Early evaluation of effects on Dual-frequency Precipitation Radar observations by the orbit boost of the GPM Core Observatory

The NASA and the JAXA performed orbit boost maneuvers in November 2023 that raised an altitude of the Global Precipitation Measurement (GPM) Core Observatory from 400 km to 435 km to extend its lifetime. Effects of the orbit boost on the spaceborne precipitation radar have been investigated in the Tropical Rainfall Measuring Mission (TRMM) performed in August 2001. This study evaluates effects on DPR observations due to the GPM orbit boost. Firstly, spacecraft altitudes of the GPM Core Observatory were analyzed during the period from 13rd October to 17th November 2023. The minimum altitudes were changed from about 400 km to about 435 km by the orbit boost. The averaged altitudes were changed from about 407 km to about 442 km by it. Thus, 407km and 442km were adopted as typical averaged satellite altitudes in pre-boost and the post-boost, respectively. Spatial resolution at the nadir and swath width is changed at 5.04km5.04km and 255.8 km at satellite altitude of 407 km to 5.48km5.48km and 277.9 km at satellite altitude of 442 km, respectively. Distances between adjacent footprints in the cross-track direction between the pre-boost and the post-boost using observation data and they confirmed that changes of the sampling were larger in the cross-track direction (about 5 km to 5.5 km at the nadir). It was found that the DPR coverage tendency was changed by the GPM orbit boost. In pre-boost, DPR achieved 100% coverage in 8 days. On the other hand, with post-boost, the coverage was still 99.9834% after 24 days, slightly less than 100%. This coverage trend is expected to change with satellite maneuvers. The maneuver is expected to change the orbit elements, thereby covering all locations. The sensitivity degradation of the DPR is expected owing to the increase of satellite altitude. Measured radar reflectivity factor (Zm) at storm top height (STH) over the ocean for is used as an indicator of the sensitivity. With analyzing Zm at STH over the ocean, the sensitivity degradation was found for about 0.8-0.9dB for KuPR, and about 0.7-0.9dB for KaPR.