Unique Earth Surface Observations Using GNSS Bistatic Radar (GNSS-R) on Spire's Constellation of CubeSats

Spire Global, Inc. operates a large and rapidly growing constellation of CubeSats performing GNSS-based science and Earth observation. In a few short years, Spire has grown from a modest CubeSat kickstarter campaign to a paradigm-shifting provider of satellite data to NOAA, NASA, and other customers of Earth observations. Spire specializes in using science-quality observations of GNSS signals (e.g., GPS, GLONASS, Galileo, QZSS, etc.) to derive valuable information about the Earth environment. Currently, these observations include radio occultations to profile the neutral atmosphere with high accuracy and vertical resolution for applications such as NWP assimilation and climate monitoring, as well as to measure ionosphere slant total electron content and scintillation indices for space weather applications. As of August 2019, and after 20 deployments, Spire now has 84, 3U Cubesats satellites capable of performing a variety of GNSS science and with plans to grow the constellation to well over 100 operational and continuously replenished satellites.

Beginning in 2018, Spire began an effort to design and build the first of many GNSS bistatic radar (or GNSS-R) missions for Earth observations for a variety of applications, including soil moisture measurement, wetlands and flood inundation mapping, sea surface roughness and winds, and sea ice characterization. Following an agile model of rapid, iterative satellite development that has been refined over a few years to produce radio occultation payloads optimized for operation on ultra-small, 3U CubeSats, we adopted a very aggressive schedule to adapt the current Spire 3U bus and STRATOS GNSS science receiver to perform GNSS-R measurements, with an expected launch in December of 2019. We will discuss the goals of the 2019 Spire GNSS-R mission, the design and operational modes of the first batch of Spire GNSS-R satellites, and plans for an iterative design effort for the next batch of Spire GNSS-R satellites to be launched in 2020.

The Spire GNSS-R effort also has a parallel path that is already harnessing existing orbiting Spire satellites used for radio occultation to additionally perform grazing angle GNSS-R measurements for high-precision, phase-delay altimetry. This presentation will additionally introduce the unique experience of adapting the current constellation of radio occultation satellites to perform these new and valuable GNSS-R Earth observations. We will introduce the concept of phase-delay altimetry and its potential to estimate surface heights on the order of 10 cm using observations of coherent GNSS signals reflected from various Earth surfaces. We will summarize the agile steps Spire took to collect these observations on-orbit within just a couple of months of conceptualization, as well as the initial inversion technique to estimate surface reflector heights with high precision. We will show promising results of estimating sea ice draft and sea surface heights using this technique and discuss plans for further investigation and calibration/validation activities. Finally, we will discuss Spire’s potential to rapidly proceed with these measurements from research to operations and to make them available as a new set of Earth observations.