On 2012-02-05 00:59 , Wolfgang S. Rupprecht wrote:
>
> "Peter H. Coffin"<(E-Mail Removed)> writes:
>> Magnetic deviation is a tricky thing: it changes depending on the
>> heading of the device, which may not be the same as the heading of the
>> car (Alan's "heading is not track" comment elsewhere), it's going to
>> vary by heading in general (it's a different mix of forces when you're
>> facing 0° versus 10° versus 20°), and shifting it by as little as an
>> inch or two in a metal-heavy vehicle like a car will result in vastly
>> different readings, often by as much as 5 or 10°.
>
> That's why I suggested a lookup table, just in case the system was
> non-linear.
>
> On the other hand, I'm hard pressed to see how it could be anything but
> a linear time-invariant system that one can use superposition on.
>
>> Good map data and good track with reasonable assumptions regarding
>> road lock are going to give you MUCH better information about
>> direction than a magnetic compass inside a box with energized
>> antennae, someplace inside a much larger, irregularly shaped highly
>> ferric box. Not generally worth the trouble when track will give so
>> much better results for effort that's already been undertaken ANYWAY,
>> when results without track don't have much of a real use for better
>> than gross answers anyway.
>
> Well, the grotty part now is that every time I come to a stop in the car
> the map spins after the stop. A smarter system would record the track
> and the magnetic heading, see that the track velocity dropped to zero
> but the magnetic heading didn't change. It could then determine that
> the heading must not have changed.

My car GPS is well behaved in that sense. It even extrapolates
positions in long tunnels based on velocity when the signal was lost.
But it does not have a heading sensor.

I agree that it should filter on track and speed and 'pin' to a heading
when the velocity gets low.

--
"We demand rigidly defined areas of doubt and uncertainty."
Douglas Adams - (Could have been a GPS engineer).

On Sun, 05 Feb 2012 09:02:08 -0500, Alan Browne wrote:

> I was really on about the notion that unless the heading system frame
> of reference is tied to where the nose of the vehicle is pointing,
> then you can't use the track vector to correct the heading system. In
> a car, with most tracks aligned with heading it may not be such an
> issue.
>
> In a "good" (aircraft) heading system, the mag heading error is
> calibrated to less than 2.5?? at all headings. The error function
> looks like a sine over 360?? with an amplitude of 2.5?? (eg:
> amplitude=error for that heading reading). It's actually 2nd order
> but grossly a sine. On one nav system program we instituted a Kalman
> Filter that uses Doppler velocity sensor inputs and GPS to real time
> calibrate that error out. Thence, if the GPS was not usable, the
> Doppler nav would be more accurate when using the magnetic heading
> disciplined gyro heading. It took a few flight tests to get working
> correctly...

A plane's a lot more regulated environment. Compass magnet is mounted
in a single place, never to be changed. Plane equipment requires
recertification of all systems when it gets changed, and airplanes in
general, when they need good mag compass readings, are generally a LONG
WAY AWAY from anything that isn't "airplane", so the environment is a
lot more controlled than a handheld GPS. And avation handhelds don't
generally come with mag compasses. (I don't rule out that there may be
some, but I've never seen one touting a mag compass as a feature, and I
bet there's BIG WARNINGS against relying on that compass instead of the
panel one for magnetic navigation.)

And (on the prior point) air nav is where you really care a lot more
about track than heading anyway. As soon as you start paying attention
to heading for the plane instead of track on a GPS or what radials
you're on, then winds suddenly get a a lot more imporant than letting
the corrections you're doing to maintain a track handle that factor
transparently.

--
91. I will not ignore the messenger that stumbles in exhausted and
obviously agitated until my personal grooming or current
entertainment is finished. It might actually be important.
--Peter Anspach's list of things to do as an Evil Overlord

On 2012-02-05 10:13 , Peter H. Coffin wrote:
> On Sun, 05 Feb 2012 09:02:08 -0500, Alan Browne wrote:
>
>> I was really on about the notion that unless the heading system frame
>> of reference is tied to where the nose of the vehicle is pointing,
>> then you can't use the track vector to correct the heading system. In
>> a car, with most tracks aligned with heading it may not be such an
>> issue.
>>
>> In a "good" (aircraft) heading system, the mag heading error is
>> calibrated to less than 2.5?? at all headings. The error function
>> looks like a sine over 360?? with an amplitude of 2.5?? (eg:
>> amplitude=error for that heading reading). It's actually 2nd order
>> but grossly a sine. On one nav system program we instituted a Kalman
>> Filter that uses Doppler velocity sensor inputs and GPS to real time
>> calibrate that error out. Thence, if the GPS was not usable, the
>> Doppler nav would be more accurate when using the magnetic heading
>> disciplined gyro heading. It took a few flight tests to get working
>> correctly...
>
> A plane's a lot more regulated environment.

Ya think?

> Compass magnet is mounted
> in a single place, never to be changed. Plane equipment requires
> recertification of all systems when it gets changed, and airplanes in
> general, when they need good mag compass readings, are generally a LONG
> WAY AWAY from anything that isn't "airplane", so the environment is a
> lot more controlled than a handheld GPS. And avation handhelds don't
> generally come with mag compasses. (I don't rule out that there may be
> some, but I've never seen one touting a mag compass as a feature, and I
> bet there's BIG WARNINGS against relying on that compass instead of the
> panel one for magnetic navigation.)

Another bonus on aircraft is that the airplane is mostly aluminum so the
distortion is less than on a ship.

> And (on the prior point) air nav is where you really care a lot more
> about track than heading anyway. As soon as you start paying attention
> to heading for the plane instead of track on a GPS or what radials
> you're on, then winds suddenly get a a lot more imporant than letting
> the corrections you're doing to maintain a track handle that factor
> transparently.

Heading is always primary for intercepting radials and maintaining or
correcting a track - the more so as x-wind increases and the difference
between track and heading grow.

IOW you don't "fly track", you "fly heading to maintain track".

The difference there is those who chase the cross track error left and
right over and over again v. those who make occasional heading changes
to re-intercept or maintain track. Much more relaxing, the later.

--
"We demand rigidly defined areas of doubt and uncertainty."
Douglas Adams - (Could have been a GPS engineer).

In article <(E-Mail Removed)>, Wolfgang S. Rupprecht wrote:
>Well, the grotty part now is that every time I come to a stop in the car
>the map spins after the stop.

I consider that an interesting and entertaining feature of my
GPS unit. :^)

--
TJH
tjhiggin.at.hiwaay.dot.net

T.J. Higgins wrote:
> In article <(E-Mail Removed)>, Wolfgang S. Rupprecht
> wrote:
>> Well, the grotty part now is that every time I come to a stop in the
>> car the map spins after the stop.
>
> I consider that an interesting and entertaining feature of my
> GPS unit. :^)

It's a sneaky sobriety test...

Mike.
--
If reply address is Mike@@mjcoon.+.com (invalid), remove spurious "@"
and substitute "plus" for +.

On Feb 4, 11:18*am, Sam Wormley <sworml...@gmail.com> wrote:
> http://www.gpsworld.com/Wireless/exp...accurate-every...
>
> > A-GNSS + Wi-Fi Positioning + Cellular Positioning + Sensors. You might have guessed it, but we think accurate everywhere location will be enabled by a combination of all the above hybrid techniques plus one more importanttechnology: sensors. Integrated sensors like accelerometers, magnetometers, and barometers enable devices to sense changes in direction, orientation,and elevation. Given an accurate starting location (for example, GNSS position fix), sensors can track location accurately for several minutes (and this will continue to get better). Location error will accumulate over time,but this can be minimized when Wi-Fi, cellular, and GNSS positioning are used in conjunction to constrain the error. Furthermore, barometers can be used to track elevation changes, thereby allowing devices to know exactly what floor of a building a user is on. Other technologies, or signals of opportunity, may be used in the future to further improve performance, but we think this mix of A-GNSS, W
>
> i-Fi, cellular, and sensor positioning is the key to accurate everywhere location in mobile devices.

Those solutions fit into the 'Maybe it's better than nothing'
category. Someone is looking for technical paper topic.

WiFi location didn't work at all for me when I tried an app to
evaluate it. Maybe my WiFi wasn't in the data base.

Cell signals can get you close enough to keep from getting lost where
there are closely spaced towers.

Barometric sensor fusing my be fine for flying when one can obtain a
recent calibration from a nearby airport. But, my ski altimeter
fluctuates hundreds of feet over the course of a ski day.

Nothing beats my GPS with WAAS for overall consistency. And I do
understand that some sort of accurate dead reckoning system that is
based upon accurate sensors can fill in the occasional voids when GPS
is masked.

Something other than a satellite system will always be required for
navigating tunnels and subways.

--- CHAS