The NASA ISS-RapidScat Mission (Invited)

After NASA's QuikSCAT satellite stopped spinning on November 2009, an observational gap opened in the international ocean surface winds constellation of observing satellites that also includes EUMETSAT's ASCAT and ISRO's OSCAT. While QuikSCAT continues to provide calibration data to the ISRO OSCAT on OceanSat-2 scatterometer, these data are sorely limited due to the satellite's hampered capabilities. Recognizing this gap, NASA has put together in a matter of a few months the ISS-RapidScat mission, which is a partnership between JPL, the ISS, ESA, KSC, and SpaceX. This mission, expected to launch in spring of 2014, is a very low-cost mission, enabled by the creative use of spare parts and a very rapid development process that nevertheless has great benefit to the science and operational weather and marine support communities. Since it uses the QuikSCAT engineering model, the RapidScat data will be quite similar to QuikSAT's in terms of data quality and spatial resolution, although modest gains in the latter are foreseen. However, due to the lower orbit and inclination, the RapidScat swath will be approximately a factor of two smaller than QuikSCAT's, and its geographic coverage will be limited to latitudes smaller than about 55 deg. Nevertheless, the unique sampling capabilities of the ISS non-sun-synchronous orbit opens up new science applications not available for typical sun-synchronous scatterometers. Foremost among these, is the ability to provide many more data that are collocated in space and time with each of the satellites in the international scatterometer constellation. Sun-synchronous satellites typically see each other with a suitably small temporal separation at high latitudes, and therefore, cross-calibration is limited in terms of the conditions that occur. RapidScat's orbit enables coincident wind observations in nearly every orbit, with a global geographical distribution, which will enable the determination of the global patterns of wind biases between all satellites in the constellation. Of great interest as well will be the ability of RapidScat to observe both the diurnal and semi-diurnal wind cycles. Previously, the diurnal cycle had been partially observed by combining scatterometer pairs, but its seasonal and yearly changes had not been characterized. Due to relative biases, the semi-diurnal cycle has not been observed from space. RapidScat will also be able to observed the diurnal cycle over land, which will improve the knowledge of scatterometer calibration targets and complement soil moisture and vegetation missions. Finally, although it has a smaller swath than QuikSCAT's, RapidScat's data will serve as a useful complement to the winds acquired from other platforms. This may be of significant advantage when covering rapidly developing systems, such as tropical hurricanes.