Improving geolocation and footprint characterisation of microwave radiometer observations using coastal crossings

Abstract. Spaceborne microwave radiometers (MWR) provide essential information on atmospheric humidity to enable wet tropospheric correction (WTC) in satellite altimetry. While WTC accuracy is generally high over the open ice-free ocean, it degrades in coastal areas due to land contamination in the radiometer's field of view. Correcting for this requires precise knowledge of the instrument's geolocation and spatial response characteristics.

This study presents a data-driven in-orbit approach to improve MWR geolocation and footprint characterisation for the Fundamental Data Record for Radiometry (FDR4RAD_V1), which constitutes the European Space Agency's (ESA) reference dataset of MWR observations from the ERS-1, ERS-2 and Envisat missions. By correlating observed brightness temperatures with effective land fractions at coastal transitions, we determine deviations between nominal and actual geolocation values, expressed as errors in along-track (ELON) and across-track (ECRO) direction, as well as footprint size, expressed as full width at half power (FWHP). A two-round iterative approach based on random-effects meta-analysis mitigates the effects of parameter interdependence and yields well-defined estimates with quantified uncertainties.

The analysis comprises more than 26,000 half-orbits sampled from ten repeat cycles per mission, spanning their combined full operational lifetimes (1991–2012). Substantial deviations between nominal and actual geolocations are observed for all instruments, with combined geolocation errors (ELON and ECRO via RSS) decreasing from 2.40 ± 0.07 km (ERS-1, 23.8 GHz) to 1.04 ± 0.08 km (Envisat, 23.8 GHz), indicating progressive improvements in positioning accuracy across missions. The standard errors of approximately 0.1 km, equivalent to an instrument pointing uncertainty of ~0.01°, reflect the precision achievable with this large-sample approach. For footprint size, the retrieved FWHP values range between 18.59 ± 0.04 km (Envisat, 36.5 GHz) and 21.78 ± 0.04 km (ERS-2, 36.5 GHz). Analysis of footprint geometry yields aspect ratios between 0.94 and 1.06, indicating small deviations from circularity. Temporal stability is confirmed for all mission–channel–parameter combinations except ELON for ERS-2 at 36.5 GHz, where a statistically significant trend coincides with documented gyroscope failures, revealing the method's sensitivity to platform changes.

The improved geolocation and footprint characterisation, provided for each instrument and channel, enables more accurate land contamination correction, enhancing wet tropospheric correction and sea level determination in coastal areas. The methodology is transferable to other near-nadir microwave radiometers, including those onboard the Sentinel-3 and Sentinel-6 missions, thereby supporting cross-mission consistency for multi-decadal sea-level records.