GIFS (Geostationary Imaging Fabry–Perot Spectrometer)
The GIFS instrument is a next-generation satellite concept within the IIP (Instrument Incubator Program) of NASA. The overall objective is to deploy GIFS on a geostationary satellite for continuous hemispheric imaging of cloud properties, including cloud top pressure, optical depth, fraction, and surface reflectance. GIFS uses an innovative tunable imaging triple-etalon FPI (Fabry–Perot Interferometer) to obtain images of high-resolution spectral line shapes of two O2 B-band lines in solar backscattered solar radiation. 1) 2) 3)
An airborne GIFS prototype and the measurement technique have been successfully demonstrated in a field campaigns onboard the NASA P3B based at Wallops Island, Virginia. The GIFS instrument team is based at JHU/APL (Johns Hopkins University Applied Physics Laboratory) and at UM (University of Michigan).
Measurement technique: The GIFS measurement technique is based on very high spectral resolution measurements of O2 Atmospheric band absorption of solar backscatter. The O2 Atmospheric A, B, and γ band transitions located at around 762, 685, and 637 nm, respectively, have absorption cross sections ideal for probing the atmospheric O2 density (and thus total density) in the Earth’s lower and middle atmosphere.
Calculations indicate that estimates of cloud top altitudes as well as optical depths can be retrieved from observations of the O2 Atmospheric bands using a moderate-resolution (0.5–6.0 nm) spectrometer. The accuracy of the estimates can be improved by measurements with higher spectral resolution and/or higher signal-to-noise ratio, where it is possible to characterize the spectral shape of an individual absorption line.
A concept for a spaceborne imaging FPI instrument, GIFS, has been developed to provide 2-D cloud properties from high–spectral resolution measurements of the O2 absorption line in solar backscattered radiation.
Figure 1: Calculated line shapes for a single O2 B-band absorption line for a series of clouds of different cloud top pressures (image credit: JHU/APL)
Legend to Figure 1: The cloud backscattered solar radiance for clouds with tops at various pressure levels is produced using line parameters from the HITRAN database and using the DISORT radiative transfer algorithm. For each trace, a cloud layer thickness of 100 mb and an optical thickness of 5 were assumed.
The GIFS instrument design builds on the heritage of FPI (Fabry–Perot Interferometer) flown on the DE-2 (Dynamics Explorer-2) spacecraft (launch Aug. 3, 1981), of HRDI (High Resolution Doppler Imager) flown on the UARS spacecraft (launch Sept. 12, 1991), and of TIDI (TIMED Doppler Interferometer) flown on TIMED (launch Dec. 7, 2001). Only 3 spaceborne FPI instruments were built so far.
GIFS is the first spaceborne FPI instrument designed to piezo-scan all three etalons. GIFS will form spatially coherent images on the CCD detector and will therefore be the first instrument to combine a tunable triple-etalon FPI (and its high–spectral resolution) with a simultaneous spatial imaging capability.
The GIFS instrument consists of a two-axis scanning telescope, a tunable triple-etalon FPI with a CCD detector, and associated electronics. Light from the telescope or from calibration sources is selected by positioning a scene selector mirror (Figure 3). Light is collimated, and passed through a narrow-band filter wheel to select any of several O2 B-band transitions, spectral calibration lines, or incandescent light. The beam is expanded and passed through a set of three piezo-electrically tuned low-, medium-, and high-resolution etalons (LRE, MRE, HRE).
Figure 2: Block diagram of the GIFS instrument (image credit: JHU/APL)
Figure 3: Optical train of the GIFS instrument (image credit: JHU/APL)
Three etalons in series reduce the sidebands that would occur with a single etalon when trying to re-solve a narrow spectral region within an atmospheric continuum. The beam is finally focused onto a CCD detector providing a two-dimensional, spectrally filtered image of the scene. Stepping the etalon gaps in resonance and acquiring a CCD image at each step produces a high-resolution spectrum at each spatial pixel.
Coupling the FPI system with a two-axis scan system allows for acquisition of a mosaic of spectral images from a three-axis stabilized spacecraft. An embedded computer controls image collection, tuning/stepping of the etalons, and the pointing system. FPI science and instrument engineering data are sent to the spacecraft via a serial link for storage and downlink.
Table 1: Specification of the GIFS instrument
Telescope scan system: Two-axis scanning is accomplished with a baseline wedge scanner approach developed for MOLA (Mars Observer Laser Altimeter), GLAS (Geoscience Laser Altimeter System) and the lidar system on JIMO (Jupiter Icy Moons). The wedge scanner uses counter-rotating refractive wedges to deflect the line of sight through any desired range of angles about two axes.
Imaging FPI: The imaging Fabry–Perot optical system consists of a triple-cavity filter (~4 cm-1 FWHM), a set of three 150 mm Spectrosil-B etalons, and a CCD detector. The gaps between each etalon are optimized to minimize the amount of white light leaked from the higher-order transmission. All three etalons have a 0.90 reflectivity ZnS–ThF4 coating at 680 nm, giving rise to a reflectivity finesse of ~30. A system finesse of 20, a conservative estimate to include optical defects, results in an instrument resolution of ~0.05 cm-1. The gaps of these three etalons are piezo-electrically controlled using the new patented capacitive feedback scheme of MAC (Michigan Aerospace Corporation - US Patent #60/268789), and they are individually tuned so that all three etalons have maximum transmission at the same frequency. The three well-tuned and optically aligned/parallel etalons along with a filter effectively attenuate the background continuum outside the narrow transmission peak.
GIFS forms a 2-D image on the detector, with each pixel mapping to a different geographical footprint. Each pixel has a peak transmission at a different resonance wavenumber following a concentric Fabry–Perot fringe pattern. The difference in the resonant wavenumber from the center pixel to another spatial pixel is determined by its incident angle to the etalon plates. For GIFS, the maximum plate incident angle is 1.345º (0.95º×0.95º etalon angular divergence), corresponding to 3.99 cm-1 difference in resonance frequency between the center pixel and the edge of GIFS FOV. - In other words, operating under this imaging mode, a single GIFS acquisition produces a spectrally filtered image of a 2-D 3.6º×3.6º scene, and a high-resolution spectrum at each spatial pixel is accomplished by stepping the etalon gaps in resonance. The Fabry–Perot etalon system is placed in a vacuum housing and thermally controlled to minimize the thermal drift.
CCD detector: The GIFS CCD detector is a passively cooled, back-thinned, 1024 x 1024 frame transfer device, the E2V 4720. The quantum efficiency is ~0.88 at 680 nm. The reimaging optics will focus the instantaneous scene onto this CCD, which will be binned on-chip to produce a 512 x 512 pixel image. The Fabry–Perot image will be read out at 1 Hz using a 12 bit A/D converter with a read noise of ~10 electrons.
Spectral scanning technique: For cloud parameter retrievals, one needs to obtain, at every image pixel, a spectral profile of an O2 absorption line spanning a spectral range of at least 2 cm-1 with a spectral sampling interval of <0.05 cm-1 (Figure 4).
Stepping the interferometer over its tuning range scans the wavenumber seen by different pixels over nearly identical ranges. However, at a given scan position the peak wavenumber varies with radius from CCD center, so there is a 4 cm-1 shift of the scan from the CCD center to the corner. For a given pixel, a 2 cm-1 tuning range centered on the O2 line visible through the shifted filter bandpass is required. A pair of O2 lines separated by 1.97 cm-1 is targeted so that every pixel scans the required interval over at least one of the lines with an etalon scan range of 3.2 cm-1. Sampling at a 0.025 cm-1 interval, each spectral scan requires 128 steps.
Figure 4: Illustration of the FPI scan technique (image credit: JHU/APL)
Legend to Figure 4: The upper three color panels represent the wavenumber resonant across the CCD for the first, middle, and last etalon steps of the 128 step scan. The lower two plots show the etalon resonance (narrow peaks) for the same three steps and their relation to the filter bandpass (black curves) for the center (center plot) and corner (lower plot) pixels. The upper plot shows the O2 spectrum, with dotted lines indicating the scan ranges of the center and corner pixels. These two pixels correspond to the extreme etalon incidence angles and therefore the extreme limits of the scan ranges.
GIFS operations concept:
GIFS is designed to map cloud properties with 4.4 km resolution over a 120º disk of the Earth (60º in zenith from the geostationary position of the instrument). Coverage can be achieved using a mosaic of these 2-D, 512 x 512 pixel images. Each pixel would project to a 4.4 km square at the Earth, and the Earth disk will be covered by 25 images in a 5 x 5 mosaic. Given the scanning scheme described, one entire mosaic takes ~1 hour to acquire.
When concentrating on limited areas for regional studies allows for a much faster mosaic acquisition. For example, North America can be covered by a 2 x 2 mosaic in ~10 minutes.
Airborne instrument incubator prototype:
Within IIP of NASA, an airborne GIFS prototype was built to demonstrate the instrument performance on an aircraft. The prototype instrument is providing high-spectral resolution measurements of individual O2 absorption lines to obtain 2-D maps of cloud top pressure and optical depth.
Like the flight GIFS, the prototype is an imaging triple-etalon system with approximately 0.05 cm-1 spectral resolution. A few significant differences from the flight system are incorporated for cost savings and to adapt to the different environment of the aircraft. First, the etalon diameters are reduced to 50 mm from 75 mm to save cost. The narrowband pre-filter diameter is reduced accordingly. Second, the exposure time is reduced to 0.2 s from 1.0 s to avoid excessive image smearing from aircraft motion. These two changes reduce signal level by a factor that is made up for by increasing the solid angle viewed by a pixel, so that the overall signal expected from the prototype is comparable to the flight system.
The spatial resolution of the scene still remains better than the planned flight instrument since the aircraft will be much closer than a geostationary satellite to the cloud deck. Divergence angles at the etalon are kept similar to the flight version so that the prototype sees comparable aperture finesse effects. Third, the detector is an off-the-shelf CCD camera, which saves the packaging and fabrication cost of an in-house detector system that has already been flight proven. Fourth, the scan system and telescope planned for the flight GIFS are omitted from the prototype. The scene is viewed directly through the narrowband filter, with imaging ultimately performed by the fringe imaging optics.
Figure 5: Photo of the optical bench of the GIFS prototype instrument (image credit: JHU/APL)
Legend to Figure 5: Light enters through the scene selector and housing (A) and passes through the narrowband filter (B) and beam expander (C). Resonance attenuators (D x 2) suppress inter-etalon reflections, and fold mirrors (E x 2) turn the path through the etalons (F x 3). Reimaging optics (G) image the scene and the fringe pattern on the camera (H).
Figure 6: Photo of the environmental chamber of the GIFS prototype instrument (image credit: JHU/APL)
The GIFS airborne instrument was completed in the fall of 2007. Laboratory testing and calibration was conducted at APL both before and after an aircraft field campaign on the NASA P3B. The GIFS prototype instrument was integrated onto the P3B and flown on a series of test flights from Wallops Island, VA, in the first half of February 2008.
The P3B flew at an altitude of 6–7 km along and just off the Atlantic coast, imaging scenes ranging from cloud-free to completely cloudy over both land and ocean surfaces. A NASA/LaRC video camera was co-manifested and used to image the scene below the aircraft and aid in the analysis of the GIFS data.
Significant portions of the science flights were flown along the CALIPSO and Aura satellite ground tracks for further opportunities for comparison (Table 2). The CALIPSO lidar provides active measurements of cloud properties and morphology that are important for the validation of the GIFS retrieval.
Table 2: Summary of early GIFS flights on the P3B aircraft
1) Jeng-Hwa Yee, Robert DeMajistre, William H. Swartz, M. Frank Morgan, John D. Boldt, Wilbert R. Skinner, Michael C. Pitts, Chris A. Hostetler, “Using Fabry–Perot Interferometer Imagery from Space for the Measurement of Clouds and Trace Gases,” ESTC2008 (Earth Science Technology Conference 2008), June 24-26, 2008, College Park, MD, USA, URL: http://esto.nasa.gov/conferences/estc2008/papers/Yee_Jeng-Hwa_B4P1.pdf
2) Jeng-Hwa Yee, M. Frank Morgan, R. DeMajistre, William H. Swartz, Elsayed, R. Talaat, James F. Garten, Wilbert R. Skinner, “Geostationary Imaging Fabry–Perot Spectrometer (GIFS),” 6th Annual NASA Earth Science Technology Conference (ESTC2006), June 27-29, 2006, College Park MD, USA, URL: http://esto.nasa.gov/conferences/estc2006/papers/a8p1.pdf
3) Jeng-Hwa Yee, M. F. Morgan, Robert DeMajistre, Elsayed, R. Talaat, James F. Garten, William H. Swartz, Wilbert R. Skinner, “Geostationary Imaging Fabry-Perot Spectrometer (GIFS),” Proceedings of SPIE, 'Instruments, Science, and Methods for Geospace and Planetary Remote Sensing ,' Eds. Carl A. Nardell, Paul G. Lucey, Jeng-Hwa Yee, James B. Garvin, Vol. 5660,, 2004, pp.14-22, Honolulu, HI, USA
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.