Flight #T2004-1 "Radar Calibration"

Date: Wed, 20 Oct 2004 23:09:16 -0500 (CDT)
Subject: Tether flight a success

Greetings.

        Well, our first tethered balloon flight in several years was a
success!  What a wonderful difference from our last attempt.  It's
amazing what proper weather conditions can do.

        To recap, this morning I phoned our FAA liason, who had talked
to the Fargo office and learned that flying a tethered balloon to
altitudes over 500' would require a waiver.  Waivers must be applied
for much farther in advance than the time remaining, so that was out.
Even if we had time, the balloon would also need to be at least 500'
below the cloud ceiling, which was 1600'.  Given that the target
height of 1km is 3280', our original flight plan was sunk.
        Instead of rescheduling the flight, Paul asked if we could
go ahead with a 500' flight, which would only require a NOTAM to be
filed with the FAA.  NOTAMs can be filed one or more hours before the
flight, so we went for it.  Paul and his students would still proceed
with their radar experiments, seeing if they could pull the target
out of the ground clutter at such a low altitude, and the flight
would also serve as a practice run for a later, higher mission.
        Dave, Ryan, and I loaded the power reel, battery, filling gear,
helium tank, balloons, radar target, and other support equipment into
a UND pickup and we were off.  We found the launch site easily and
were set up to start filling at about 1:30.  We used about half a large
tank of helium to fill a single 350g latex balloon.  The 6" aluminum
sphere that would provide the radar target was hung from a 10' lanyard
attached to the balloon nozzle.  Another lanyard was attached to the
end of the tether and the balloon was launched.  We attached long
strips of red paper streamer using small plastic clamps at intervals
along the tether to make it visible to aircraft, as per FAA part 101
regulations.  About halfway to 500', our laser range finder started
flaking out and giving occasonal nonsense readings for balloon altitude.
A backup method must
be determined before the next flight in case this happens again,
although the suspect in this case is a weak battery in the laser
unit (my fault for not bringing a spare).
        When the balloon was at altitude, the nearly windless day
allowed it to deflect only about 30 degrees from vertical, and Paul
and his students were able to identify it on the radar and perform
their experiments.  They were analysing the data the last I checked with
them, so I don't know if they were successful or not.  He did report
that the target was low, showing up on the radar at an elevation of
only 0.5 degrees off the ground.
        A group of four students visited the launch site and took
quite a few pictures, then left for a warmer location.  Everyone
was treated to the strange singing the tether made from the wind
blowing over it, and the changing wind directions and
intensities made visible by the streamers.  After about
two hours and a few phone calls, Paul finally let us know he had
completed his experiments and we started cranking down the balloon
at around 4:00 pm.  That power reel is totally cool, anyone who
battled our old hand-cranked kite reel years ago would have loved
this thing!  We retrieved the clamps and streamers, and finally
uhooked the sphere and balloon.  Everything was stowed and we were
back on the road by 4:15.

        Lessons learned: bring spare batteries (yeah, I'm supposed to
know this already).  Have an alternate method to determine altitude
(mark the tether, count reel revolutions, etc).  If the weather is
REALLY calm, tethered balloon work is possible in North Dakota :).
Using small balloons and filling them with lots of gas helps make
them more aerodynamic and less likely to be blown around (we used
1200g balloons before that, when tethered, fluttered and billowed like
sails, catching the wind and getting blown all over the place).

        I'll update the website soon, and add some pics I took during
the missiion.  Thanks to Dave and Ryan for braving the cold and
providing invaluable assistance!  Thanks to Paul for suggesting the
flight an paying for it.  Thanks to George at the FAA for helping
clarify things and work out the kinks, and thanks to the students
who came out to help and observe.  I had serious reservations about
doing ANY more tethered balloon flights, but with everybody's help
this one came off smoothly.