*4.2. Sampling of a Landfalling Hurricane*

To illustrate the scientific implications of the sampling properties of an optimized constellation of GNSS-R satellites, a specific example is considered. Hurricane Ida made landfall along the U.S. Louisiana coast on 29 August 2021 at 1655 UTC. The measurements that would have been made on that day by two different GNSS-R constellations have been simulated and overlaid with GOES imagery of the storm. One constellation consists of 8 satellites arranged in a common orbit plane at an inclination of 30◦, similar to the CYGNSS configuration. The other constellation consists of 8 satellites in each of 3 orbit planes with RAANs of 0◦, 120◦ and 240◦ and a common 30◦ inclination. The global coverage with this configuration was shown in Figure 2. All spacecraft in both constellations are assumed to be capable of 16 simultaneous specular reflection measurements. Results of the simulation are shown in Figure 10. In the figure, four 6-hourly intervals of time are considered, consistent with the initialization time increment between operational numerical hurricane forecasts. All samples made during each 6 h interval are shown, together with the GOES image of the storm taken at the center of the four time intervals, namely at 0600, 1200, 1800 and 2400 UTC on 29 August.

In Figure 10, the right column corresponds to measurements made by the 24 spacecraft distributed over 3 orbit planes. In each 6 h interval, the storm is well sampled both in its inner core region and across the surrounding wind field in all four principle quadrants. Inner core measurements support the determination of maximum sustained winds and hurricane intensity, while coverage of the wider wind field supports the determination of storm size and duration during landfall. The left-hand column in Figure 10 corresponds to measurements made by the 8 spacecraft constellation. The storm is quite well sampled throughout the inner core and surrounding wind field during only one of the four 6 h intervals (1500–2100 UTC). In two of the other intervals, samples are made over only two of the four principle quadrants of the storm, with little of the inner core sampled in either case. The fourth 6 h interval has no samples whatsoever of the storm.

**Figure 10.** *Cont*.

**Figure 10.** Simulated measurements by two GNSS-R constellations of Hurricane Ida during landfall on 29 August 2021. The left column corresponds to an 8-satellite constellation and the right to one with 24 satellites, all at 30◦ inclination. Rows correspond to 6 h time intervals centered on 0600, 1200, 1800 and 2400 UTC. GNSS-R tracks are overlaid on GOES-16 Band 8 (6.17 μm) images at the center times. Landfall occurred at 1655 UTC.
