outside the cone of confusion).

Do the indications read in the correct sense when the aircraft is completely upside down?  

Rgds
JHW

[Hardy Heinlin]

Do the indications read in the correct sense when the aircraft is completely upside down?  

I hope so :-)

Aside from the plain geometry stuff, I've also added some pseudo-random fuselage effects. I think it now feels like a realistic sloppy jumpy "What's-it-doing-now?"-ADF.

ILS = Precision
VOR = Non-precision
NDB = Less-than-non-precision


Cheers,

|-|ardy

[Will]

I think it now feels like a realistic sloppy jumpy "What's-it-doing-now?"-ADF.

Ha!  Excellent.  Although when you simulate a system that doesn't work very well, you have to be be prepared for people who blame you for the difference between realism and perfection.  

Another (realistic) idea... you could have the effective range change as a function of the time of day: greater reception in the hours of darkness!

[Hardy Heinlin]

Another (realistic) idea... you could have the effective range change as a function of the time of day: greater reception in the hours of darkness!

Right! I'll add that ...

[Jeroen D]

What about weather? I assume weather, heavy rain/snow has a deterioting effect on radio performance as well?
Jeroen

[Hardy Heinlin]

There may be electro-static effects in rain drops, but I don't know how that works and if it would be noticeable at all. However, like in PS1, I've added some disturbance effects caused by lightning in thunderstorms.

|-|

[Will]

For electrical storms, the ADF points towards nearby lightning bolts (which emit lots of radio waves).  So you would see a bright flash outside, then the needle would start swinging towards the flash, and then a quarter of a second later, the needle would swing back to the main station.  If there's lots of nearby lightning, the needle will swing quite a bit.  But in practice, most lightening bolts are far enough away that the magnitude of the swing isn't more than a few degrees.  Only the really bright, close flashes will swing the needle 45 or 90 degrees.

[Holger Wende]

Hi Hardy,
I wonder whether the radar display in WX+T mode should display more terrain reflections on the left with 90° bank to the left?
Particularly at low altitudes I would expect to see significant terrain signals?
Regards, Holger

[John H Watson]

Up to certain limits, radar systems can compensate for aircaft pitch and roll, but what happens beyond these limits? Is the display removed?

A quote from the Boeing AMM for an Allied Signal weather radar system:

"The antenna is stabilized in roll up to +/- 40 degrees and
stabilized in pitch up to +/-25 degrees. The antenna can be
manually tilted +/- 15 degrees from the horizontal (zero-
degree) and is stabilized over a combined tilt, pitch, and roll
input of +/- 45 degrees"

[Peter Sagar]

For what they're worth, a couple of excerpts from "Radio Navigation Volume 2, ADF, Automatic Direction Finding with the RADIO COMPASS", by W.R. EVANS, 1980, ISBN 0 85552 083 3 (my old CPL text);

"An NDB uses a non-directional vertical antenna. Because of the design of the aerial system, nearly all of its energy is radiated in the form of vertically-polarised ground waves and, as a result, most of the transmitted energy travels more or less parallel to the surface of the earth."

The text has only a general reference to "turning errors";

"When an aeroplane turns, particularly if the turn is reasonably steep, this too can cause the bearing indicator needle to give an inaccurate reading. The errors are more pronounced with fixed-looped systems. Pilots experiencing problems should remember that the radio compass is only a navigational aid and not a flight instrument."

Unfortunately nothing more specific on turning errors.

Peter.

[Hardy Heinlin]

Hi Holger,

you mean the left side should be partially black? Do you have some real world examples?

A few things to consider:

- When the aircraft bank is 90°, the radar antenna bank is 60° (see AMM quote)
- WX radar is in AUTO with WX+T, so ground clutter is suppressed
- Auto-tilt near the ground is +5°, so add this to the nose, multiplied by cos(60°)
- The 90° bank can be maintained only for a few seconds (relevance for sim?)
- A radar sweep takes 2 seconds (any relevance for instrument modelling?)
- Is ground clutter suppression inop within such bank angles and time frames?
- The demo is supposed to show ADF math, not to make the 90° per se relevant
- Is the WX radar image removed from the ND when bank exceeds 40°?
- How much development time is needed to simulate this (yet unknown) effect?
- ...


Cheers,

|-|ardy


Edit: Thanks Peter. Messages crossed.

[Jeroen Hoppenbrouwers]

... when I came back to this Forum after just having watched http://www.youtube.com/watch?v=koY6kXhQDQo  I could not help but laugh about the failure of the ADF bank   :mrgreen:

[Zinger]

In the older rotating loop antennas, only the two almost vertical loop portions are used for reception, their reception pattern is perpendicular. This might suggest that with bank their direction finding ability gradually diminishes due to antenna characteristics. I'm not sure the newer goniometer, non moving loop-like antennas share the same characteristics.

[Hardy Heinlin]

I recently read that the newer non-rotating antennas are more sensitive to bank errors than the older types are. I guess this suggests that they receive signals also at steeper bank angles, but with it they also make the geometric error more noticeable.

[Holger Wende]

...Do you have some real world examples?
...relevance for sim

I didn't want to be a nitpicker :oops: and I do not have real world examples. And I did not know about roll/pitch compensation, at least not up to this magnitude.

But again I learned a few things...  

Regards, Holger

[Matt Sheil]

For what they're worth, a couple of excerpts from "Radio Navigation Volume 2, ADF, Automatic Direction Finding with the RADIO COMPASS", by W.R. EVANS, 1980, ISBN 0 85552 083 3 (my old CPL text);
Peter.

Russ Evans, I knew him
Here is some trivia, he was a check and training officer for the department of Aviation in about 1983, as part of their currency he had to be checked yearly in a Lear35, so he could then check other pilots on the LearJet.
So I was working at the airport at the time and knew Russ and he invited me on the check ride he was doing.
I can remember him doing unusual attitudes in the lear. the examineer would tell Russ to close his eyes and would throw the plane around left right, power on power off and so on, then he would put the plane in a 90 deg bank, fulll nose up, full nose down trim, and say you have the plane.
It was awesome.
 :mrgreen:

[Hardy Heinlin]

You are no nitpicker, Holger :-) The reason why I post these things is to check if I got something wrong. Every day I get something wrong and learn new things. So please keep posting if you have any doubts or questions on something. It's a discussion forum :-)


Cheers,

|-|ardy

[Peter Sagar]

For what they're worth, a couple of excerpts from "Radio Navigation Volume 2, ADF, Automatic Direction Finding with the RADIO COMPASS", by W.R. EVANS, 1980, ISBN 0 85552 083 3 (my old CPL text);
Peter.

Russ Evans, I knew him
Here is some trivia, he was a check and training officer for the department of Aviation in about 1983, as part of their currency he had to be checked yearly in a Lear35, so he could then check other pilots on the LearJet.
So I was working at the airport at the time and knew Russ and he invited me on the check ride he was doing.
I can remember him doing unusual attitudes in the lear. the examineer would tell Russ to close his eyes and would throw the plane around left right, power on power off and so on, then he would put the plane in a 90 deg bank, fulll nose up, full nose down trim, and say you have the plane.
It was awesome.
 :mrgreen:

Ah, so that's where your love for unusual attitudes came from.  :mrgreen:

Peter.