On 4 Sep 2003 21:11:38 -0700, (Jason Armistead)
wrote:

(Kirk Stant) wrote in message om...
James,

Interesting hobby!

You could use a GPS logger (like a Volkslogger or Colibri), set to
minimum logging interval (1 or 2 seconds). You would have to download
the trace afterwards and use a program to view it. I would recommend
SeeYou, but some other more terrain-oriented programs might have
better terrain resolution (which would be important for you, I
assume).

This setup would require a small battery hooked up to the logger, but
with the short flight duration it shouldn't be too big.

Some of the newer Garmin handhelds also log altitude, they may be a
lot cheaper and just as useful. The key is probably going to be the
software you use to look at the trace.

Let us know how it works out.

Kirk
LS6-b "66"

Given the relatively short flight time, and the inherent inaccuracy of
GPS (even with Selective Availability switched off by the US
Government), you might find that the error in the GPS fix is
insufficient for accurate speed measurements.

Try setting up your GPS in a fixed location, and then leave it there
logging data for a few minutes. You will see that the position fix
moves around slightly due to the inherent inaccuracy of the system.

For a discussion of Selective Availability and accurace of GPS, see
the following site:

http://www.ngs.noaa.gov/FGCS/info/sans_SA/

and the linked-to Accuracy Comparison pages, especially the most
recent one

http://www.ngs.noaa.gov/FGCS/info/sa...mpare/ERLA.htm

which suggests 6.3 metre accuracy with SA off and 45 metres with SA
on.

Small, perhaps, but maybe important when you're taking short-period
differentials (1st derivative of position give speed). If you're
moving 60 km/h (approx 16.6 m/sec) then an error of 1m in your
position fix (well within the error limits of GPS) will give you a
derived speed now of between 15.6 m/sec (56 km/h) and 17.6 m/sec (63.3
km/h). Try repeating those calculations with a worst-case inaccuracy
of 6.3 metres and you get speeds between 37 km/h and 82.4 km/h !

I would perhaps consider going to a Differntial GPS (DGPS) system,
where a known fixed local station broadcasts supplemental position
correction information that improves the accuracy of the satellite
location fix.

For a good discussion of accuracy between GPS and DGPS systems, see

http://www.gpscontrol.com/php/suppor...l/accuracy.php

In summary, my take on all this is that GPS is good for averaged
velocity based on samples over a period of several seconds (our
minutes, like gliding !), rather than instantaneous velocity based on
sample-to-sample differentiation where GPS accuracy can cloud the
results (like your free-falling bird man attempts).

Good luck and watch those landings !


Cheers

Jason


Jason,

GPS does not use the sample to sample differences to give you
velocities. The 3 D velocities are done from Doppler shifts.

Mike Borgelt