Compass Questions ?

[Bob]

Hello,

Not a Pilot, but always interested in the engineering aspects.

This question came up, and I realize that I am not sure of a correct answer.

Regarding the compass heading that shows in the glass displays:

How is it derived ?
GPS ?

If not GPS, what ?

Is it then Magnetic or True ?

Does the Pilot have to correct for variation/declination changes during
a flight ?

Does aviation use True or Magnetic when discussing or flying ?

etc. ?

Thanks,
Bob

[Larry Dighera]

Hello Bob,

I'm unable to provide the answer as to the technology employed by
computer-generated displays to display magnetic information, but I would expect
something like a Hall device might be employed. As you implied, it could as
easily be derived from GPS data corrected for magnetic variation at the
particular location.

Magnetic/compass headings are not true, but magnetic. Navigation Sectional
charts <http://skyvector.com/> are depicted in True coordinates, and the
pilot/navigator must correct for magnetic variation in drafting his flight
plan. ATC expects all course/heading numbers to be magnetic.

You'll find more information here:
<http://www.faa.gov/regulations_policies/handbooks_manuals/>, and here:
<http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2015.pdf>.

Hope that helps.

Best regards,
Larry Dighera



On Mon, 13 Jul 2015 18:36:54 -0400, Bob > wrote:

>Hello,
>
>Not a Pilot, but always interested in the engineering aspects.
>
>This question came up, and I realize that I am not sure of a correct answer.
>
>Regarding the compass heading that shows in the glass displays:
>
>How is it derived ?
>GPS ?
>
>If not GPS, what ?
>
>Is it then Magnetic or True ?
>
>Does the Pilot have to correct for variation/declination changes during
>a flight ?
>
>Does aviation use True or Magnetic when discussing or flying ?
>
>etc. ?
>
>Thanks,
>Bob

[Dave Doe]

In article >, , Bob
says...
>
> Hello,
>
> Not a Pilot, but always interested in the engineering aspects.
>
> This question came up, and I realize that I am not sure of a correct answer.
>
> Regarding the compass heading that shows in the glass displays:
>
> How is it derived ?
> GPS ?

Not sure what you really mean by glass display, but I'll assume old
school (real) compass - a magnetic pointer).

They point North! - magnetic north, always, of course! :)

I believe a LAME checks and swings the compass (Licenced Aircraft
Mechanical Engineer). (Ie. a mechanic! - but a plane mechanic! :) )
On doing so, they may adjust a placard beside the compass to indicate
local deviation, normally quadrants and sub-quadrants only. So maybe +1
for East for example. This is of course, local to each plane.

> If not GPS, what ?
>
> Is it then Magnetic or True ?

A compass always points magnetically.

> Does the Pilot have to correct for variation/declination changes
during
> a flight ?

Yes, both chart variation, and compass deviation (per above). On top of
that, there's wind to adjust for.

> Does aviation use True or Magnetic when discussing or flying ?

Both of course. Charts are True, the compass is Magnetic.

--
Duncan.

[Bob]

Hi,

Thanks for replies and help.

What I was referring to are the new, modern, "glass" displays where most
everything shown is derived
from transducers located elsewhere.

e.g. when a display shows a Heading of, e.g., 075, it is of course
getting this info. from some kind of transducer, located elsewhere.

Is it derived directly from GPS ?
A Hall effect compass ?
or,... ?

Thanks again; interesting subject.

Bob
-------------------------------------------------------------------------------------------------------------

On 7/13/2015 6:36 PM, Bob wrote:
> Hello,
>
> Not a Pilot, but always interested in the engineering aspects.
>
> This question came up, and I realize that I am not sure of a correct
> answer.
>
> Regarding the compass heading that shows in the glass displays:
>
> How is it derived ?
> GPS ?
>
> If not GPS, what ?
>
> Is it then Magnetic or True ?
>
> Does the Pilot have to correct for variation/declination changes during
> a flight ?
>
> Does aviation use True or Magnetic when discussing or flying ?
>
> etc. ?
>
> Thanks,
> Bob

[Dave Doe]

In article >, , Bob
says...
>
> Hi,
>
> Thanks for replies and help.
>
> What I was referring to are the new, modern, "glass" displays where most
> everything shown is derived
> from transducers located elsewhere.
>
> e.g. when a display shows a Heading of, e.g., 075, it is of course
> getting this info. from some kind of transducer, located elsewhere.
>
> Is it derived directly from GPS ?
> A Hall effect compass ?
> or,... ?
>
> Thanks again; interesting subject.

Well I'm not sure on that - perhaps others will reply. However,
cellphones usually use one or more magnetometers (hope I spelt that
right :) ). A GPS can't get a magnetic heading, only True.

I play with Arduino's a bit - and Googling 'arduino magentometer' gives
a few results, eg...
http://bildr.org/2011/01/hmc6352/
http://bildr.org/2012/02/hmc5883l_arduino/
http://eclecti.cc/hardware/hmc5843-magnetometer-library-for-arduino

I *assume* that the compass in a glass display uses the same sort of
technology.

--
Duncan.

[Dave Doe]

In article >,
, Dave Doe says...
>
> In article >, , Bob
> says...
> >
> > Hi,
> >
> > Thanks for replies and help.
> >
> > What I was referring to are the new, modern, "glass" displays where most
> > everything shown is derived
> > from transducers located elsewhere.
> >
> > e.g. when a display shows a Heading of, e.g., 075, it is of course
> > getting this info. from some kind of transducer, located elsewhere.
> >
> > Is it derived directly from GPS ?
> > A Hall effect compass ?
> > or,... ?
> >
> > Thanks again; interesting subject.
>
> Well I'm not sure on that - perhaps others will reply. However,
> cellphones usually use one or more magnetometers (hope I spelt that
> right :) ). A GPS can't get a magnetic heading, only True.
>
> I play with Arduino's a bit - and Googling 'arduino magentometer' gives
> a few results, eg...
> http://bildr.org/2011/01/hmc6352/
> http://bildr.org/2012/02/hmc5883l_arduino/
> http://eclecti.cc/hardware/hmc5843-magnetometer-library-for-arduino
>
> I *assume* that the compass in a glass display uses the same sort of
> technology.

Apologies to the apostrophe police - that's terrible!

--
Duncan.

[Don Poitras]

Dave Doe > wrote:
> In article >,
> , Dave Doe says...
> >
> > In article >, , Bob
> > says...
> > >
> > > Hi,
> > >
> > > Thanks for replies and help.
> > >
> > > What I was referring to are the new, modern, "glass" displays where most
> > > everything shown is derived
> > > from transducers located elsewhere.
> > >
> > > e.g. when a display shows a Heading of, e.g., 075, it is of course
> > > getting this info. from some kind of transducer, located elsewhere.
> > >
> > > Is it derived directly from GPS ?
> > > A Hall effect compass ?
> > > or,... ?
> > >
> > > Thanks again; interesting subject.
> >
> > Well I'm not sure on that - perhaps others will reply. However,
> > cellphones usually use one or more magnetometers (hope I spelt that
> > right :) ). A GPS can't get a magnetic heading, only True.
> >
> > I play with Arduino's a bit - and Googling 'arduino magentometer' gives
> > a few results, eg...
> > http://bildr.org/2011/01/hmc6352/
> > http://bildr.org/2012/02/hmc5883l_arduino/
> > http://eclecti.cc/hardware/hmc5843-magnetometer-library-for-arduino
> >
> > I *assume* that the compass in a glass display uses the same sort of
> > technology.

> Apologies to the apostrophe police - that's terrible!

> --
> Duncan.

The display in a glass panel is GPS course. You need a magnetic compass to know
your heading. The difference between the two is wind correction, magnetic
variation and compass deviation.

While I don't doubt that someone could create a device that would send
magnetic heading info to a glass panel, it would have to be some obvious
secondary display. You want to be looking at your true course if you want
to get from A to B.

--
Don Poitras

[Dave Doe]

In article >, , Don
Poitras says...
>
> Dave Doe > wrote:
> > In article >,
> > , Dave Doe says...
> > >
> > > In article >, , Bob
> > > says...
> > > >
> > > > Hi,
> > > >
> > > > Thanks for replies and help.
> > > >
> > > > What I was referring to are the new, modern, "glass" displays where most
> > > > everything shown is derived
> > > > from transducers located elsewhere.
> > > >
> > > > e.g. when a display shows a Heading of, e.g., 075, it is of course
> > > > getting this info. from some kind of transducer, located elsewhere.
> > > >
> > > > Is it derived directly from GPS ?
> > > > A Hall effect compass ?
> > > > or,... ?
> > > >
> > > > Thanks again; interesting subject.
> > >
> > > Well I'm not sure on that - perhaps others will reply. However,
> > > cellphones usually use one or more magnetometers (hope I spelt that
> > > right :) ). A GPS can't get a magnetic heading, only True.
> > >
> > > I play with Arduino's a bit - and Googling 'arduino magentometer' gives
> > > a few results, eg...
> > > http://bildr.org/2011/01/hmc6352/
> > > http://bildr.org/2012/02/hmc5883l_arduino/
> > > http://eclecti.cc/hardware/hmc5843-magnetometer-library-for-arduino
> > >
> > > I *assume* that the compass in a glass display uses the same sort of
> > > technology.
>
> > Apologies to the apostrophe police - that's terrible!
>
> > --
> > Duncan.
>
> The display in a glass panel is GPS course. You need a magnetic compass to know
> your heading. The difference between the two is wind correction, magnetic
> variation and compass deviation.
>
> While I don't doubt that someone could create a device that would send
> magnetic heading info to a glass panel, it would have to be some obvious
> secondary display. You want to be looking at your true course if you want
> to get from A to B.

The OP is asking about just about such a device. And it's clear they do
exist in a glass display (a compass that is).

Take for example, the Cessna website. If you navigate to say the Cessna
TTx - and check out the glass Avionics section, and look at Equipment
Details (rather than Standard Features), it specifies a Magnetometer ...
"GMU-44 Magnetometer (dual)"

Albiet a compass is almost obsolete and uncessary with todays navigation
equipment, but let's not forget runways - still lined up and named
magnetic. Makes me wonder when that will change.

--
Duncan.

[Don Poitras]

Dave Doe > wrote:
> In article >, , Don
> Poitras says...
> >
> > Dave Doe > wrote:
> > > In article >,
> > > , Dave Doe says...
> > > >
> > > > In article >, , Bob
> > > > says...
> > > > >
> > > > > Hi,
> > > > >
> > > > > Thanks for replies and help.
> > > > >
> > > > > What I was referring to are the new, modern, "glass" displays where most
> > > > > everything shown is derived
> > > > > from transducers located elsewhere.
> > > > >
> > > > > e.g. when a display shows a Heading of, e.g., 075, it is of course
> > > > > getting this info. from some kind of transducer, located elsewhere.
> > > > >
> > > > > Is it derived directly from GPS ?
> > > > > A Hall effect compass ?
> > > > > or,... ?
> > > > >
> > > > > Thanks again; interesting subject.
> > > >
> > > > Well I'm not sure on that - perhaps others will reply. However,
> > > > cellphones usually use one or more magnetometers (hope I spelt that
> > > > right :) ). A GPS can't get a magnetic heading, only True.
> > > >
> > > > I play with Arduino's a bit - and Googling 'arduino magentometer' gives
> > > > a few results, eg...
> > > > http://bildr.org/2011/01/hmc6352/
> > > > http://bildr.org/2012/02/hmc5883l_arduino/
> > > > http://eclecti.cc/hardware/hmc5843-magnetometer-library-for-arduino
> > > >
> > > > I *assume* that the compass in a glass display uses the same sort of
> > > > technology.
> >
> > > Apologies to the apostrophe police - that's terrible!
> >
> > > --
> > > Duncan.
> >
> > The display in a glass panel is GPS course. You need a magnetic compass to know
> > your heading. The difference between the two is wind correction, magnetic
> > variation and compass deviation.
> >
> > While I don't doubt that someone could create a device that would send
> > magnetic heading info to a glass panel, it would have to be some obvious
> > secondary display. You want to be looking at your true course if you want
> > to get from A to B.

> The OP is asking about just about such a device. And it's clear they do
> exist in a glass display (a compass that is).

> Take for example, the Cessna website. If you navigate to say the Cessna
> TTx - and check out the glass Avionics section, and look at Equipment
> Details (rather than Standard Features), it specifies a Magnetometer ...
> "GMU-44 Magnetometer (dual)"

> Albiet a compass is almost obsolete and uncessary with todays navigation
> equipment, but let's not forget runways - still lined up and named
> magnetic. Makes me wonder when that will change.

> --
> Duncan.

You're right. I was wrong. It makes sense that the glass HSI would need
to work like the mechanical one and that gets set manually to the
current compass setting.

--
Don Poitras

[Dallas]

Bob > wrote:
>
> Regarding the compass heading that shows in the glass displays:


The heading information from the PFD comes from a directional gyro. The
directional gyro's drift is constantly corrected (slaved) by a magnetic
compass located in the tailcone on the airplane. The output of this compass
is electrically "enhanced" and amplified through a "flux gate".

Bottom line: A very accurate, stable, compass reading that never needs
adjustment.

--
Dallas

[Skywise]

Dallas > wrote in
-
september.org:

> Bob > wrote:
>>
>> Regarding the compass heading that shows in the glass displays:
>
>
> The heading information from the PFD comes from a directional gyro. The
> directional gyro's drift is constantly corrected (slaved) by a magnetic
> compass located in the tailcone on the airplane. The output of this compass
> is electrically "enhanced" and amplified through a "flux gate".
>
> Bottom line: A very accurate, stable, compass reading that never needs
> adjustment.

And to expand on that a bit...

https://en.wikipedia.org/wiki/Attitude_and_heading_reference_system

Brian
--
http://www.earthwaves.org/forum/index.php - Earth Sciences discussion
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Sed quis custodiet ipsos Custodes?

[Brian Whatcott]

Oh my, a reasonable question, attracting answers over the next two days,
of which just one had it right - and that referred to the AHRS
attitude heading system. What used to be an expensive collection of
gyros and accelerometers is now commercially available (for model
aircraft and drones) for $20 on up. Glass cockpits can often select
heading in TRUE or MAG form.

For those of us flying on the cheap, a moving map application like APIC
from Approach Systems (an update costs $10) loaded on a personal
assistant like the HP IQue which cost around $300 when new uses its
internal GPS to provide a track made good (over the ground) which can be
presented as Mag or True. It is convenient to set the direction
indicator to the MAG track indicated, though this ignores the difference
between Heading and Track - the former being needed to make good the
latter, despite a crosswind.

Sincerely

Brian W


On 7/13/2015 5:36 PM, Bob wrote:
> Hello,
>
> Not a Pilot, but always interested in the engineering aspects.
>
> This question came up, and I realize that I am not sure of a correct
> answer.
>
> Regarding the compass heading that shows in the glass displays:
>
> How is it derived ?
> GPS ?
>
> If not GPS, what ?
>
> Is it then Magnetic or True ?
>
> Does the Pilot have to correct for variation/declination changes during
> a flight ?
>
> Does aviation use True or Magnetic when discussing or flying ?
>
> etc. ?
>
> Thanks,
> Bob