*Article* **On the Reliability of Surface Current Measurements by X-Band Marine Radar**

**Katrin G. Hessner 1,\*, Saad El Naggar 2, Wilken-Jon von Appen <sup>2</sup> and Volker H. Strass <sup>2</sup>**


**\*** Correspondence: hessner@oceanwaves.de; Tel.: +49-4131-699-5822

Received: 22 February 2019; Accepted: 18 April 2019; Published: 30 April 2019

**Abstract:** Real-time quality-controlled surface current data derived from X-Band marine radar (*MR*) measurements were evaluated to estimate their operational reliability. The presented data were acquired by the standard commercial off-the-shelf MR-based *sigma s6* WaMoS® II (WaMoS® II) deployed onboard the German Research vessel *Polarstern*. The measurement reliability is specified by an *IQ* value obtained by the WaMoS® II real-time quality control (*rtQC*). Data which pass the *rtQC* without objection are assumed to be reliable. For these data sets accuracy and correlation with corresponding vessel-mounted acoustic Doppler current profiler (ADCP) measurements are determined. To reduce potential misinterpretation due to short-term oceanic variability/turbulences, the evaluation of the WaMoS® II accuracy was carried out based on sliding means over 20 min of the reliable data only. The associated standard deviation σ*WaMoS* = 0.02 m/s of the mean WaMoS® II measurements reflect a high precision of the measurement and the successful *rtQC* during different wave, current and weather conditions. The direct comparison of 7272 WaMoS® II/ADCP northward and eastward velocity data pairs yield a correlation of *r* ≥ 0.94, with |*bias*Δ| ≤ 0.06 m/s and σ*<sup>S</sup>* = 0.05 m/s. This confirms that the MR-based surface current measurements are accurate and reliable.

**Keywords:** X-Band radar; marine radar current measurement; quality control; measurement reliability; accuracies; precision; WaMoS® II; vessel mounted acoustic Doppler current profiler
