At 08:21 04 November 2008, 309 wrote:
On Nov 3, 1:30=A0am, Tim Newport-Peace
wrote:

UTC=3DGPS+Leap Seconds.

For a UTC (=3DGMT) reference try =A0http://wwp.greenwichmeantime.com/

The Volkslogger uses 'GPS Time' in the GPS Engine, but adds Leap
Seconds
before recording the time.


Thanks...I'll try the "event marker" experiment next chance I get.

FYI, my watch (synchronized to the Garmin GPS unit) matches pretty
damn closely to the grenwichmeantime.com link (and the US atomic clock
and the US Naval Observatory clock...) and this is ~14 seconds BEHIND
the reading of the VL. Using the equation above, Leap seconds would
be negative (i.e., the clock with the leap second correction - GMT-
reads an earlier time than one without leap seconds).

So at the same moment in time, the readings are as below:
Garmin =3D 00:01:15 =3D watch =3D GMT =3D US Atomic =3D USNO (???)
VL =3D 00:01:30

Yes, I see the seconds in the IGC file...I'm using that for performing
airspeed system calibrations.

I'll try to see how the event marker synch's with GMT vs. VL vs.
Watch...

The point to remember is that when you add in a Leap Second, it makes the
minute one second longer and delays the start of the next minute. This
will result in UTC being slower by one second for each Leap Second added.




Now here's a different question: does anybody know how to take an IGC
file (or SeeYou treatment of it) and derive north-south and east-west
velocities? This would be useful in determining the pitot-static
errors by flying a constant bank angle circle: Plot north-south/east
west velocities and you get a circle -- the center gives you the wind
and the radius is the true airspeed. If you want more info on that,
say so and I'll try to find the references online. For now, I've been
using the GPS-Horseshoe Heading technique...which uses more gas, but
gives a good opportunity to measure cruise performance...for a tug or
other power plane.

-Pete

This is probably better in a new thread, however...

The B-record in the IGC file (records starting with the letter B) contain
Time (6 bytes), Latitude (8 bytes), Longitude (Longitude (9 bytes) and a
Fix Valid flag.

Ignore records where the fix valid flag is "V" only use records where
the fix valid flag is "A".

You can now extract the Time, Latitude and Longitude differences between
two records and calculate the N-S and E-W speed in degrees/second.

If you want to translate degrees into metres, it is too complex to explain
in a newsgroup but look at Ed Williams' Aviation Formulary at
http://williams.best.vwh.net/avform.htm
-
Tim Newport-Peace >
Skype: specialist_systems
http://www.spsys.demon.co.uk/icom.htm