Gageteers, we need a digital heading sensor

"Derrick Steed" wrote in message
...
Isn't there some form of flux gate compass which can be used? The GPS's we
use originated in the maritime market (after NMEA is a maritime standard),
surely there is a flux gate compass with NMEA output.

Rgds,

Derrick.

A flux gate compass is just another kind of magnetic compass. It eliminates
the jiggling of a pivoting compass card but it is still subject to
installation, variation and dip errors. The dip error means that it only
provides useful data when the wings are level. Still, they are probably
cheaper than GPS compasses.

GPS compasses are elegant in that they point to true north.

Bill Daniels

Dave,

Posting to the group in hopes you answer might serve others...

My SN-10B serves me well. However, I am having a small problem based
on airspeed error. I have noted on ridge flights that after prolonged
high-speed flight without turns, the instrument starts counting up the
wind speed. On one leg, I saw a 25 knot increase. Clearly I need to
fix the asi error table, but if the SN-10 is counting up, which way do
I need to go in the table?

OC

Marine users have been using electronic compasses for some time. These are
fluxgate compasses and most have NEMA output so what you want is not
impossible. The marine ones are expensive circa $400 but there are a new
bread of digital compasses some of which are included in GPS handhelds but I
cannot see if any have NEMA outputs. In addition there ae some aviation
sensors coming n the market (MEMS) (see
http://www.willow.co.uk/html/heading.html) You will need a NEMA sentance
output of HDM or better HDG from such a device and a NEMA concatenator to
meld the NEMA output into the IPAQ datastream.

On the Plus side
You would have a compass that had little lag - easier to roll out of
thermals onto heading
You would get your wind data

On the downside
Expensive as one-off
Yet more battery drain
Device would need A solid state memory for an electronic version of
deviation card
Device would have to be installed in tail as they are V sensitive
You only get Magnetic heading from device - GPS is essentially True and if
flying in areas of local magnetic anomalies you will get strange results.

Given most gliders fly Track as opposed to bearing it would be easier simply
to note magnetic heading & speed once on track and make sure that the two
remain constant if they diverge then your wind direction has changed and you
can either do the mental maths (or use a flight computer) or put in a 360.

What we are all really waiting for is a Touch Single panel display system
that would include something like this along with a Head-Up display. But the
market is just too small to make something like this a real economic
proposition at this time along with the associated battery drain. You would
be lucky to sell more than 500 units.

rgds
stephen


"Bill Daniels" wrote in message
news:_y8Ac.53766$0y.15362@attbi_s03...
Dave, can it give highly accurate, real-time wind data, second by second
on
a straight one hour glide?

My last flight showed this problem during a long glide. For 30 miles the
wind data showed SW winds at 20 knots but then I noticed the glider
drifting
to the west. Two 360 turns and the computer showed wind at 090 at 10
knots.
When I crossed the wind shift line I had a chance to change strategy. 30
miles later when the wind error became obvious it was too late.

More and more I think we need very accurate real-time wind data with no
requirement to be constantly changing heading. A cheap, reliable heading
sensor would make this possible.

Bill Daniels

"Dave Nadler YO" wrote in message
m...
Bill - The heading is not required. In the ILEC SN10 we do this
without heading, and it normally has the wind by the time you're
off tow. Pilot feedback and measurements indicate that we do
it quite accurately. We just use TAS and GPS ground track over
multiple observations (TAS requiring accurate IAS, press, temp).
Best Regards, Dave "YO"

"Bill Daniels" wrote in message
news:gw%zc.59643$HG.25574@attbi_s53...
I've been analyzing the IGC files from my flights and comparing them
to
others.

Generally, in good conditions, I'm circling less than 25% of the time.
Other, far better pilots than I, are circling less than 15% of the
time.
This means that wind data that depends on the glider flying circles
gets
way
out of date during the long glides. OK, "S" turns can give the
computer
a
chance to compute wind if you remember to fly them but there are still
a
lot
of long, straight glides.

To compute real-time wind data we need Groundspeed, Ground Track (any
GPS
gives these)True Airspeed (Some computers do this if they have OAT,
Pressure
Altitude and IAS) and accurate heading data. Heading data it the
tough
nut
to crack.

So, how do we get Heading? One way might be to put a GPS antenna in
each
wing and look at the carrier phase difference. If the computer saw a
constant track, it could assume the wings are level and determine a
heading
value. This heading value might update an aerospace quality MEMS
heading
gyro that would maintain a flow of heading data to the glide computer
during
times when the wings were banked.

With good heading data we could get uninterrupted high quality wind
data
regardless of the gliders' maneuvering or lack of it.

OK, RAS gageteers, how about it?

Bill Daniels

Hi Chris - Clearly you need our compass option ;-)
Give a call over the weekend to discuss your issue,
and also to tell me where you've located such a long ridge,
Best Regards, Dave "YO"

(Chris OCallaghan) wrote in message . com...
Dave,

Posting to the group in hopes you answer might serve others...

My SN-10B serves me well. However, I am having a small problem based
on airspeed error. I have noted on ridge flights that after prolonged
high-speed flight without turns, the instrument starts counting up the
wind speed. On one leg, I saw a 25 knot increase. Clearly I need to
fix the asi error table, but if the SN-10 is counting up, which way do
I need to go in the table?

OC

One thing that a lot of the UAV guys are doing, is to use a magnetic
heading sensor coupled with a MEMS gyro, or if more money is available,
a FOG. The magnetic sensor can be used to periodically determine heading
and to correct for signal drift from the gyro. For instance, the gyro
can be used as the primary source for heading info, while the magnetic
sensor is used during long, stable glides. Very similar to how the DG
and compass are used in power planes. Of course, the gyro would have to
be either mechanically gimbaled to remain parallel to the horizon, or,
alternatively, several gyros on different axes can be used together to
determine actual heading change, rather than yaw rate.

Using some basic Kalman filtering, it should be possible to get fairly
accurate (5-8 degree) heading info. This will, of course, be magnetic
heading, and subject to the usual accuracy issues. However, it might be
possible to correct the data to true heading by loading a map of lines
of deviation and magnetic anomalies (which can also be fed into the
Kalman filter). Simply feed the wind component data from the GPS, the
raw heading info from the magnetic sensor, and the stabilized data from
the gyro into a Kalman filter, and pretty stable and accurate heading
data should be available.

Definitely do-able. Let's do it! :O)

BTW, check out www.rotomotion.com. These guys have several fairly cheap
AHRS', as well as several cheap IMU's. They've also published a bunch of
open source code on Sourceforge under the Autopilot project
http://autopilot.sourceforge.net/ with an effective Kalman filter to
integrate GPS, IMU, and magnetic sensor data.

- Robert

Derrick Steed wrote:
Isn't there some form of flux gate compass which can be used? The GPS's we use originated in the maritime market (after NMEA is a maritime standard), surely there is a flux gate compass with NMEA output.

Rgds,

Derrick.

Hi, Bill.

The Garmin eTrex Vista includes GPS, barometric altitude, and electronic
compass. IIRC all the above data are output on the serial interface as
NMEA or Garmin sentences.

Can you get by with +- 2 degrees heading accuracy? No idea how big the
dip error is when wings banked, but maybe your flight computer could be
smart enough to use the compass when straight & level and switch to
turning-base wind calculation when banked?

http://www.garmin.com/products/etrexVista/spec.html

Dave

"Dave Houlton" wrote in message
...
Derrick Steed wrote:
Isn't there some form of flux gate compass which can be used? The GPS's
we use originated in the maritime market (after NMEA is a maritime
standard), surely there is a flux gate compass with NMEA output.

Rgds,

Derrick.

Hi, Bill.

The Garmin eTrex Vista includes GPS, barometric altitude, and electronic
compass. IIRC all the above data are output on the serial interface as
NMEA or Garmin sentences.

Can you get by with +- 2 degrees heading accuracy? No idea how big the
dip error is when wings banked, but maybe your flight computer could be
smart enough to use the compass when straight & level and switch to
turning-base wind calculation when banked?

http://www.garmin.com/products/etrexVista/spec.html

Dave

The "compass" on many handheld GPS units really shows ground track although
I think some of them have electronic flux gate compasses. To compute true
wind data you need four inputs: True Heading, True Airspeed, Ground Speed
and Ground Track. We only need to add heading to get true wind info.

There are fluxgate and other magnetoresistive sensors that use the earth
magnetic field to determine heading. The $500 NMEA marine units would
probably work but they suffer from all the problems that any magnetic
compass has. There are other clever devices that detect the magnetic field
in three axes and, with the GPS position data and a look-up table, subtract
the variation and dip errors to output true north.

I still like the idea of a double-headed GPS receiver with two antennas a
meter apart on the fuselage centerline. These should be good to a half
degree accuracy. If you add MEMS gyros for a "coast through" in the event
of a GPS signal interruption, you have an elegant heading sensor.

Bill Daniels

"Dave Houlton" wrote in message
...
Derrick Steed wrote:
Isn't there some form of flux gate compass which can be used? The GPS's
we use originated in the maritime market (after NMEA is a maritime
standard), surely there is a flux gate compass with NMEA output.

Rgds,

Derrick.

Hi, Bill.

The Garmin eTrex Vista includes GPS, barometric altitude, and electronic
compass. IIRC all the above data are output on the serial interface as
NMEA or Garmin sentences.

Can you get by with +- 2 degrees heading accuracy? No idea how big the
dip error is when wings banked, but maybe your flight computer could be
smart enough to use the compass when straight & level and switch to
turning-base wind calculation when banked?

http://www.garmin.com/products/etrexVista/spec.html

Dave

The "compass" on many handheld GPS units really shows ground track although
I think some of them have electronic flux gate compasses. To compute true
wind data you need four inputs: True Heading, True Airspeed, Ground Speed
and Ground Track. We only need to add heading to get true wind info.

There are fluxgate and other magnetoresistive sensors that use the earth
magnetic field to determine heading. The $500 NMEA marine units would
probably work but they suffer from all the problems that any magnetic
compass has. There are other clever devices that detect the magnetic field
in three axes and, with the GPS position data and a look-up table, subtract
the variation and dip errors to output true north.

I still like the idea of a double-headed GPS receiver with two antennas a
meter apart on the fuselage centerline. These should be good to a half
degree accuracy. If you add MEMS gyros for a "coast through" in the event
of a GPS signal interruption, you have an elegant heading sensor.

Bill Daniels

"Dave Houlton" wrote in message
...
Derrick Steed wrote:
Isn't there some form of flux gate compass which can be used? The GPS's
we use originated in the maritime market (after NMEA is a maritime
standard), surely there is a flux gate compass with NMEA output.

Rgds,

Derrick.

Hi, Bill.

The Garmin eTrex Vista includes GPS, barometric altitude, and electronic
compass. IIRC all the above data are output on the serial interface as
NMEA or Garmin sentences.

Can you get by with +- 2 degrees heading accuracy? No idea how big the
dip error is when wings banked, but maybe your flight computer could be
smart enough to use the compass when straight & level and switch to
turning-base wind calculation when banked?

http://www.garmin.com/products/etrexVista/spec.html

Dave

The "compass" on many handheld GPS units really shows ground track although
I think some of them have electronic flux gate compasses. To compute true
wind data you need four inputs: True Heading, True Airspeed, Ground Speed
and Ground Track. We only need to add heading to get true wind info.

There are fluxgate and other magnetoresistive sensors that use the earth
magnetic field to determine heading. The $500 NMEA marine units would
probably work but they suffer from all the problems that any magnetic
compass has. There are other clever devices that detect the magnetic field
in three axes and, with the GPS position data and a look-up table, subtract
the variation and dip errors to output true north.

I still like the idea of a double-headed GPS receiver with two antennas a
meter apart on the fuselage centerline. These should be good to a half
degree accuracy. If you add MEMS gyros for a "coast through" in the event
of a GPS signal interruption, you have an elegant heading sensor.

Bill Daniels

I have no idea how three copies of the previous post got sent. I swear I
hit the send button only once.

Bill Daniels