Electric field measurements inside thunderstorms are essential to our
understanding of thunderstorm charge structure, electrification, and
lightning initiation. However, most existing measurements have been made
by single instruments carried aloft by weather balloon, thus providing
measurements made at a single point that moves through the storm on a
timescale of tens of minutes. It is therefore difficult to interpret
such data, since a change in observed field strength may be due to
motion of the balloon into a region with different field or due to
overall evolution of the storm's electrical structure with time.
Separation of such spatial and temporal variability requires
simultaneous measurements at multiple locations within the storm. This
can be accomplished with a single weather balloon by carrying multiple
independent electric field dropsondes aloft and releasing them one at a
time, separated by short time intervals. The balloon payload design is
optimized for low mass and use of off-the-shelf components whenever
possible, releasing each dropsonde by a hot wire cut-down mechanism.
Each dropsonde spins as it falls, measuring electric field as it rotates
and sends data to a ground station in real-time. The dropsondes are
designed to fall and rotate stably by use of aerodynamic simulations,
with internal components robustly connected along the axis of the
instrument to ensure the desired balance and alignment of the principal
axes of the moment of inertia. The telemetry transmitters use simple
low-cost low-power all-in-one transmitter chips. The telemetry ground
station receives signals simultaneously from all dropsondes by a single
software-defined radio receiver. Robust long-range communication is
enabled by use of spread spectrum techniques and error correcting codes.