Stratospheric Wind Interferometer For Transport Studies Research Articles

Scientific Assessment of the SWIFT Instrument Design

Abstract The Stratospheric Wind Interferometer for Transport Studies (SWIFT) is a proposed satellite instrument. SWIFT is an imaging field-widened Doppler Michelson interferometer. It observes a thermal IR atmospheric emission line in a limb-viewing geometry in order to measure stratospheric winds and stratospheric ozone concentration profiles with global coverage during both day and night. SWIFT has the capability of improving the knowledge of the dynamics of the stratosphere and global distribution of and global transport of ozone. The target wind and ozone accuracies are 3 m s−1 and 5%–10%, respectively. The instrument is a follow up to the highly successful Canada–France Wind-Imaging Interferometer (WINDII) instrument on NASA's Upper Atmosphere Research Satellite (UARS). To assess the suitability of the method of Doppler imaging Michelson interferometry for the measurement of stratospheric wind and ozone using the SWIFT instrument, a scientific assessment of the instrument performance was undertaken using forward and inverse modeling and error analyses. This paper is aimed at determining the technical and scientific feasibility of the SWIFT instrument and its ability to meet the science requirements. This paper also briefly describes the SWIFT experiment, the data retrieval algorithms, and technical challenges in stratospheric wind measurements. Meeting the wind accuracy requirement imposes tight requirements on instrument thermal stability, filter monitoring, and determination of reference phase calibration. The SWIFT instrument design shows a strong level of dependence on the knowledge of atmospheric N2O concentration. The presence of N2O as an interfering species degrades the SWIFT performance at all altitudes with the largest impact especially for altitudes below 30 km.

Design requirements for the SWIFT instrument

The Stratospheric Wind Interferometer for Transport studies (SWIFT) instrument is a proposed limb-viewing satellite instrument that employs the method of Doppler Michelson interferometry to measure stratospheric wind velocities and ozone densities in the altitude range of 15–45 km. The values of the main instrument parameters including filter system parameters and Michelson interferometer parameters are derived using simulations and analyses. The system design requirements for the instrument and spacecraft are presented and discussed. Some of the retrieval-imposed design requirements are also discussed. Critical design issues are identified. The design optimization process is described. The sensitivity of wind measurements to instrument characteristics is investigated including the impact on critical design issues. Using sensitivity analyses, the instrument parameters were iteratively optimized in order to meet the science objectives. It is shown that wind measurements are sensitive to the thermal sensitivity of the instrument components, especially the narrow filter and the Michelson interferometer. The optimized values of the main system parameters including Michelson interferometer optical path difference, instrument visibility, instrument responsivity and knowledge of spacecraft velocity are reported. This work also shows that the filter thermal drift and the Michelson thermal drift are two main technical risks.

Integration of Spatial Heterodyne Spectroscopy with the Stratospheric Wind Interferometer For Transport studies (SWIFT)

The Doppler Michelson Interferometer (DMI) approach developed for atmospheric wind measurement was first flown as the WIND Imaging Interferometer (WINDII), launched on NASA's Upper Atmosphere Research Satellite in 1991. WINDII obtained global winds over the altitude range of 80–300 km, using airglow emission as a Doppler target. The same concept was proposed for the Stratospheric Wind Interferometer For Transport studies (SWIFT), destined for flight on the CSA Chinook mission. SWIFT was intended for measurement of winds in the altitude range of 20–45 km where the only available emission for Doppler measurement is thermal radiation from minor constituents. An ozone line at about 8.8 µm was adopted as the target. The challenge is to isolate this line from other lines in this complex spectrum to measure its phase with sufficient accuracy. The demands on the filter fabrication and the knowledge of its characteristics in flight, including thermal drifts, became a major consideration. Spatial Heterodyne Spectroscopy (SHS) also uses a Michelson interferometer but with the mirrors replaced by diffraction gratings; this produces a spatial interferogram across an imaging detector for a narrow spectral band. By using a longer path difference in one arm, as is done in DMI, in a configuration called DASH (Doppler Asymmetric Spatial Heterodyne), it is possible to measure the phases of each of these lines in the band separately, which significantly reduces the demands on filter fabrication and knowledge. A development model is now being implemented within the Centre for Research in Earth and Space Science at York University (CRESS); the concept is described and a report on progress is provided.

Onboard calibration and monitoring for the SWIFT instrument

The SWIFT (Stratospheric Wind Interferometer for Transport studies) instrument is a proposed space-based field-widened Doppler Michelson interferometer designed to measure stratospheric winds and ozone densities using a passive optical technique called Doppler Michelson imaging interferometry. The onboard calibration and monitoring procedures for the SWIFT instrument are described in this paper. Sample results of the simulations of onboard calibration measurements are presented and discussed. This paper also discusses the results of the derivation of the calibrations and monitoring requirements for the SWIFT instrument. SWIFT's measurement technique and viewing geometry are briefly described. The reference phase calibration and filter monitoring for the SWIFT instrument are two of the main critical design issues. In this paper it is shown that in order to meet SWIFT's science requirements, Michelson interferometer optical path difference monitoring corresponding to a phase calibration accuracy of ∼10−3 radians, filter passband monitoring corresponding to phase accuracy of ∼5 × 10−3 radians and a thermal stability of 10−3 K s−1 are required.

On the feasibility of a fast forward model for Doppler interferometry in the infrared

AbstractA fast forward radiative transfer model to compute the emitted limb radiances for use in recovering line‐of‐sight Doppler winds from thermal line emission has been developed and evaluated. This study was formulated around the Stratospheric Wind Interferometer for Transport Studies (SWIFT) instrument—a limb imaging, field widened, phase‐stepping Michelson interferometer. The fast forward model is required to simulate a four‐point interferogram from which line‐of‐sight winds may be computed. The model, RT‐SWIFT (Radiative Transfer for SWIFT), is part of an effort to develop the capability of providing simultaneous measurements of horizontal wind velocity vectors and ozone concentration in the stratosphere for improving our understanding of global stratospheric dynamics and for studies in ozone transport.Based in part on RT‐MIPAS (Radiative Transfer for the Michelson Interferometer for Passive Atmospheric Sounding), RT‐SWIFT uses a linear regression algorithm to parametrize the effective layer optical depths and can simulate the effect of variable ozone, nitrous oxide, water vapour and atmospheric winds. Trace gases are included as fixed climatological profiles.The model development involved the selection of two suitable databases consisting of appropriate atmospheric absorber and wind profiles, and the accurate line‐by‐line modelling of their transmittances; one for generating regression coefficients and one consisting of simulated measurements for independent evaluation. The development also required selecting suitable predictors for the absorbers, winds and viewing geometry.One‐dimensional inversion results using RT‐SWIFT with simulated measurements indicate that the model leads to wind and ozone error levels of about 3 to 5 m s−1 and 3 to 15% respectively for altitudes above ∼24 km. While further investment in model development and optimization is warranted, this demonstration study indicates that a sufficiently accurate fast forward model for near real‐time assimilation, and inversion, of horizontal Doppler wind measurements from thermal line emission is feasible. © 2011 Crown in the right of Canada. Published by John Wiley & Sons Ltd.

Laser spectroscopic study of ozone in the 100←000 band for the SWIFT instrument

A complete spectroscopic study of 15 strong ozone lines in the 1132.5–1134.5 cm −1 spectral range has been undertaken in the framework of the development of the stratospheric wind interferometer for transport studies (SWIFT), led by the Canadian Space Agency. Measurements have been performed with an interferometrically stabilized tunable diode laser spectrometer. Absolute line positions and intensities have been determined with high accuracy (4×10 −5 cm −1 and 1–2% respectively). Self- and air-broadening coefficients at 296 K have been obtained with an accuracy of 1%. The air-shifting coefficient and its temperature dependence have also been measured for unblended lines together with the temperature dependence of the air-broadening. Line intensities have been calibrated by simultaneously performed UV absorption measurements at 253.7 nm. Our IR/UV comparison supports a previously reported inconsistency between recommended IR intensities (HITRAN08) and UV absorption cross-sections and indicates that current IR intensities are too small by ∼3%.

Satellite Measurement of Stratospheric Winds and Ozone Using Doppler Michelson Interferometry. Part I: Instrument Model and Measurement Simulation

Abstract This paper presents an instrument model and observation simulations for the measurement of stratospheric winds and ozone concentration using a satellite instrument employing imaging and the Doppler Michelson interferometery technique. The measurement technique and instrument concept are described. The instrument model and simulations are based on initial design characteristics of the Canadian Stratospheric Wind Interferometer for Transport Studies (SWIFT) satellite instrument. SWIFT employs an imaging array and a field-widened Michelson interferometer. It will measure stratospheric winds and ozone densities using the wind-induced phase shifts of interferograms from atmospheric limb radiance spectra in the vicinity of the vibration–rotation ozone line at 1133.4335 cm−1. The measurement simulation and analysis tools have been developed to assess the SWIFT instrument performance and to evaluate the impact of instrument and measurement characteristics on expected wind and ozone errors. Sample results of the measurement simulation and the related line-of-sight wind error noise levels are presented.

Satellite Measurement of Stratospheric Winds and Ozone Using Doppler Michelson Interferometry. Part II: Retrieval Method and Expected Performance

Abstract This paper is about the retrieval of horizontal wind and ozone number density from measurement simulations for the Stratospheric Wind Interferometer for Transport Studies (SWIFT). This instrument relies on the concept of imaging Doppler Michelson interferometry applied to thermal infrared emission originating from the stratosphere. The instrument and measurement simulations are described in detail in the first of this series of two papers. In this second paper, a summary of the measurement simulations and a data retrieval method suited to these measurements are first presented. The inversion method consists of the maximum a posteriori solution approach with added differential regularization and, when required, iterations performed with the Gauss–Newton method. Inversion characterization and an error analysis have been performed. Retrieval noise estimates have been obtained both from derived covariance matrices and sample inversions. Retrieval noise levels for wind and ozone number density of ∼1–3 m s−1 and <1% have been obtained over the altitude range of 20–45 km with Backus–Gilbert resolving lengths of ∼1.5 km. Retrieval noise levels over the extended altitude range of 15–55 km are less than 10 m s−1 and 2%. The sensitivity to other error sources has been examined through a few sample realizations. The contributions from these other errors can be as important as or more so than retrieval noise. An error budget identifying contributing wind and ozone error levels to total errors of 5 m s−1 and 5% for altitudes of 20–45 km has been prepared relying on the retrieval errors and knowledge of the instrument design.

Performance evaluation of a thermal Doppler Michelson interferometer system

The thermal Doppler Michelson interferometer is the primary element of a proposed limb-viewing satellite instrument called SWIFT (Stratospheric Wind Interferometer for Transport studies). SWIFT is intended to measure stratospheric wind velocities in the altitude range of 15-45 km. SWIFT also uses narrowband tandem etalon filters made of germanium to select a line out of the thermal spectrum. The instrument uses the same technique of phase-stepping interferometry employed by the Wind Imaging Interferometer onboard the Upper Atmosphere Research Satellite. A thermal emission line of ozone near 9 microm is used to detect the Doppler shift due to winds. A test bed was set up for this instrument that included the Michelson interferometer and the etalon filters. For the test bed work, we investigate the behavior of individual components and their combination and report the results.

The stratospheric wind interferometer for transport studies (swift)

Abstract The Stratospheric Wind Interferometer For Transport studies (SWIFT) is an instrument intended to measure winds to an accuracy of 5 m s −1 or better in the stratosphere, during both day and night, as well as ozone concentrations. It is based on WINDII, the WIND Imaging Interferometer on the UARS satellite, but there are a number of important differences. WINDII operated in the visible region, with widely-spaced airglow emission lines, a field-widened Michelson interferometer that uses glass combinations to provide thermal stability, and a CCD detector. SWIFT uses the thermal emission from an ozone line near 8.9 μm, a region in which the choice of refractive materials is very limited. Through a careful search for a suitable line several were found of appropriate strength that were adequately isolated, but only with a combination of etalon filters. Fortunately, HgCdTe array detectors are available so the detector is not a problem. By measuring both winds and ozone concentration it is possible to measure ozone fluxes. SWIFT will study ozone transport, transport across the sub-tropical mixing barrier, equatorial dynamics and data assimilation. The latter is an important tool for the execution of the scientific objectives.