How can GNSS-R altimetry from the Spire constellation complement institutional altimetry missions?

Spire Global operates a constellation of smallsats equipped with an advanced GNSS receiver designed to collect radio occultation observations and reprogrammed to detect surface reflections. Nguyen et al. [2020] demonstrated the feasibility of phase delay altimetry using coherent reflections, following a number of prior studies (e.g., Martín-Neira [1993], Cardellach et al. [2004]). Since then, Level 2 grazing angle altimetry products have been generated on an operational basis. As of January 2023, over 20 satellites continuously measure GNSS-R grazing angle reflection events in areas of high-coherence, i.e., calm waters and glaciated surfaces, yielding nearly 500,000 kms of altimetry tracks every day. The vertical resolution is on the order of decimeters, as demonstrated by comparisons against reference surface models composed of an MSS and ocean tides. 

 

A growing number of studies have utilized these datasets to examine ocean surfaces, sea ice, ice caps, and inland water bodies. An overview of these scientific applications enabled by Spire’s products is provided, along with a description of the advantages and current disadvantages of GNSS-R altimetry as observed after nearly three years of operations. 

 

We will show a comparison between the cryospheric products from ICESat-2 and CryoSat-2 against Spire products with a focus on sea ice. The Spire constellation provides improved temporal and spatial coverage, due GNSS-R's bistatic geometry and the range of receiver orbits. The retrievals show particular sensitivity in the smoother young-ice surfaces. Decimeter-level sea ice signals are routinely detected and validated with SAR imagery. The Spire sea ice product could complement existing cryospheric products, particularly in light of the potential gap in measurements before the launch of CRISTAL. We will also discuss ongoing development to several aspects of the algorithm such as phase unwrapping and systematic, polar-wide sea ice freeboard extraction.

 

References

 

Cardellach, E., Ao, C. O., De la Torre Juárez, M., & Hajj, G. A. (2004). Carrier phase delay altimetry with GPS-reflection/occultation interferometry from low Earth orbiters. Geophysical Research Letters, 31(10), L10402.

Martín-Neira, M., (1993), A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry, ESA J., vol. 17, no. 4, pp. 331–355.

Nguyen, V. A., et al. (2020), Initial GNSS Phase Altimetry Measurements From the Spire Satellite Constellation, Geophys. Res. Letters, vol. 47, no. 15.