The Cross-track Infrared Sounder (CrIS) is one of the mission-critical instruments onboard the National Polar-orbiting Operational Environmental Satellite System (NPOESS). CrIS develops vertical profiles of moisture, temperature, and pressure in the earth's atmosphere by measuring upwelling atmospheric infrared radiation at very high spectral resolution. This paper describes initial test results for the CrIS Engineering Development Unit #3 (EDU3).
The Cross-track Infrared Sounder (CrIS) is one of the mission-critical instruments onboard the National Polar-orbiting Operational Environmental Satellite System (NPOESS). CrIS develops vertical profiles of moisture, temperature, and pressure in the earth’s atmosphere by measuring upwelling atmospheric infrared radiation at very high spectral resolution. This paper presents the current design status and performance projections of the CrIS instrument, which is approaching a Critical Design Review. Preliminary results from tests of an Engineering Development Unit will also be presented.
For the proper radiometric calibration of the CrIS sensor it is imperative to provide high quality characterizations of the ILS and the spatial response of the sensor. The methodology for determining the self-apodized ILS using both a laser and gas cell measurements is discussed with emphasis on the updates to the gas cell methodology. These methods for characterizing the self-apodized ILS have recently been exercised using the CrIS Engineering Development Unit. A description of the test configuration, data collection and data analysis is presented. Finally, the results are presented for the ILS determination are presented.
In the CrIS Acceptance Test Program two of the most important sets of requirements to verify are the spatial and spectral requirements of the sensor. The development of the test program to verify these requirements starts with the understanding of the system level requirements and how they are allocated to the sensor and module levels. Knowing the requirements and the verification method, the test developer selects the most appropriate test methodology to verify each requirement form the list of test concepts that were proposed. For the chosen concept the developer derives the requirements for the test and test equipment needed to verify the system requirements. The collimator serves as an example of the flowdown of system requirements for the spatial and spectral uncertainties to test requirements and test performance data on the collimator are reviewed. Models and simulations are also key tools for developing the tests. Examples of models used for the spatial spectral requirements are an ILS model, a gas absorption model and an edge response model. This paper serves as a summary of the processes used and requirements needed to develop the characterizations for the CrIS sensor.
For a complex remote sensor like the NPOESS Crosstrack Infrared Sounder (CrIS), the process of requirements flowdown is extremely important to the success of the project. When there is both an algorithm and a sensor, the task of allocating requirements between the sensor and the algorithm becomes a challenge. This is where the use of system models and simulations has been an invaluable tool. Complex requirements such as radiometric uncertainty and Instrument Line Shape (ILS) uncertainty have utilized system models and simulations for the allocation of requirements. For radiometric uncertainty the sensor model in conjunction with the algorithm which handles the calibration of the sensor was used to assess the contribution of parameters such as component and detector temperature stability on radiometric uncertainty. Variation of the parameter values within the sensor model allowed us to compute the impact on radiometric uncertainty and allocate requirements appropriately. Examples of how the model and simulations were used to develop requirements for the CrIS radiometric uncertainty will be presented. For the assessment of ILS uncertainty a model for predicting the ILS of a Michelson interferometer was employed. The model calculates the ILS and associated spectral shift based upon a set of input parameters. By varying the input parameters the sensitivity of the ILS to the specific parameters could be determined and used to allocate the requirements from a top level down to the module level. A description of the model, the input parameters and results for the CrIS requirements development will be presented.
The Total Ozone Mapping Spectrometer (TOMS) provides daily global mapping of the total column ozone in the earth’s atmosphere. It does this by measuring the solar irradiance and the backscattered solar radiance in 6 spectral bands falling within the range from 308.6 nm to 360 nm. The accuracy of the ozone retrieval is highly dependent on the knowledge of the transfer characteristics and center wavelength for each spectral band. A 0.1 nm wavelength error translates to a 1.6% error in ozone. Several techniques have historically been used to perform the wavelength calibration of the TOMS instruments. These methods include the use of film and reference spectra from low-pressure spectral line lamps and the use of continuum sources with a narrow-band scanning monochromator. The spectral transfer characteristic of the Flight Model 5 instrument for the QuikTOMS mission was calibrated using a new technique employing a frequency doubled tunable dye laser. The tunable laser has several advantages that include a very narrow spectral bandwidth; accurate wavelength determination using a wavemeter; and the ability to calibrate the instrument system level of assembly (prior methods required that the calibration be performed at the monochromator sub assembly level). The technique uses the output from a diode-pumped solid state Nd:V04 laser that is frequency doubled to provide a continuous wave 532 nm pump laser beam to a Coherent Model 899-01 frequency doubled ring dye laser. The output is directed into the entrance port of a 6-inch diameter Spectralon integrating sphere. A GaP photodiode is used to monitor the sphere wall radiance while a Burleigh Wavemeter (WA-1500) is used to monitor the wavelength of the visible output of the dye laser. The TOMS field of view is oriented to view the exit port of the integrating sphere. During the measurement process the response of the instrument is monitored as the laser source is stepped in 0.02-nm increments over each of the six TOMS spectral bands. Results of the new technique allow establishing the wavelength center to a precision of better than 0.1 nm. In addition to the spectral band measurements, the laser provided a means to calibrate the radiometric linearity of the QuikTOMS instrument and yield new insights into the stray light performance of the complete optical system.
This paper presents the design and initial test results of the laboratory Wide Field-of-View Imaging Spectrometer (WFIS). The WFIS is a patented optical design intended for use in remote sensing of the Earth and the Earth's atmosphere in the hyperspectral imaging mode. It is meant to operate as a pushbroom imager to provide coverage of the Earth from low Earth orbit without scanning mechanisms. The optical system occupies a volume measuring less than 20 cm X 18 cm X 13 cm. The laboratory unit covers the 500 nm to 1000 nm wavelength range over a cross-track field of view of 70 degrees. The image is focused onto a CCD area array such that the spatial component falls along the horizontal direction and the spectral information is dispersed along the vertical direction. The system's focal length is 7.5 mm with an effective focal ratio of 3.7. A holographic grating produced on a unique convex substrate is the dispersing element. A key feature of the WFIS is an all-reflective optical path, allowing the basic design to be adapted to wavelength regions from the UV to the IR. Presented are the initial test results of the laboratory spectrometer that characterize its spatial and spectral performance over a 70 degree X 0.08 degree field of view.
More accurate weather forecasting requires improvements in vertical temperature profiling.T He increase in vertical temperature resolution along with wind distributions will provide important information on vertical motion fields for Mesoscale weather predictions. Such information is particularly valuable for short-term forecasts and storm tracking. New techniques beyond what can be achieved with the current filter wheel sounders are required. A Michelson interferometer is proposed for the next generation of GOES Sounders. The interferometric spectrometer will greatly increase the spectral resolution of the sounder over the filter wheel instruments, improving its ability to measure temperature and water vapor profiles. This paper describes the current baseline design for the interferometer-equipped GOES Sounder, known as the GOES High-Resolution Interferometric Sounder.
The modulation transfer function is one of the GOES Imager and Sounder system's key optical performance indicators. Though the visible images from GOES-8 and GOES-9 are excellent, the MTF values measured during ground tests are below expectations. The 4490 cycle per radian frequency MTF is particularly interesting because it is not significantly affected by small changes in focus or aberrations of the optical system. The primary contributors to the ow MTF values which are within the instrument are the ghost reflections from the transmissive components and the detector and its package. The ghost reflections that exist are certainly a factor in the stray light. An estimate of each ghost's intensity produces a worst case effect on MTF of approximately 2.5 percent. The detector itself appears to be the largest contributor to the stray light. There are several ways that the detector and its package contribute to the stray light. The most prominent among them is the reflection from the aluminum coated substrate combined with the closeness of the detector window. The stray light form the detector reduces the MTF by approximately 4 percent. Instantaneous geometric field of view (IGFOV) curves verify the existence of the stray light and calculations show approximately the magnitude of difference between measured and expected MTF data. A new detector design appropriate for the Imager visible channel or the Sounder star sense channel incorporates the changes recommended to reduce stray light within the detector THe first of the new detectors qualifies for the Sounder star sense detector. IGFOV curves show the reduction in stray light levels by about a factor of 2 to 4. Sounder star sense optical MTF data shows approximately 8 percent increase in the MTF at 4490 cycles per radian compared to SN05 Sounder.
Starting with SN05 Imager the optical MTF of the visible optics assembly has been measured at subsystem and at system level testing. For the visible detector MTF at 18,000 cycles/radian relative differences of 16 percent have been observed between system and subsystem data. The test setups are quite different, however, the MTF values should be dominated by the detector's field of view MTF and thus the differences should be small. Descriptions of the two configurations are given along with a discussion of some of the important differences. The investigation into the differences led to the testing of the imager and test collimator with a parabolic mirror as collimator and an MTF analyzer. The MTF values using the parabola with the instrument were about 12 percent better than with the test collimator. Observations of the image of a pinhole at the Cassegrain focus led to the analysis of spider vane diffraction which accounts for 2.7 percent MTF reduction. In addition analysis has also been performed on the collimator interferogram that indicates a reduction in MTF due to a number of steep zones within the collimator wavefront. The analysis predicts a 9.7 percent reduction in MTF, at 18,000 cycles/radian due to the test collimator of which 1.4 percent is a result of the collimator secondary spider vanes. The verification of the analysis awaits a new collimator with slope errors reduced to less than 0.12 waves/inch.