Four institutions are collaborating to design and build three near identical R ~2700 cross-dispersed near-infrared spectrographs for use on various 5-10 meter telescopes. The instrument design addresses the common observatory need for efficient, reliable near-infrared spectrographs through such features as broad wavelength coverage across 6 simultaneous orders (0.8 - 2.4 microns) in echelle format, real-time slit viewing through separate optics and detector, and minimal moving parts. Lastly, the collaborators are saving money and increasing the likelihood of success through economies of scale and sharing intellectual capital.
Recent results of the vicarious calibration of the Landsat-7 ETM+ sensor are presented based on the reflectance-based vicarious method using results from a smaller test site local to the University of Arizona area. This test site is not as bright, nor as spatially-uniform and as large as typical sites. However, the proximity of the site allows for more frequent calibrations and hopefully a better understanding of the calibration as a function of time. The selection of the test site, its properties, and example results of calibrations at this site are presented. The results from seven dates are presented and show that the ETM+ sensor has been stable to better than 5% since launch. The results from these seven dates have larger variability than those from the large test sites, but agree for the most part to better than 5% with the large test sites.
Ground-reference techniques for the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7 are described. The techniques are similar to those used for many years for Landsat-5 Thematic Mapper (TM). Recent results with the Landsat-5 TM are presented, including comparisons with hyperspectral, airborne imaging data. These results show that the Landsat sensor has remained stable within the 5% uncertainty of the ground- reference methods for the last five years. The airborne imagery is also used to show uncertainties due to registration errors, spectral differences, and spatial resolution differences in cross-comparison techniques planned for Earth Observing System sensors. In addition to the use of the traditional methods and test sites, a smaller test site local to the University of Arizona area is being evaluated for use with ETM+. This site, while not as bright, spatially- uniform and large as typical sites, allows more frequent calibrations and hopefully a better understanding of the calibration as a function of time. The selection of the test site, its properties, and example results of calibration of Landsat-5 TM are presented.
The Remote Sensing Group at the University of Arizona has provided vicarious calibration results for all three SPOT- series satellites carrying the HRV cameras. The HRVs are high spatial-resolution sensors that are well-suited to the reflectance-, irradiance-, and radiance-based methods. The SPOT-4 platform has a similar sensor, the HRVIR camera, that now includes a band at approximately 1630 nm. The SPOT-4 platform also has a new sensor, Vegetation, that has much lower spatial resolution than the HRVIR sensor, and thus poses a challenge for vicarious calibration. This work presents the modifications that must be made to reflectance- , irradiance-, and radiance-based approaches in order to use them for the Vegetation. This paper also describes a proposed procedure for such a calibration along with an uncertainty analysis due to spectral mismatch and spatial misregistration. The work also shows how such cross- calibration can be done using a cross-calibration between Landsat-5 TM and SPOT-3 HRV using White Sands Missile Range. The results show TM can be cross-calibrated using HRV to provide calibration coefficients which are within 1 percent of those obtained from a reflectance-based approach.
Errors can occur in laboratory measurements when the response of a bandpass-filtered radiometer extends into an atmospheric absorption region. Atmospheric models, such as MODTRAN3, can be valuable tools that allow optical measurement in these regions to be accurately analyzed. Comparisons of MODTRAN3-predicted and laboratory-measured atmospheric transmittance have been made to help establish the validity of MODTRAN3 for use in modeling short-path length, low resolution, optical effects over the absorption band near 1380 nm. MODTRAN3-predicted transmittance is shown to be within 4 percent of the measured data and well within 2 percent foremost of the water band. The spectroradiometric measurement of the water-vapor absorption band, its description, and its comparison to the MODTRAN3 prediction are presented. Also presented are examples of errors that can occur when an instrument response extends into this region.
Airborne radiometric instruments are often used to collect calibrated radiance data, whether for producing remotely- sensed imagery, for use in vicarious calibration, or for atmospheric correction. Typically, these radiometers are calibrated in a laboratory environment using source whose spectral outputs are traceable to some established, man-made standard. In the field, these devices are used with a different source: solar radiation. The use of solar radiation as a calibration source should therefore be considered when calibration radiometers used to collect energy in the solar reflective region. This paper presents a novel method of calibration which makes use of scattered solar radiation as the source. This technique is particularly applicable for airborne radiometers intended to view low-reflectance surfaces, since the magnitude and spectral distribution of the collected energy is very similar to that of skylight, especially at shorter visible wavelengths. The method is applied to visible and near-IR bands of a Barnes Modular Multispectral 8-channel Radiometer. A sensitivity study was performed for the method and an associated uncertainty analysis is presented. The calibration results are compared to a second, more established solar-based method whose source is directly- transmitted solar irradiance.
A spectral polarimeter with an autotracking mount to obtain atmospheric parameters required for the vicarious calibration of satellite sensors has been modified to work with anew computer and electronic components. The instrument has 12 bands covering the visible through the short-wave IR. There are 9 bands from 400 nm to 1100 nm which use a silicon detector, and 3 bands from 1100 nm to 2500 nm which use a temperature-stabilized, lead-sulfide detector. The instrument's operation was verified by using it as a solar radiometer and collecting Langley plot data. These were compared to data taken concurrently by a well-characterized, manually-pointed radiometer with 10 visible and near-IR channels. In addition, the effect of the gaseous transmittance on the retrieved optical depths of the short- wave IR bands are presented. The data are obtained by finding the band-averaged transmittance for each filter under several atmospheric and view conditions using the output from MODTRAN3.