Airborne Scanning Microwave Limb Sounder
Science background measurement needs
The Upper Troposphere and Lower Stratosphere (UT/LS, the region from ~10–20 km altitude) plays several important roles in Earth’s atmosphere. It is in this region where radiatively active species such as water vapor (the strongest greenhouse gas) and ozone exhibit steep vertical gradients and large spatial and temporal variability, and where changes in their abundances strongly influence global climate. Lofting of near-surface air by deep convection substantially modulates the UT/LS, and the fast winds and long chemical lifetimes characterizing this region facilitate global pollution transport. Further, the UT/LS is the gateway through which air enters the stratosphere, a region that will continue to undergo severe ozone destruction so long as anthropogenic halogen levels remain high. In addition, long-term trends and considerable but poorly understood variability in lower stratospheric water vapor significantly affect surface temperature and the ozone layer.
Continued improvements of our understanding of processes affecting the UT/LS and the impacts of these processes demands both:
- A continued long-term record of daily near-global vertically observations of atmospheric composition profiles, extending the highly valuable observational datasets from instruments such as the Microwave Limb Sounder (MLS) on NASA’s Upper Atmosphere Research Satellite (UARS, 1991–2000) and Aura (2004–) spacecraft.
- Select high resolution observations to improve our understanding of key smaller-scale processes that influence UT/LS composition and its future evolution in a changing climate.
The Airborne Scanning Microwave Limb Sounder instrument (A-SMLS) serves both to test and prove next-generation technologies needed to economically meet need #1, while also providing measurements aligned with need #2.
The Microwave Limb Sounding approach
In situ sampling and (for some molecules) lidar instruments can provide atmospheric composition measurements with the vertical resolution needed. However, for spaceborne instruments that can achieve global coverage, the only means to make measurements with the needed resolution is to view the atmospheric limb (i.e., look at the atmosphere “edge on”). This viewing geometry provides not only the needed vertical resolution, but also a longer path length through the atmosphere, giving a stronger signal for tenuous trace gases. Additionally, limb sounding in the microwave brings an additional advantage over limb sounding at shorter wavelengths (e.g., infrared, visible and ultraviolet) in that the observations are unaffected by aerosols and by all but the thickest clouds. This is especially important in the tropical upper troposphere where clouds are prevalent, and the probability of encountering a “cloud free scene” a very low, particularly when looking essentially horizontally through the atmosphere. A-SMLS is an airborne follow on to the UARS and Aura MLS instruments that measure a wealth of species from low-earth orbit with the daily near-global coverage needed to track key processes affecting the upper troposphere and stratosphere and provide unbiased measures of long term variability.
The years since the Aura launch have seen dramatic increases in technology capabilities in the radio frequency signal processing arena, mainly driven by advances in the communications industry. A-SMLS is a path finder for future MLS-class instruments capitalizing on these technologies.
Most notable among these advances is the new digital spectrometer technology, which enables replacement of the ~600 individual analog channels in Aura MLS with a small number (no more than 12 needed with current technology/needs) of CMOS chips each measuring 4096 individual channels with improved accuracy and greatly simplified calibration requirements. In addition, new submillimeter Low Noise Amplifier technology enables development of microwave receivers that perform as well as Aura MLS at room temperature but, when cooled (e.g., to ~50K), offer dramatic (8-fold or better) improvements in signal to noise. This in turn enables dramatic reductions in the integration time needed to accomplish a given signal to noise, potentially enabling future MLS-like instruments to scan in azimuth as well as elevation, providing true “3-D tomography” of the atmospheric limb.
The Airborne Scanning Microwave Limb Sounder instrument
The Airborne Scanning Microwave Limb Sounder (A-SMLS) is a pathfinder for such instruments, using the latest generation receivers and spectrometers, to demonstrate these technologies in advance of them being proposed to future spaceflight opportunities. In addition to this pathfinder role, A-SMLS can provide high spatial resolution measurements of UT/LS composition needed to improve our understanding of key processes in this region including:
- Convective uplift of air and associated trace gases from the surface to the upper troposphere
- Long range transport of pollution plumes (including those uplifted by the convective transport described above)
- Transport of air from the troposphere to the stratosphere and vice versa
Table 1 details the key A-SMLS capabilities and parameters.
A-SMLS was originally developed under NASA’s Instrument Incubator Program and flew on the WB-57 aircraft in July and September 2012. This version of the instrument used 4 K-cooled superconducting microwave receivers, requiring a liquid Helium-filled dewar. The instrument was subsequently adapted for the NASA ER-2 aircraft, on which it flew in March 2015.
A-SMLS is currently being updated to use new 340 GHz microwave receivers that can operate both at room temperature, or cooled to 50 K using a mechanical cooler (removing the previous need for cryogen for 4-K operation). Test flights of this new configuration are planned for Summer/Fall 2018.
Table 1: Key A-SMLS parameters.
Vertical profiles of atmospheric composition from ~8–20 km altitude
Primary target gases:
Ozone (O3), Water vapor (H2O) and Carbon Monoxide (CO)
Additional observable species:(a)
HNO3, N2O, H2CO, ClO, HOCl, BrO, HO2, OH, CH3CN, HCN, CH3Cl, CH3OH, SO2, Temperature, Cloud Ice
Sideband separating (I/Q frontend, analog or digital separation)
2 splitters, 4(b) bandpass filter / attenuator / amplifier chains
Currently: two 3-GHz wide digital spectrometer System on Chip devices with analog sideband separation.
Plan to augment with two wideband digital polyphase spectrometers including sideband separation in the digital domain.
Signal to noise:
1700 K per sideband or better system noise temperature at ambient.
Completely programmable. Capable of scanning ±30° horizontally either side of the flight direction, with a ±4° vertical scan.
Horizontal scan gives a 300 km-wide swath from ER-2 flight altitudes.
Accommodated in fore- and mid-body of NASA ER-2 aircraft wing pod.
(a) Currently, with the 3-GHz wide spectrometers, observations of some of these molecules require minor
instrument retuning. With the planned-for wideband spectrometers, all molecules would be observed
Science lead: Nathaniel Livesey
Instrument lead: Robert Stachnik