Frequency Comb Spectrometer for Satellite Atmospheric Remote Sensing (FCS-SARS)

The dual frequency-comb spectrometer has recently emerged as a novel instrument to support precise retrievals of atmospheric trace gas column densities over long paths without systematic drifts. In this work, we propose the development of dual comb spectroscopy (DCS) for satellite measurements to 1) perform active-source integrated vertical column measurements to ground stations, which will substantially improve coverage in the high and low latitudes and provide ultrahigh-resolving-power spectra for TCCON and OCO-2 calibration and 2) perform active satellite-to-satellite occultation measurements to obtain vertical profiles of gases in the upper troposphere and stratosphere. Specifically, we propose to develop a dual-comb spectrometer in the 2.1 um region for measurements of CH4, 13CH4, CO2, C18OO, 13CO2, H2O, N2O, and O3, as well as temperature and line-of-sight wind. When incorporated into a satellite mission, this novel instrument could yield data on the integrated columns and vertical profiles of these gases and atmospheric parameters, which in turn could support global transport model validation, contribute to greenhouse gas measurements and source attribution, and finally monitor the recovery of the stratospheric ozone layer and the ozone hole. Additionally, the column measurements will enable a cross-calibration of satellite missions such as OCO-2 and GOSAT with TCCON and the WMO standard. Our proposed instrument is designed for active vertical column DCS as well as satellite-to-satellite DCS, as highlighted in a recent Keck Institute for Space Science report. DCS rests on the Nobel-prize winning technology of frequency comb lasers, whose collimated output comprises a set of evenly spaced narrow comb "teeth" covering a very broad spectrum. In DCS, these comb teeth are transmitted through the air and the resulting absorption is "read out" on a comb tooth-by-tooth basis with high accuracy and negligible instrument lineshape. This results in a resolving power of >10,000,000 -- orders-of-magnitude higher than achievable with a Fourier Transform or grating spectrometer. The sample point spacings of ~200-MHz (0.006 cm-1) can easily fully resolve narrow molecular features at high altitudes. Moreover, unlike conventional spectrometers, this exquisitely narrow instrument lineshape is not fundamentally linked to the instrument size. In fact, the receiver consists only of a telescope and photoreceiver, so that DCS could provide precise column gas measurements in a cost-effective and scalable approach. Finally, because DCS uses an active light source, rather than the sun, it can provide information at high/low latitudes and during night, unlike nadir-looking scattered-sunlight instruments. In this program, we will 1) develop the DCS instrument at the 2.1 um spectral region, selected to overlap with the above listed critical gas species and current technology, 2) evaluate its operation over a 35-km open-path from the Maua Loa to Mauna Kea observatory, 3) conduct a trade study of orbit and satellite system configuration, and finally 4) evaluate the feasibility of future DCS satellite missions to support Earth science applications. A satellite-based DCS system would increase NASA's measurement capability for several Explorer-level Targeted Observables: Greenhouse Gases, and Ozone and Trace Gases. The proposed missions will allow for global and regional CO2 and methane trends over the seasonal and multi-year scales and would enable vertical profiles on the regional and global scales for the Explorer level trace gases H2O, N2O, O3, and CH4, as called for in the 2017 NASA Earth Science Decadal Survey. We propose a period of performance of 18 months. While we have operated DCS in the field at 1.6-um band for ground-based CO2/CH4 measurements (TRL level 5), the translation to the 2.1-um region and future satellite-based measurements is a new technology with an entry TRL level of 2 and exit TRL level of 4.

Increase scientific understanding of natural phenomena using remote sensing

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