Atmospheric Pressure Sounder

Photo of Atmospheric Pressure Sounder
  • Project Type: Project
  • Class: Instrument

The long-term goal of this activity is to demonstrate the feasibility of a laser sounder instrument capable of measuring the surface-pressure field for the entire air column from satellite-to-ground with global coverage.

The earth's surface pressure is a vital component of a variety of important scientific measurements, which are being undertaken at Goddard. Accurate knowledge of the surface pressure can enable calibration of 2-D measurements of CO2 content in the atmosphere and greatly improve the fidelity of surface water redistribution measurements from time-varying gravity fields. It is also important in weather prediction and atmospheric modeling.

The measurement approach uses differential absorption spectroscopy of O2 in the 770 nm wavelength region to derive atmospheric pressure. The planned instrument architecture would be a nadir-viewing laser instrument that analyzes the ground reflections from the earth. Applying DIAL to the oxygen absorption for monitoring pressure was pioneered at Goddard by Korb, et al. However, in contrast to previous laser-instruments for this application, our laser sounder instrument leverages new telecommunication laser-diode and fiber-optic amplifier technology that is low-cost, high power, high efficiency, high reliability and robust.

This technology allows a direct instrument development pathway to space flight deployment. Using the 770 nm oxygen feature in the peak of the silicon absorption for photo-detectors means we can use state-of-the-art, high quantum efficiency, photon counting receivers with spaceflight heritage like those used on the ICESat mission.

In addition, the active laser sounding technique has advantages over passive spectrometers in its high (MHz)spectral resolution and stability, the ability to measure at night and in daytime, a narrow measurement swath, and the ability to simultaneously detect and exclude measurements with clouds or aerosols in the path. O2 absorption lines are free from contamination from other gases and are within the tuning range of lasers. When averaging over 50 seconds, calculations show an SNR of> 1000 appears achievable for each on-and-off line measurement. Such a mission can furnish global maps of the lower tropospheric O2 column abundance at dawn and dusk. Global coverage with an accuracy of a few mbar appears achievable.

To demonstrate the feasibility of this instrument design,we will first characterize and optimize the laser transmitter for this application. Our transmitter consists of a fiber coupled, tunable, 1540 nm, DFB laser diode,amplified by an erbium-doped fiber amplifier (EDFA) and then frequency doubled to 770 nm. We are currently exploring the limits of power, pulse width,duty cycle, and spectral width and stability for different EDFAs. We will then optimize the frequency conversion efficiency. This technology development can also be applied to two-color frequency doubled systems for a normalized difference vegetation index lidar instrument. Once the laser transmitter has been demonstrated, it will be integrated into a laboratory-based laser sounding instrument to make oxygen measurements. The lab-based instrument is capable of measuring oxygen content in both a gas cell and a 300-meter atmospheric path. This allows us to fully calibrate the measurements as well as test the instrument capability in the atmosphere.                                                                                                                                                                                         

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