<jats:title>Abstract</jats:title><jats:p>Climate warming makes an increasing thin-ice fraction likely to occur in the Arctic, underpinning the need for its regular observation. synchronous helicopter-borne measurements of the sea-ice thickness and like-polarized L-band radar backscatter carried out along identical flight tracks north of svalbard during winter are combined to develop an algorithm to estimate the thin-ice thickness solely from the L-band backscatter co-polarization ratio (LCPR). Airborne ice-thickness and LCPR data are smoothed along track (to reduce noise), co-located and compared. A linear and a logarithmic fit are applied using thickness values between 0.0 and 0.6m and 0.0 and 1.0 m, respectively. The thin-ice thickness is derived from the LCPR data using these fits, first for dependent data (used to obtain the fits) and subsequently for independent data. The results are compared to airborne ice-thickness measurements for ice-thickness values between 0.0 and 0.6m using linear regression. The logarithmic fit gives the most reliable results, with a correlation of 0.72 and a rms difference of 8 cm. It permits us to derive the thickness of thin ice (below 50–60cm thickness) from airborne LCPR data with an uncertainty of about 10 cm.</jats:p>