Instrument Architecture of the Advanced Ultra-high Resolution Optical Radiometer (AURORA) Pathfinder

Space-borne microwave and millimeter-wave radiometers have been the workhorses of Earth remote sensing providing critical global vertical profiles of temperature and water vapor that inform our understanding of upper atmospheric physics. The 2017 National Academie’s Earth science decadal survey has identified Earth’s planetary boundary layer (PBL) as an important observable critical to understanding local as well as meso-scale atmospheric physics that influence the Earth’s weather and climate. Radiometers are needed to meet the requirements of PBL remote sensing as well, while continuing to provide temperature and water vapor profile of the upper atmosphere for context. Successfully implementing space-based passive sensing for PBL science requires instrument sensitivity deep into the atmospheric layers near the planet’s surface. Due to limitations of conventional RF architectures and signal detection schemes it is a difficult engineering challenge to optimally develop a space-borne radiometer that can profile as well as provide sensitivity to the PBL. Complex multi-frequency instruments would thus be required so that dedicated narrow band channels can separately satisfy the needs of atmospheric profiling and PBL sensing.

Recent advances in photonics and application specific integrated circuit (ASIC) technologies have opened the door to ultra-wideband RF architectures in the micro- and millimeter-wave frequency bands that can simultaneously profile the upper atmosphere while being sensitive to PBL atmosphere. Such architectures now make it possible to retrieve PBL height and water vapor profiles with global coverage and moderate temporal resolution from space-borne platforms. The Advanced Ultra-high Resolution Optical Radiometer (AURORA) instrument is a pathfinder technology demonstration project funded by NASA’s Earth science technology office (ESTO). The main goal of this project is to mature the technology readiness level (TRL) of RF-photonics and high-speed ASIC based technologies incubated at JPL, GSFC and private industry partners to TRL-6, and to showcase their benefits for atmospheric remote sensing with emphasis on PBL sensing. The AURORA pathfinder project will be implemented by a combined team from NASA’s Jet Propulsion Laboratory (JPL) and the NASA Goddard Space Flight Center (GSFC). The millimeter-wave radiometer developed as part of the AURORA pathfinder project is a wide-band instrument spanning the molecular absorption spectrum from 115 through 185 GHz, thus simultaneously covering both the 118 GHz oxygen line, the 183 GHz water vapor line absorption line as well as the absorption continuum in between. In fact, radiances detected in the absorption continuum frequency regions (also called as the window regions) from 125-175 GHz are signatures of temperature and water vapor in the PBL and provide the means for retrieving various structural information such as height and water vapor content therein.

In this presentation we will showcase the overall design strategy and capabilities of the AURORA instrument, and performance of the various subsystems. Wide band-width coverage near the absorption lines (113-125 GHz and 164-185 GHz) at as low as 0.5 MHz frequency resolution is made possible with high speed ASICs uniquely designed for such high-speed spectrometry. These ASICs can be combined to allow up to 18 GHz of signal digitization on both the upper- and lower-sidebands of the two absorption lines. The ultra-wideband coverage of up to 50 GHz between 125 to 175 GHz is implemented with a common millimeter-wave front-end features wide-band antenna optics, and low noise amplifiers. The RF-photonics subsystem implemented down the chain after the RF front-end is a significant departure and upgrade from conventional heterodyning architectures and is one of the main novelties in the AURORA instrument design. Using an elctro-optical mixer the RF energy is modulated onto an optical carrier signal. This optical carrier is then split into 1-4 GHz wide sub-bands in a waveguide grating chip and detected with photo-detectors. The benefits of this scheme become apparent when one compares the simplicity of this single modulation scheme to the multi-component heterodyning scheme that would otherwise be required to convert such a wide-band signal to base-band for digitization. The calibration challenges of both the ASIC and photonic signal chain and their mitigation will also be discussed in this presentation.