REMOTE SENSING & BATHYMETRY // SIGNAL TO GRID

Mapping: Making Ice Data Legible

A colour-coded map of Arctic sea-ice thickness or seabed topography looks uniform and complete on a screen. Yet every pixel is an inversion of electromagnetic or acoustic travel times, dependent on assumptions about snow density, brine volume, and water-column sound speed.

ORBITAL ALTIMETRY // FREEBOARD TO THICKNESS

How Radar and Laser Altimeters Weigh Floating Ice

Roughly 88% to 90% of a floating sea-ice floe sits submerged beneath the waterline. Because neither radar waves nor optical lasers can penetrate salty sea ice and seawater to bounce off the bottom of a floe from space, satellites measure the small fraction sticking above the water—known as freeboard.

ESA’s CryoSat-2 carries a Ku-band (13.575 GHz) Synthetic Aperture Interferometric Radar Altimeter (SIRAL), which under cold winter conditions penetrates dry snow cover to reflect near the snow–ice interface (radar freeboard). NASA’s ICESat-2 uses green (532 nm) photon-counting laser beams that reflect off the top of the snowpack (total freeboard). By combining both satellites, researchers can estimate snow depth from space—or apply modified Warren climatologies—and multiply the freeboard by the density ratio of seawater (~1,024 kg/m³) to sea ice (~917 kg/m³ for first-year ice, ~882 kg/m³ for porous multi-year ice).

Soft-lit 3D render of a surface sea-ice mass balance beacon and acoustic snow-depth pingers on a plain background

FIG 4.1 — Sea-Ice Mass Balance Thermistor Chain & Sonic Pinger Illustration

ERROR PROPAGATION // WHERE UNCERTAINTY LIVES

Why Ice Thickness Maps Stop During Summer Melt

Between May and September, liquid meltwater coats snow grains and pools in surface depressions. Wet snow blocks radar penetration, while bright specular reflections from both melt ponds and open leads confuse waveform retrackers.

Snow Depth Bias

The Largest Error Term

A 10 cm unmodelled snow layer changes hydrostatic equilibrium enough to skew derived ice thickness by 30 to 50 cm. Airborne EM bird surveys (Operation IceBridge / AWI IceBird) provide the ground truth needed to bound this error.

SAR Sea-Ice Charts

Sentinel-1 & RADARSAT

C-band Synthetic Aperture Radar measures surface roughness and dielectric backscatter (sigma-nought), enabling national ice services (MET Norway, CIS, DMI) to draw tactical polygon charts for icebreakers.

Upward-Looking Sonar

Moored Ice Profiling Sonar

Submerged moorings (such as the Beaufort Gyre Exploration Project) ping upward at 420 kHz every 2 seconds, measuring ice draft directly to ±5 cm accuracy as the pack drifts overhead.

Soft-lit 3D render of a hull-mounted multibeam bathymetric sonar array on a plain background

FIG 4.2 — 1° × 1° Swath Multibeam Echosounder Array Illustration

SEABED TOPOGRAPHY // IBCAO & SEABED 2030

Mapping Ridges and Troughs Beneath the Pack

Deep Atlantic inflow follows bathymetric contours along the continental slope, while sills across the Lomonosov and Gakkel Ridges control how abyssal waters exchange between basins. Yet because icebreakers cannot steer straight lawnmower survey lines through heavy pack ice, over 80% of the Arctic Ocean floor in the International Bathymetric Chart of the Arctic Ocean (IBCAO Version 4.0, gridded at 200 × 200 m) relies on interpolation between sparse ship tracks, declassified submarine soundings, and satellite gravity anomalies.

Furthermore, acoustic rays bend whenever water temperature or salinity changes with depth. Without regular Expendable Bathythermograph (XBT) or CTD sound-velocity profiles, outer beams of a multibeam swath curl upward or downward ("smile" or "frown" artifacts), misplacing depth contours by tens of metres at 4,000 m depth. Every IBCAO release therefore publishes a companion Type Identifier (TID) grid so researchers can verify whether a feature was directly ensonified or mathematically inferred.

See How Maps Guide Conservation