Ultra-wideband RF photonics spectrometer for atmospheric boundary layer sensing

Most of the energy exchange in the atmosphere occurs in the atmospheric boundary layer (ABL) with respect to solar heating and evaporation. Remote sensing in the ABL from space is challenging because of its proximity to the surface and potential sharp gradients in air properties. Passive microwave instruments can be a critical component to a ABL observing system since they provide thermodynamic information in both cloudy and clear air, including cloud and precipitation properties, and surface flux information such as temperature and wind speed. However, conventional passive microwave systems fall well short of being optimized for near surface sensing due to limited number of spectral channels and coarse spectral resolution covering only a small portion of the spectrum of interest for ABL sensing.

The near surface thermodynamic structure information is encoded on the microwave spectrum between and on the shoulders of the water vapor absorption and oxygen lines near 183 GHz and 118 GHz respectively. The ultra-wideband photonic spectro-radiometer instrument is funded by NASA ESTO under ACT-20 program to combine low-noise wideband InP RF technology with a novel photonic integrated circuit (PIC) design for obtaining large bandwidth (>50 GHz) with enhanced channel resolution (<1 GHz). A high-speed, low-loss electro-optic modulator is used to convert radio frequency energy into sidebands on an optical carrier. The designed PIC includes an input star-coupler that divides the optical power transmitted from the optical modulator among N waveguides monotonically increasing in length within an arrayed waveguide grating (AWG) that provides chromatic dispersion, an output star-coupler that forms an image of the optical spectrum, and an array of photodiodes that convert the optical power to electrical signals. The combination of a low-noise wide-band RF radiometer with an RF Photonics backend system is a key technology development allowing unprecedented ability to spectrally resolve the complete microwave spectrum which is critically needed for the planetary boundary layer sensing. In this paper, we will describe the capabilities of this system for measuring the thermodynamic structure in the lower ~2km of the atmosphere which is not available with the current state of the art technology launched in the space.