Measuring GNSS Radio Occultation (GNSS-RO) Profiles Using High Altitude Balloons

GNSS-RO is a remote sensing technique in which a GNSS receiver located on an airborne or space-based platform measures the changes in the phase of the GNSS signal, as they pass through the atmosphere. Specifically, during what is known as an occultation event, signals from a GNSS satellite that is rising or setting behind Earth's atmospheric limb (horizon) are bent due to refraction in the Earth's atmosphere. Measuring the Doppler shift of these signals allows us to estimate the bending angle and refractivity of the Earth's atmosphere, which in turn can be used to arrive at vertical temperature, density and pressure distribution profiles in the Earth's atmosphere. Since 1995, space-based GNSS-RO has made significant impacts on weather forecasting, climate prediction, and space weather research. However, it is difficult to remote sense the planetary boundary layer from space-based ROs due to geophysical and technological limitations (e.g., low signal-to-noise ratio, atmospheric ducting, and antenna gain loss). In addition, space-based RO systems lack the agility to provide on-demand RO data for instance in areas with impending natural disasters. The high cost and time associated with launching LEO constellations also makes them unsuitable for adapting to the ever-changing network of GNSS constellations or incorporating new advances in receiver technology.

In this paper, we propose a balloon-based platform with COTS (commercial off-the-shelf) receivers for carrying out GNSS-RO measurements. We derive the instrument specifications from science measurement requirements and explain the design of our receiver along with the platform developed for hosting the payload on high altitude balloons. We show preliminary results from our ground-based validation campaign, a controlled hot-air balloon launch and a high altitude helium balloon launch. We also discuss the lessons learned in terms of changes that have to be made to our system in terms of receiver performance, platform stability etc. to fully deploy a network of balloon-borne receivers for science-worthy GNSS-RO measurements in the future.