"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