Comparison of the Effective Isotropic Radiated Power Parameter in CYGNSS v2.1 and v3.0 Level 1 Data and Its Impact on Soil Moisture Estimation

Abstract

AbstractThe effective isotropic radiated power (EIRP) is the measured radiated power of an antenna pointed in a specific direction. For the Global Positioning System (GPS), the EIRP is a function of the transmitted power and the gain of the transmitting antenna. It is a fundamental observation used for estimating surface reflectivity that can be used to estimate near-surface soil moisture. Most investigations of GPS EIRP for soil moisture used level 1 version 2.1 data from the eight satellites of the Cyclone Global Navigation Satellite System (CYGNSS) mission. The newer version 3.0 introduces a dynamic EIRP calibration algorithm with the variations in GPS transmit power being tracked using the direct signal power measured by the navigation receivers. In this paper we compare the estimated EIRP from versions 2.1 and 3.0 for the year of 2020. We correlate the estimated surface reflectivity with reference soil moisture observations from the Soil Moisture Active Passive (SMAP) mission provided on a 9x9 km grid using the bistatic radar equation for coherent reflections. The correlation of CYGNSS with SMAP is slightly improved using version 3.0 versus 2.1 with average of 0.10 and maximum of 0.30. The advantage of version 3.0 was most noticeable in areas where soil moisture retrieval is challenging, such as the arid and densely vegetated regions of the world.