Abstract. The Global Navigation Satellite System (GNSS) radio occultation (RO) technique has proven to be an effective tool for Earth atmosphere profiling. Traditional spaceborne RO satellite constellations are expensive with relatively low sampling rates for individual satellites. Airborne RO platforms can provide much higher spatial and temporal sampling of ROs around regional weather events. This paper explores the capability of a low-cost and scalable Commercial-Off-The-Shelf (COTS) GNSS receiver onboard high-altitude balloons. The refractivity retrievals from balloon-borne RO payloads obtained from two flight campaigns (World View and ZPM-1) are presented. The balloon-borne RO soundings from the World View campaign show high-quality refractivity profiles in the troposphere with near-zero median difference (~2.3 % median-absolute-deviation) from the colocated ECMWF ERA5 reanalysis data. Soundings from the ZPM-1 campaign show a relatively large positive bias (~2.5 %). In summary, the low-cost COTS RO payloads onboard balloon platforms are worth further improvement for dense targeted atmospheric soundings to improve regional weather forecasts.

Abstract. The Global Navigation Satellite System (GNSS) radio occultation (RO) technique has proven to be an effective tool for Earth atmosphere profiling. Traditional spaceborne RO satellite constellations are expensive with relatively low sampling density for specific regions of interest. In contrast, in-atmosphere RO platforms can provide much higher spatial and temporal sampling of ROs around regional weather events. This study explores the capability of a low-cost and scalable commercial off-the-shelf (COTS) GNSS receiver on board high-altitude balloons. The refractivity retrievals from balloon-borne RO payloads obtained from two flight campaigns (World View and ZPM-1) are presented. The balloon-borne RO soundings from the World View campaign show refractivity profiles between 6 and 19 km, with overall near-zero median difference from colocated ECMWF ERA5 reanalysis data and variability comparable to spaceborne RO missions (∼ 2.3 % median absolute deviation or MAD). Soundings from the ZPM-1 campaign show a relatively large positive refractivity bias (∼ 2.5 %). In summary, low-cost COTS RO payloads on board balloon platforms are worth further engineering and study in order to provide capabilities for dense, targeted atmospheric soundings that can improve regional weather forecasts via data assimilation.

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GNSS radio occultation (RO) leverages GNSS satellites rising and setting on the horizon to extract refractivity in the troposphere and the ionosphere. Traditionally, highly specialized scientific instruments have been deployed on low-Earth-orbiting (LEO) platforms to collect GNSS-RO soundings where the first LEO GPS-RO experiment was performed in 1995 by the Microlab 1 satellite (Ware et al., 1996). Subsequently, there have been numerous successful missions that utilize GNSS signals for atmospheric and ionospheric soundings (Yue et al., 2011). The joint U.S.-Taiwan Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) was launched in 2006 and represents the first dedicated LEO constellation of six satellites employing GNSS-RO techniques (Anthes et al., 2008;Kumar, 2006). The success of COSMIC demonstrated the operational value of these soundings in weather prediction, space weather monitoring, and geodesy.

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.