  In recent decades, GNSS Radio Occultation soundings have proven an invaluable
input to global weather forecasting. The success of government-sponsored
programs such as COSMIC is now complemented by commercial low-cost cubesat
implementations. The result is access to more than 10,000 soundings per day and
improved weather forecasting accuracy. This movement towards commercialization
has been supported by several agencies, including the National Aeronautics and
Space Administration (NASA), National Oceanic and Atmospheric Administration
(NOAA) and the U.S. Air Force (USAF) with programs such as the Commercial
Weather Data Pilot (CWDP). This has resulted in further interest in
commercially deploying GNSS-RO on complementary platforms. Here, we examine a
so far underutilized platform: the high-altitude weather balloon. Such
meteorological radiosondes are deployed twice daily at over 900 locations
globally and form an essential in-situ data source as a long-standing input to
weather forecasting models. Adding GNSS-RO capability to existing radiosonde
platforms would greatly expand capability, allowing for persistent and local
area monitoring, a feature particularly useful for hurricane and other severe
weather monitoring. A prohibitive barrier to entry to this inclusion is cost
and complexity as GNSS-RO traditionally requires highly specialized and
sensitive equipment. This paper describes a multi-year effort to develop a
low-cost and scalable approach to balloon GNSS-RO based on
Commercial-Off-The-Shelf (COTS) GNSS receivers. We present hardware prototypes
and data processing techniques which demonstrate the technical feasibility of
the approach through results from several flight testing campaigns.
