*4.3. Wind Speed*

This variable presents gaps in the revisit time (<3 h) and latency time (<1 h). The required revisit time is 3 h at 10-km spatial resolution, with an accuracy of 0.5 m/s. Typical resolutions for microwave radiometer winds are about 25 km [15] or 12.5 km [16] over oceans; the measurement range is 0–50 m/s [17], and the accuracy is from about 2–10 m/s (depending on the rain flag) [18]. Microwave radiometers infer wind speed from frequencies near 6.8, 10.7, 19 and 37 GHz. Another technology that can be used to measure wind speed is SAR. The current Copernicus infrastructure has a constellation of two SARs, but the revisit time is between one and two days for high latitudes and the high resolution dataset (1 km) [19]. Another solution is to infer the wind vector using radar scatterometers, with a 10-km spatial resolution over the oceans.


**Table 3.** Mapping of the potential technologies to cover measurements with gaps. The technology limitations from the perspective of the measurement requirements are presented.

*<sup>a</sup>* Marginal relevance; *<sup>b</sup>* medium relevance; *<sup>c</sup>* high relevance .

The use of novel techniques using Signals of Opportunity (SoOp), such as those from Direct Broadcast Satellite (DBS) television signals at the Ku- or X-band [33], can be potentially exploited in the future to measure precipitation and winds over the sea surface at higher revisit times. These signals are potentially sensitive to detecting fluctuations of the sea surface roughness and light precipitation. As compared to GNSS-R systems, the spatial resolution will be better (higher frequency) and the Signal-to-Noise Ratio (SNR) higher as transmitters transmit more power. In this regard, a receiver of signals of opportunity at the Ku- or X-band can provide cost benefits and high quality data, but these techniques have yet to be developed.
