TAS = True Air Speed

With TAS data and GPS derived ground speed SeeYou can calculate wind
using the effect of heading changes and not need to rely on thermal
circle drift.

The IGC file format is modular and somewhat extensible and pretty well
documented. You can see what additional data is being written to the
file by looking at the file header.

In the case of a SeeYou Mobile generated file fed by a device like a
Cambridge 302 that passes TAS data in the NMEA stream then SeeYou
Mobile will include some of that data in the IGC file and IGC file
header will have a J options record like

I043640TAS4145GSP4648TRT4953VAR

This describes the format of additional data stored in each B record
(the standard time/position/altitude record). e.g. here "3640TAS"
characters 36 to 40 in a B record encode the TAS. They are also
encoding the Ground Speed (GSP) and True Track (TRT) but these are all
derivable by SeeYou so I'm not sure what SeeYou uses and I'm too lazy
to do the experiment. A possible reason to want to use these files is
these may be better numbers than you can derive from the IGC file if
the sample times are really coarse.

Now it also turns out SeeYou Mobile will write what it thinks the
winds are in the IGC file. It does that in a optional K records
interspersed amongst the B records. I think I have done the experiment
of deliberately erasing this data and I think I recall SeeYou seems to
want to recalculate the its own wind from the TAS not pull them out of
the J record, but I could be wrong on that.

The header will have an optional record J definition that defines the
I record. Something like...

J020810WDI1115WVE

That says there are 02 data fields in a K record.

The first fields is WDI (Wind Direction) in characters 08 to 10
The second field is WVE (Wind Velocity in km/h) in characers 11 to 15
(I guess Naviter did not get the memo about using WSP :-))
The preceding characters are a standard time stamp

Then amongst the B records you'll see things like

K20253329400074

Anyhow bottom line is IGC files can be quite different and if you want
good non-circling wind you *won't* get it from an IGC file from a
simple GPS output converted to IGC or from a Colibri like flight
recorder.

You can open any IGC file with a text editor and look for the
existence of I and J records to show if TAS or wind data is written to
the file or not. Given modern compute power it really seems a waste
not to be logging this stuff in detail at 1 Hz rates, makes for great
play back in SeeYou.

Darryl


On Sep 28, 7:32*pm, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
What is TAS data?

Mike Schumann
[snip]