experimenting with using LED-based
photometers on near space missions.
There’s a problem with this
experiment, however. The near
spacecraft is not a stable platform.
This means each time a photometer
takes a light intensity measurement,
the sun is in a different position
relative to the photometer and this
strongly affects the light intensity. So,
on mission NearSys 15L, I tried
something different: turning the
photometer downwards.
I didn’t think this would be
significant, but the data shows
otherwise as you can see in Figure 8
when you compare upward and
downward photometers. Mission
NearSys 15C flew the UV-B
photometer facing upwards, and
mission NearSys 15L flew the
photometer facing downwards. It
appears the downward photometer is
measuring the scattering of ultraviolet
off the atmosphere, which should be
a good proxy for the intensity of solar
ultraviolet. Better yet, the atmosphere
is such a good scattering source that
the pointing direction of the
photometer is irrelevant.
So, that was GPSL 2015 in a
nutshell. Several things were learned
and I have another idea to research
for the future. If you’re in the Texas
area next year and interested in
amateur near space exploration, then
check out Granbury June 16–19.
That’s when ARBONET (Amateur
Radio Balloons Over North East
Texas) will host the next GPSL. You
can learn more about ARBONET at
www.arbonet.net.
You can read about this year’s
GPSL host, ROBOMO at
robomo.com.
Onwards and Upwards,
Your near space guide NV
October 2015 65
In Jeff Ducklow’s picture of mission
NearSys 15M (Figure 3), the orange
module is Ann Boes’ tracking
module which is a cube measuring
one foot across in every dimension.
Using that as a reference, the
balloon appears to be 17 feet
across. At launch, the balloon was
only six feet across and about seven
feet tall (for an average of 6. 3 feet).
That means the balloon in the
image has increased its volume 19. 7
times. It’s pretty cold at 52,000 feet,
something like - 60°F. A temperature
that cold will decrease the volume
of the balloon by roughly 20%.
Therefore, the volume of the
balloon at 52,000 feet without the
effects of the cold is closer to 15. 8
times greater than it was at launch.
That kind of expansion requires an
air pressure that’s 6.3% of the air
pressure at the time of launch.
Actual measurements made during
near space missions show that the
pressure is actually closer to 7.7% of
surface air pressure at 52,000 feet.
This back of the envelope
calculation is pretty good for
measuring air pressure with a
photograph and using Charle’s and
Boyle’s Laws.
