Comparing Spectral Characteristics of Landsat-8 and Sentinel-2 Same-Day Data for Arctic-Boreal Regions

The Arctic-Boreal regions experience strong changes of air temperature and precipitation regimes, which affect the thermal state of the permafrost. This results in widespread permafrost-thaw disturbances, some unfolding slowly and over long periods, others occurring rapidly and abruptly. Despite optical remote sensing offering a variety of techniques to assess and monitor landscape changes, a persistent cloud cover decreases the amount of usable images considerably. However, combining data from multiple platforms promises to increase the number of images drastically. We therefore assess the comparability of Landsat-8 and Sentinel-2 imagery and the possibility to use both Landsat and Sentinel-2 images together in time series analyses, achieving a temporally-dense data coverage in Arctic-Boreal regions. We determined overlapping same-day acquisitions of Landsat-8 and Sentinel-2 images for three representative study sites in Eastern Siberia. We then compared the Landsat-8 and Sentinel-2 pixel-pairs, downscaled to 60 m, of corresponding bands and derived the ordinary least squares regression for every band combination. The acquired coefficients were used for spectral bandpass adjustment between the two sensors. The spectral band comparisons showed an overall good fit between Landsat-8 and Sentinel-2 images already. The ordinary least squares regression analyses underline the generally good spectral fit with intercept values between 0.0031 and 0.056 and slope values between 0.531 and 0.877. A spectral comparison after spectral bandpass adjustment of Sentinel-2 values to Landsat-8 shows a nearly perfect alignment between the same-day images. The spectral band adjustment succeeds in adjusting Sentinel-2 spectral values to Landsat-8 very well in Eastern Siberian Arctic-Boreal landscapes. After spectral adjustment, Landsat and Sentinel-2 data can be used to create temporally-dense time series and be applied to assess permafrost landscape changes in Eastern Siberia. Remaining differences between the sensors can be attributed to several factors including heterogeneous terrain, poor cloud and cloud shadow masking, and mixed pixels.

The Sydney University Stellar Interferometer: A Major Upgrade to Spectral Coverage and Performance

A new beam-combination and detection system has been installed in the Sydney University Stellar Interferometer working at the red end of the visual spectrum (λλ 500–950 nm) to complement the existing blue-sensitive system (λλ 430–520 nm) and to provide an increase in sensitivity. Dichroic beam-splitters have been introduced to allow simultaneous observations with both spectral systems, albeit with some restriction on the spectral range of the longer wavelength system (λλ 550–760 nm). The blue system has been upgraded to allow remote selection of wavelength and spectral bandpass, and to enable simultaneous operation with the red system with the latter providing fringe-envelope tracking. The new system and upgrades are described and examples of commissioning tests presented. As an illustration of the improvement in performance the measurement of the angular diameter of the southern F supergiant δ CMa is described and compared with previous determinations.

Design Parameters for an Optimized Flame/Furnace Infrared Emission (FIRE) Radiometer

Several design aspects of an optimized flame/furnace infrared emission (FIRE) radiometer are discussed. For a source in thermal equilibrium, the optimum excitation temperature for CO2 is predicted to be 2500 K, based on the stability of the molecule and the population of excited states as functions of temperature. The influence of spectral bandpass on the magnitude of flame background was studied for several wavelengths within the profile of the 4.4-µm emission band of CO2. For the dispersive FIRE radiometer used in this study, the system was found to be detectornoise limited over the range of spectral bandwidths accessible by the instrument (0.01–0.08 µm). When a hydrogen/air flame was used as the excitation source, the maximum signal-to-background ratio was obtained at 4.40 µm with a spectral bandpass of 0.08 µm. For this same experimental arrangement, a mathematical model was used to predict that the maximum signal-to-noise ratio would occur at 4.40 µm with a spectral bandpass of about 0.25 µm. The influence of chopping frequency on detector noise was determined for two different types of wavelength isolation. When a room-temperature bandpass filter was employed, a modulation frequency of 600–1000 Hz was satisfactory for avoiding 1/ f noise from the PbSe detector. However, when a monochromator was employed, chopping frequencies beyond 1000 Hz produced some additional reduction in 1/ f noise. The specific detectivity for the PbSe detector (6.2 × 108 cm Hz0.5 W−1), calculated from the measured noise at a modulation frequency of 600 Hz, is in good agreement with values reported in the literature for 4.40 µm under the same experimental conditions.