Enhanced Spatial & Temporal Sampling of Air/Sea Interaction in Tropical Cyclones by the NASA CYGNSS Mission

The NASA Cyclone Global Navigation Satellite System (CYGNSS) is a new spaceborne mission scheduled for launch in 2016 that is focused on tropical cyclone (TC) inner core process studies. CYGNSS addresses the deficiencies with current TC intensity forecasts that result from inadequate observations and modeling of the inner core. The inadequacy in current observations results from two causes: 1) Much of the inner core ocean surface is obscured from current spaceborne remote sensing instruments by intense precipitation in the eye wall and inner rain bands. 2) The rapidly evolving genesis and intensification stages of the TC life cycle are poorly sampled in time by conventional polar-orbiting, wide-swath surface wind imagers. CYGNSS is specifically designed to address these two limitations by combining the all-weather performance of GNSS-R bistatic ocean surface scatterometry with the enhanced sampling properties of a constellation of satellites. CYGNSS consists of a constellation of eight small observatories carried into orbit on a single launch vehicle. The eight satellites comprise a constellation that flies closely together to measure the ocean surface wind field with unprecedented temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of wind speeds experienced in a TC.

CYGNSS will provide surface wind measurements of the TC inner core that could not previously be measured from space. Mission simulations predict a median(mean) revisit time of 3(6) hours. The CYGNSS wind fields, when combined with as-frequent precipitation fields (e.g. produced by the upcoming Global Precipitation Measurement mission), will resolve the evolution of both the precipitation and underlying wind fields throughout the TC life cycle. They will provide near simultaneous and continuous observations and enable new insights into TC inner core dynamics and energetics.

The 8 CYGNSS observatories will fly in 500 km circular orbits at a common inclination of ~35°. Each observatory includes a Delay Doppler Mapping Instrument (DDMI) consisting of a modified GPS receiver capable of measuring surface scattering, a low gain zenith antenna for measurement of the direct GPS signal, and two high gain nadir antennas for measurement of the weaker scattered signal. Each DDMI is capable of measuring 4 simultaneous bi-static reflections, resulting in a total of 32 wind measurements per second across the globe by the full constellation.

The use of a constellation of satellites results in spatial and temporal sampling properties that are markedly different from conventional imagers. Simulation studies will be presented which examine the sampling as functions of various orbit parameters of the constellation. Historical records of TC storm tracks are overlaid onto a simulated time series of the surface wind sampling enabled by the constellation. For comparison purposes, a similar analysis is conducted using the sampling properties of several past and present conventional spaceborne ocean wind scatterometers. Differences in the ability of the sensors to resolve the evolution of the TC inner core will be examined. The signal-to-noise ratio of the measured scattered signal and the resulting uncertainty in retrieved surface wind speed are also examined.

The CYGNSS observatories are currently in Phase B development. In parallel with the hardware development, a detailed end-to-end simulator has been constructed in software that is designed to reproduce the expected measurements made on orbit by CYGNSS and, in particular, when it overflies TCs. The simulator includes realistic nature run models of the TC, realistic representations of the orbital geometries and measurement configurations of the GPS transmitting satellites and the CYGNSS receiving satellites, physically based electromagnetic scattering models for the surface interaction, and simulations of the on-board signal processing performed by CYGNSS to form the Level 1 science data products – the Delay Doppler Maps (DDM). Wind speed retrieval algorithms are in development to make full use of the information contained in the DDMs.

An update on the current status of the mission will be presented, with emphasis given to the wind speed retrieval algorithm development and its predicted performance, and to the expected spatial and temporal sampling properties of the retrieved winds.