A first approach towards dual-hemisphere sea ice reference measurements from multiple data sources repurposed for evaluation and product intercomparison of satellite altimetry

Sea ice altimetry currently remains the primary method for estimating sea ice thickness from space, however, time series of such satellite-derived estimates are of limited use without having been quality-controlled against reference measurements. Such reference measurements (a term encapsulating in situ observations and remotely sensed measurements from ground, air, and below the ice) for validation of altimetry measurements over sea ice in the polar regions are sparse and rarely presented in a manner where the time-space averaging matches that of the satellite-derived products. Here, an approach to a published comprehensive collection of sea ice reference measurements repurposed for satellite altimetry observations over sea ice is presented, which includes estimates of freeboard, thickness, draft and snow depth from sea ice-covered regions in the Northern Hemisphere (NH) and the Southern Hemisphere (SH), all of which are relevant for comparison with altimetry estimates. The measurements have been collected using airborne sensors, autonomous drifting buoys, moored and submarine-mounted upward-looking sonars, and visual observations. The data package has been prepared to match the spatial (25 km for NH and 50 km for SH) and temporal (monthly) resolutions of conventional satellite altimetry-derived sea ice thickness data products for a direct evaluation of these, and the code is publicly available and distributed for users to modify depending on their aim. This data package, also known as the Climate Change Initiative (CCI) sea ice thickness (SIT) Round Robin Data Package (RRDP), was produced within the ESA CCI Sea Ice project. The current version of the CCI SIT RRDP covers the polar satellite altimetry era (1993–2024) and has ongoing efforts aimed at continuously updating the datasets. The CCI SIT RRDP has been collocated with satellite-derived sea ice thickness products from CryoSat-2, Envisat, and ERS-1/2 produced within the ESA CCI and the Fundamental Data Records for Altimetry (FDR4ALT) projects to demonstrate the overlap and inter-comparison between the reference measurements and satellite-derived products. Here, the CCI SIT RRDP is introduced along with examples of its use as a validation source for satellite altimetry products, where the averaging, collocation and uncertainty methodology is presented, and advantages and limitations are discussed. The CCI SIT RRDP dataset is available at https://doi.org/10.11583/DTU.24787341 (Olsen and Skourup, 2026a).