Long-term measurements of the global distributions of clouds, trace gases, and surface reflectance are needed for
the study and monitoring of global change and air quality. The Geostationary Imaging Fabry-Perot Spectrometer
(GIFS) instrument is an example of a next-generation satellite remote sensing concept. GIFS is designed
to be deployed on a geostationary satellite, where it can make continuous hemispheric imaging observations of
cloud properties (including cloud top pressure, optical depth, and fraction), trace gas concentrations, such as tropospheric
and boundary layer CO, and surface reflectance and pressure. These measurements can be made with
spatial resolution, accuracy, and revisit time suitable for monitoring applications. It uses an innovative tunable
imaging triple-etalon Fabry-Perot interferometer to obtain very high-resolution line-resolved spectral images of
backscattered solar radiation, which contains cloud and trace gas information. An airborne GIFS prototype and
the measurement technique have been successfully demonstrated in a recent field campaign onboard the NASA
P3B based at Wallops Island, Virginia. In this paper, we present the preliminary GIFS instrument design and
use GIFS prototype measurements to demonstrate the instrument functionality and measurement capabilities.
Long-term measurements of the global distribution of clouds and the surface reflectance are needed to provide inputs to climatological models for global change studies. The Geostationary Imaging Fabry-Perot Spectrometer (GIFS) instrument is a next-generation satellite concept, to be deployed on a geostationary satellite for continuous hemispheric imaging of cloud properties, including cloud top pressure, optical depth, fraction, and surface reflectance. This is an ideal approach to make these cloud property measurements with desired spatial resolution, accuracy, and revisit time. It uses an innovative tunable imaging triple-etalon Fabry-Perot interferometer to obtain images of high-resolution spectral line shapes of two O<sub>2</sub> B-band lines in the backscattered solar radiation. The GIFS remote sensing technique takes advantage of the pressure broadening information embedded in the absorption line shapes to better determine cloud properties, especially for those clouds below 5 km. We present a preliminary instrument design, including the general instrument requirements.
Space-based observation of tropospheric trace species has been identified as a high-priority atmospheric science goal. In particular, global and regional measurements of lower atmosphere ozone concentrations are critical to both enhancing scientific understanding and to expanding capabilities for pollution monitoring. The interferometer addressed here will be a spatially imaging, spectrally tunable airborne sensor focused on making such important tropospheric ozone measurements, and is designed to be a risk-reduction and proof-of-concept test-bed for developing the corresponding orbiting instrument also based upon a dual etalon Fabry-Perot interferometer. We present herein details of the airborne
instrument design and development process, including parameter specifications for the interferometer and other enabling subsystems, as well as plans for integration, test, and characterization in the laboratory.
The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds in the mesosphere and thermosphere (60-180 km) as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm and rotational lines in the O<sub>2</sub>(0-0) Atmospheric band. Wind measurement accuracies approach 3 ms<sup>-1</sup> in the mesosphere and 15 ms<sup>-1</sup> in the thermosphere. The TIDI instrument’s performance during the first year and a half of operation is discussed in this paper. Many subsystems are working as designed. The thermal control system is holding the instrument temperatures at
their desired set-points. The CCD detector is working as expected with no changes observed in the gain, bias or read noise. The instrument suffers from a light leak that causes the background to be elevated and increases the uncertainty in the wind measurement. Nothing can be done to eliminate this problem but modeling of the background has eliminated any systematic effect. Water outgassing from the spacecraft or instrument has deposited as ice on some part of the optics and reduced the instrument’s sensitivity. This
problem has been reduced by two spacecraft rolls which pointed the TIDI radiator to view more of the earth causing the optics to warm up and sublimate much of the ice.
The High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) has been measuring winds in the stratosphere, mesosphere and lower thermosphere since November, 1991. The winds are determined by measuring the Doppler shift of emission and absorption lines in the O<sub>2</sub> Atmospheric Band that are located between 630 and 762 nm. HRDI is a triple-etalon Fabry-Perot interferometer that has a resolution of ~0.05 cm<sup>-1</sup> and very good white light rejection. A multi-channel detector with 31 channels is used to examine a spectral region 0.5 cm<sup>-1</sup> wide and an adjustable filter wheel permits the selection of any one of 13 spectral bands. The long life of this instrument has presented many challenges in keeping the calibrations current and in compensating for inevitable degradations in instrument and spacecraft performance. Some of the problems with the UARS spacecraft the affect HRDI operations are: limited power due to the solar array drive failure; loss of data resulting from a failure of the tape recorders, and loss of attitude knowledge caused by the failure of the star trackers. HRDI has shown little loss in capability over the years
with only a decrease in the azimuth rate of the telescope motor a significant sign of aging. This paper will discuss some of these challenges and how they have been met.
Monitoring tropospheric chemistry from space is the next frontier for advancing present-day remote sensing capabilities to meet future high-priority atmospheric science measurement needs. Paramount to these measurement requirements is that for tropospheric ozone, one of the most important gas-phase trace constituents in the lower atmosphere. Such space-based observations of tropospheric trace species are challenged by the need for sufficient horizontal resolution to identify constituent spatial distribution inhomogeneities (that result from non-uniform sources/sinks and atmospheric transport) and the need for adequate temporal resolution to resolve daytime and diurnal variations. Both of these requirements can be fulfilled from a geostationary Earth orbit (GEO) measurement system. The Tropospheric Trace Species Sensing Fabry-Perot Interferometer (TTSS-FPI) was recently selected for funding within NASA’s Instrument Incubator Program (IIP). Within this project we will develop and demonstrate a multispectral imaging airborne system to mitigate risk associated with an advanced atmospheric remote sensor intended for geostationary based measurement of tropospheric ozone and other trace species. The concept is centered about an imaging Fabry-Perot interferometer (FPI) observing a narrow spectral interval within the strong 9.6 micron ozone infrared band with a spectral resolution ~0.07 cm<sup>-1</sup>. This concept is also applicable to and could simplify designs associated with atmospheric chemistry sensors targeting other trace species (which typically require spectral resolutions in the range of 0.01 - 0.1 cm<sup>-1</sup>), since such an FPI approach could be implemented for those spectral bands requiring the highest spectral resolution and thus simplify overall design complexity. The measurement and instrument concepts, approach for development and demonstration within IIP, and a summary of progress-to-date will all be reported.
The High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite has been providing measurements of the wind field in the stratosphere, mesosphere, and lower thermosphere since November 1991. Mesospheric temperatures, ozone and O(<SUP>1</SUP>D) densities, and stratospheric aerosol extinctions coefficients, are also retrieved. The instrument characteristics have been carefully monitored by frequent calibrations during the nearly eight years of operation. The instrument sensitivity showed a significant decrease (close to 50% in some cases) during the first seven and a half years of operation which was caused by the piezoelectric-controlled etalons slowly drifting from a parallel state. A recalibration of the etalons in late 1998 resulted in close to a complete recovery of the instrument sensitivity. The loss of sensitivity was linear with time, with discrete changes occurring at times. Careful modeling of the data permits a determination of the sensitivity as a function of time, allowing the data to be corrected for this systematic effect.
The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds, temperatures, and constituents in the mesosphere and thermosphere (60 - 300 km) region of the atmosphere as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm, OI 630.0 nm, OII 732.0 nm, O<SUB>2</SUB>(0-0), O<SUB>2</SUB>(0-1), Na D, OI 844.6 nm, and OH in the spectral region 550 - 900 nm. Wind measurement accuracies will approach 3 ms<SUP>-1</SUP> in the mesosphere and 15 ms<SUP>-1</SUP> in the thermosphere. The TIDI instrument has several novel features that allow high measurement accuracies in a modest-sized instrument. These include: an optical system that simultaneously feeds the views from four scanning telescopes which are pointed at plus or minus 45 degrees and plus or minus 135 degrees to the spacecraft velocity vector into a high-resolution interferometer, the first spaceflight application of the circle-to-line imaging optic (CLIO), and a high quantum efficiency, low noise CCD.
The high resolution doppler imager (HRDI) on the upper atmosphere research satellite has been providing measurements of the wind field in the stratosphere, mesosphere and lower thermosphere since November 1991. Mesospheric temperatures, ozone and O(<SUP>1</SUP>D), as well as stratospheric aerosol extinctions, are also recovered. The instrument characteristics have been carefully monitored during the nearly five years of operation. The instrument thermal and long-term drifts can be removed from the data, and wind biases are less than about 2 m/s. The interferometer sensitivity has varied by about 3 percent, most likely due to changes in the parallelism of one of the etalons. There is not indication that either the radiator or thermal blankets have shown any significant degradation. Recently, the azimuth slew rate of the telescope has displayed some variation, which may indicate an increase of bearing friction.
Doppler lidars can be separated into two main categories by the detection technique used, coherent or incoherent. This paper focuses on the differences between the two types of Doppler lidar and the effect of speckle on each. Coherent Doppler lidars make use of a local oscillator heterodyning system similar in principle to a Doppler radar to determine Doppler shift while incoherent lidars typically use an interferometric technique to resolve the small spectral shift. The effect of laser speckle on lidar measurements in general has become a topic of great interest in recent years because in many cases it limits the ultimate resolution or accuracy of the measurement. In the case of Doppler lidars, speckle has very different effects on coherent and incoherent Doppler detection systems. Due to the single-shot nature of coherent lidar measurements and the narrow field of view required, many shots of data are lost due to speckle. The larger fields of view used for incoherent Doppler lidar as well as the inherent multiple pulse averaging make speckle significantly less of a problem for these systems.
An incoherent (direct detection) Doppler lidar is developed that operates in the middle of the visible spectrum and measures wind and aerosol profiles during the day and night from the planetary boundary layer to the lower stratosphere. The primary challenge of making a lidar measurement in the visible spectrum during daylight hours is the strong presence of background light from the sun. To make a measurement of this type, the laser line must be isolated spectrally to the greatest extent possible. This has been accomplished through the use of a multiple étalon Fabry-Pérot interferometer in combination with a narrow-band filter. The incoherent technique and system are a modified version of the Fabry-Pérot interlerometer and image-plane detector technology developed for an earlier Doppler lidar developed at the University of Michigan and for the High-Resolution Doppler Imager (HRDI) now flying on the Upper Atmosphere Research Satellite. The incoherent Doppler analysis is discussed and sample measurements are shown. Winds are measured in the boundary layer with 100-m vertical resolution and 5-mm temporal resolution with 1 to 3 m s<sup>-1</sup> accuracy.
Horizontal winds in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O<SUB>2</SUB> atmospheric band. The validity of the measurements depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind induced Doppler shift. These positions have been determined to an accuracy of better than 5 ms<SUP>-1</SUP> from the comparison of winds measured by HRDI with those obtained by MF radars and rockets. In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effect both on the amplitude of the HRDI derived diurnal tidal amplitude and the variance of the wind differences with correlative data.
A conceptual space-based incoherent Doppler lidar wind measurement system is described. The system employs a Fabry-Perot interferometer to detect the Doppler shift of the backscattered laser line, and uses two channels, one for aerosol and one for molecular backscatter. Previous investigations have considered only the aerosol backscatter as the means to determine the Doppler shift. Several studies have demonstrated that aerosol backscatter, particularly over the oceans and in the southern hemisphere, can be extremely low in the free troposphere. The two channel configuration permits acceptable measurements regardless of the aerosol loading. The system operates in the near UV, which is eye safe and provides a large molecular backscatter. With a 20 Watt laser, 1 meter diameter collecting telescope, and 5 seconds integration time, the horizontal line of sight wind errors would be less than 1 m/s with aerosols typical of a continental loading from the surface to the stratosphere. Areas of low aerosol loading would have errors of about 3 m/s.
The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite has been providing measurements of the wind field in the stratosphere, mesosphere, and lower thermosphere since November 1991. Examination of various calibration data indicates the instrument has remained remarkably stable since launch. The instrument has a thermal drift of about 30 m/s/ degree(s)C (slightly dependent on wavelength) and a long-term temporal drift that has amounted to about 80 m/s since launch. These effects are removed in the data processing leaving an uncertainty in the instrument stability of approximately 2 m/s. The temperature control of the instrument has improved significantly since launch as a new method was implemented. The initial temperature control held the instrument temperature at about +/- 1 degree(s)C. The improved method, which holds constant the temperature of the optical bench instead of the radiator, keeps the instrument temperature at about 0.2 degree(s)C. The calibrations indicate very little change in the sensitivity of the instrument. The detector response has shown no degradation and the optics have not changed their transmittance.
The University of Michigan's Space Physics Research Laboratory has constructed a mobile high-spectral-resolution Doppler lidar capable of measuring wind and aerosol loading profiles in the troposphere and lower stratosphere. The system uses a 3-W pulsed frequency-doubled Nd:YAG laser operating at 532 nm as the active source. Backscattered signal is collected by a 44.4-cm-diameter Newtonian telescope. A two axis mirror scanning system allows the instrument to achieve full sky coverage. A pair of Fabry-Perot interferometers in combination with a narrowband (0.1nm) interference filter are used to filter daylight background and provide a high spectral resolving element to measure the Doppler shift. In addition, the aerosol and molecular scattered components of the signal can be separated, giving a measure of the relative aerosol loading. Measurements have been made day and night in the boundary layer with vertical resolution of 100 m and a temporal resolution of approximately 5 minutes. Accuracy of the wind velocity is on the order of 1 to 2 m/s in the boundary layer.
The use of a (CCD) with an interferometer, such as a Fabry-Pérot, poses some problems because of the circular symmetry with the interferometer and the rectangular arrangement of the CCD. Integrating the fringe pattern along a row or column results in a signature that can be inverted to recover the fringe. The methods of constrained linear inversion are used to significantly improve the quality of the inversion.