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[REPORTED]Mag/True bearings


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I've noticed that when switching from Magnetic to True setting in the HSI>DATA>AC page, the HUD cue to a specific point (e.g. TACAN) is moved with the heading scale while it should remain in the same position (only the heading scale should change).

 

Example:

Mag var. E 6°

Magnetic heading: 000°

Bearing (and HUD cue) to TACAN: 000° (right in front of me)

 

When switching to true what happens in the sim is:

True heading: 006°

HUD cue to TACAN remains at 000° (it moves left along with the heading scale)

 

In reality, only the heading scale should change. The cue to TACAN should be at 006° since the TACAN is still right in front of me.

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  • 4 months later...

Sorry guys, bumping it up again since this has not been fixed (or aknowledged)

 

To summarize and elaborate a bit more, when switching from HDG MAG to HDG TRUE in the HSI>DATA>AC page:

 

1) If a waypoint is selected in the HSI page, the bearing cue to the waypoint in the HUD remains where it is and only the compass tape moves (changing from magnetic to true and viceversa) - this is the correct behaviour

2) If a TACAN station is selected in the HSI page, the bearing cue to the TACAN station moves with the compass tape (as if the direction where the TACAN station changed) - this is not the correct behaviour

 

Cheers! :)

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I think that might actually be correct. Hornet is receiving the current TACAN radial, and presenting it on HSI. The TACAN's transmissions are oriented (assume) to local Magnetic North (Station Declination). The fundamental enhancement of the VOR/TACAN signal vs the standard radio beacon signal is that they are directional (contains direction information). Hence why classical radio beacons are called NDB's (Non-Directional Beacon). This signal itself does not contain direction information. The receiver must Direction Find to determine it. If the process is Automatic Direction Finding, then the equipment is called ADF.

 

A lot of my assumptions are based off civilian VOR's and RMI indications, but they should be nearly identical to TACAN, as the pilot indications are identical (only the methods of transmission and interpretation, and security are different).

 

When you are South of the TACAN, and see it's bearing as 000°M, you are on the 180° Radial from the TACAN. The TACAN is transmitting this information to the Hornet, which is calculating via the signal pattern alone that it is on the 180° Radial from the TACAN. The signal is directional. The Hornet's radio doesn't care the direction from which it's received (this is why it's more accurate). So, since the Hornet knows the Radial is 180, it knows the bearing is 000, and drives the pointer to 000. No matter if the current north reference chosen by the Hornet pilot.

 

The magnetic variation is applied at the transmitter, not at the receiver.

 

When you are on the 180 Radial facing 000M, you see the bearing pointer for TACAN straight ahead. When you change to True heading, your heading is now 006T. But your equipment still receives the 180 signal from TACAN, so the bearing pointer shifts left of your nose by 6 degrees, to still show 000.

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I think that might actually be correct. Hornet is receiving the current TACAN radial, and presenting it on HSI. The TACAN's transmissions are oriented (assume) to local Magnetic North (Station Declination). The fundamental enhancement of the VOR/TACAN signal vs the standard radio beacon signal is that they are directional (contains direction information). Hence why classical radio beacons are called NDB's (Non-Directional Beacon). This signal itself does not contain direction information. The receiver must Direction Find to determine it. If the process is Automatic Direction Finding, then the equipment is called ADF.

 

A lot of my assumptions are based off civilian VOR's and RMI indications, but they should be nearly identical to TACAN, as the pilot indications are identical (only the methods of transmission and interpretation, and security are different).

 

When you are South of the TACAN, and see it's bearing as 000°M, you are on the 180° Radial from the TACAN. The TACAN is transmitting this information to the Hornet, which is calculating via the signal pattern alone that it is on the 180° Radial from the TACAN. The signal is directional. The Hornet's radio doesn't care the direction from which it's received (this is why it's more accurate). So, since the Hornet knows the Radial is 180, it knows the bearing is 000, and drives the pointer to 000. No matter if the current north reference chosen by the Hornet pilot.

 

The magnetic variation is applied at the transmitter, not at the receiver.

 

When you are on the 180 Radial facing 000M, you see the bearing pointer for TACAN straight ahead. When you change to True heading, your heading is now 006T. But your equipment still receives the 180 signal from TACAN, so the bearing pointer shifts left of your nose by 6 degrees, to still show 000.

 

This is reported by ED years ago. There is nothing correct here. Reading OP might help seeing the issue.

Stay safe

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Based on my understanding of VOR/TACAN, the receiver's understanding of relative position has nothing to do with the actual direction of the signal. The position information of the signal is determined by comparison of the features of the radiation pattern. If the signal says the airplane is east of the station then the bearing pointer points to the west even if the airplane is not east of the station.

 

You see TACANs are calibrated such that their north part of the signal is in the magnetic north direction sometimes but not always. There exist true north calibrated facilities, rare as they are. Not that any station would be deliberately calibrated this way but there's nothing preventing the north direction to be in any direction from the station.

 

Basically if the radio signal says the direction to the station is X then the airplane will believe that even if that's not true. It's perfectly reasonable for the F-18 to say "I'm on the 180 radial, so the station must be 360" even if the station is not 360. It's possible for the airplane to be designed to assume that when you change to display true heading that the pilot is still using magnetic-calibrated TACAN and to sort out all the indications to compensate. But that's not necessarily how the Hornet is designed. It could very well be designed to assume that when the pilot switches to true heading mode (presumably to operate in the arctic circle) it is also using true-calibrated TACANs.

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Thanks all for the support or contribution.

 

I never though about the VOR relation but it seems to me a bit strange since it is the bearing to the station itself that changes and not the input radial (or course)...

 

Later today I'll do a test and see if by switching MAG/TRUE the position of the TACAN symbol in the HSI changes. If it does (I believe this is the case) then it is not a VOR radial related issue...

 

A clarification from ED on this would be appreciated. :helpsmilie:

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Ok, I ran the test and the bug is clearly on the bearing caret on the HUD.

 

I input a TACAN station with HDG MAG and set the HSI range so that the TACAN symbol was within HSI range. Switching HDG MAD/TRUE back and forth, both the TACAN symbol and the TACAN bearing caret (T within a triangle) on the outer perimeter of the HSI stayed at the same position and only the compass rose shifted. On the HUD instead, the bearing caret (which should be linked to the bearing caret on the HSI) moved with the compass rose.

 

Result: the TACAN bearing caret on the HUD is not linked to the TACAN bearing caret on the HSI once HDG TRUE is selected.

 

A bug in my opinion. :thumbup:

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Ok, I ran the test and the bug is clearly on the bearing caret on the HUD.

 

I input a TACAN station with HDG MAG and set the HSI range so that the TACAN symbol was within HSI range. Switching HDG MAD/TRUE back and forth, both the TACAN symbol and the TACAN bearing caret (T within a triangle) on the outer perimeter of the HSI stayed at the same position and only the compass rose shifted. On the HUD instead, the bearing caret (which should be linked to the bearing caret on the HSI) moved with the compass rose.

 

Result: the TACAN bearing caret on the HUD is not linked to the TACAN bearing caret on the HSI once HDG TRUE is selected.

 

A bug in my opinion. :thumbup:

 

Thanks for taking the time, but your original post was more than clear and it was never a tacan thing. Reading OP was enough to see that the biphase method used in vor/tacan has nothing to do with this.

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Stay safe

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Thanks for taking the time, but your original post was more than clear and it was never a tacan thing. Reading OP was enough to see that the biphase method used in vor/tacan has nothing to do with this.

 

Oh well, I had to run the test to clear the field from doubts. I hope Bignewy will add "REPORTED" to the title of this thread. :thumbup:

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