First Results of Grazing Angle GNSS-R Altimetry from Sea Ice and Ocean Surfaces Using the Spire CubeSat Constellation

Dallas Masters (Spire Global, Inc., United States)

CoAuthors

Vu Nguyen (Spire Global, Inc., United States); Takayuki Yuasa (Spire Global Singapore PTE Ltd., Singapore); Oleguer Nogués-Correig (Spire Global UK Ltd., UK); Linus Tan (Spire Global Singapore PTE Ltd., Singapore); Timothy Duly (Spire Global, Inc., USA)

Event: 2019 Ocean Surface Topography Science Team Meeting

Session: The Future of Altimetry

Presentation type: Type Oral

Spire Global, Inc. operates the world’s largest and rapidly growing constellation of cubesats performing GNSS based science and Earth observation. Currently, the Spire constellation consists of 75+ 3U cubesats, with 25+ satellites capable of performing a variety of GNSS science, including radio occultation, ionosphere measurements, and precise orbit determination. These satellites have been primarily tasked to perform radio occultation (RO) to produce accurate profiles of atmospheric temperature, pressure, and water vapor and to collect millions of daily ionospheric total electron content measurements. Previous work has shown that grazing angle reflections of GNSS signals off of ocean and sea ice surfaces serendipitously collected during radio occultation measurements had the potential for being developed into new Earth observations of sea surface heights and sea ice. These GNSS-R techniques using the phase observables from grazing angle reflections collected by orbiting RO receivers did not get much traction, most likely because the RO missions were either not capable of operationally collecting these reflections or that mission priorities precluded their collection in favor of conventional RO measurements.

Recently, with the launch of dedicated GNSS-R missions, such as TechDemoSat-1 (TDS-1) and the eight-satellite CYGNSS constellation, phase observations of grazing angle reflections have again garnered interest and shown that the technique can perform 10 cm-level precision altimetry over sea ice, lakes, and ice sheet surfaces. But these measurements have been limited to a few collections using the raw intermediate frequency (IF) modes on these satellites and have not been able to produce reliable statistics of the measurements, e.g., the likelihood of coherent reflections and their spatial and temporal distributions across various Earth surface types. To address this issue and to pursue an operational system for collection of grazing angle GNSS reflections, Spire recently reprogrammed its STRATOS GNSS science receiver to perform this measurement on currently orbiting RO satellites. To accomplish this, the open loop tracking used in RO collection was modified to perform open loop prediction and tracking of grazing angle reflections between 5-30 deg elevation. This software mode was subsequently uploaded to one orbiting satellite and tested successfully on the first attempt, acquiring 50 Hz in-phase and quadrature samples of GNSS reflections from sea ice and ocean surfaces. Initial results confirm coherency of reflections over sea ice surfaces and some open ocean surfaces. Full altimetric processing has been performed on a few cases, confirming the precision of the technique over sea ice and open oceans where reflections were coherent. In comparing the height estimates of in areas of sea ice to ice thickness estimates from SMOS, we noted similar gradients, supporting the theory that the L-band GNSS signals penetrate the sea ice and are mostly reflecting from the water-ice interface under the ice. This would make the technique a sensor of sea ice draft, which would potentially complement other sensors such as ICESat-2 and Cryosat-2 that sense the top layer of sea ice. A production period has now begun on multiple Spire satellites that will result in large quantities of diverse measurements from space in a relatively short time. We will present further results of this new and potentially revolutionary technique to use existing orbiting RO satellites to perform grazing angle GNSS-R altimetry.
 

Oral presentation show times:

Room Start Date End Date
The Forum Thu, Oct 24 2019,12:00 Thu, Oct 24 2019,12:15
Dallas Masters
Spire Global, Inc.
United States
dallas.masters@spire.com