Role of the impact parameter in exoplanet transmission spectroscopy

Context. Transmission spectroscopy is a promising tool for the atmospheric characterization of transiting exoplanets. Because the planetary signal is faint, discrepancies have been reported regarding individual targets. Aims. We investigate the dependence of the estimated transmission spectrum on deviations of the orbital parameters of the star-planet system that are due to the limb-darkening effects of the host star. We describe how the uncertainty on the orbital parameters translates into an uncertainty on the planetary spectral slope. Methods. We created synthetic transit light curves in seven different wavelength bands, from the near-ultraviolet to the near-infrared, and fit them with transit models parameterized by fixed deviating values of the impact parameter b. First, we performed a qualitative study to illustrate the effect by presenting the changes in the transmission spectrum slope with different deviations of b. Then, we quantified these variations by creating an error envelope (for centrally transiting, off-center, and grazing systems) based on a derived typical uncertainty on b from the literature. Finally, we compared the variations in the transmission spectra for different spectral types of host stars. Results. Our simulations show a wavelength-dependent offset that is more pronounced at the blue wavelengths where the limb-darkening effect is stronger. This offset introduces a slope in the planetary transmission spectrum that becomes steeper with increasing b values. Variations of b by positive or negative values within its uncertainty interval introduce positive or negative slopes, thus the formation of an error envelope. The amplitude from blue optical to near-infrared wavelength for a typical uncertainty on b corresponds to one atmospheric pressure scale height and more. This impact parameter degeneracy is confirmed for different host types; K stars present prominently steeper slopes, while M stars indicate features at the blue wavelengths. Conclusions. We demonstrate that transmission spectra can be hard to interpret, basically because of the limitations in defining a precise impact parameter value for a transiting exoplanet. This consequently limits a characterization of its atmosphere.

Development and assessment of a higher-spatial-resolution (4.4 km) MISR aerosol optical depth product using AERONET-DRAGON data

Since early 2000, the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA's Terra satellite has been acquiring data that have been used to produce aerosol optical depth (AOD) and particle property retrievals at 17.6 km spatial resolution. Capitalizing on the capabilities provided by multi-angle viewing, the current operational (Version 22) MISR algorithm performs well, with about 75 % of MISR AOD retrievals globally falling within 0.05 or 20 % × AOD of paired validation data from the ground-based Aerosol Robotic Network (AERONET). This paper describes the development and assessment of a prototype version of a higher-spatial-resolution 4.4 km MISR aerosol optical depth product compared against multiple AERONET Distributed Regional Aerosol Gridded Observations Network (DRAGON) deployments around the globe. In comparisons with AERONET-DRAGON AODs, the 4.4 km resolution retrievals show improved correlation (r = 0. 9595), smaller RMSE (0.0768), reduced bias (−0.0208), and a larger fraction within the expected error envelope (80.92 %) relative to the Version 22 MISR retrievals.

MODIS 3 km aerosol product: applications over land in an urban/suburban region

MODerate resolution Imaging Spectroradiometer (MODIS) instruments aboard the Terra and Aqua satellites have provided a rich dataset of aerosol information at a 10 km spatial scale. Although originally intended for climate applications, the air quality community quickly became interested in using the MODIS aerosol data. However, 10 km resolution is not sufficient to resolve local scale aerosol features. With this in mind, MODIS Collection 6 includes a global aerosol product with a 3 km resolution. Here, we evaluate the 3 km product over the Baltimore–Washington D.C., USA, corridor during the summer of 2011 by comparing with spatially dense aerosol data measured by airborne High Spectral Resolution Lidar (HSRL) and a network of 44 sun photometers (SP) spaced approximately 10 km apart, collected as part of the DISCOVER-AQ field campaign. The HSRL instrument shows that AOD can vary by over 0.2 within a single 10 km MODIS pixel, meaning that higher resolution satellite retrievals may help to better characterize aerosol spatial distributions in this region. Different techniques for validating a high-resolution aerosol product against SP measurements are considered. Although the 10 km product is more statistically reliable than the 3 km product, the 3 km product still performs acceptably with nearly two-thirds of MODIS/SP collocations falling within an expected error envelope with high correlation (R > 0.90), although with a high bias of ~ 0.06. The 3 km product can better resolve aerosol gradients and retrieve closer to clouds and shorelines than the 10 km product, but tends to show more noise, especially in urban areas. This urban degradation is quantified using ancillary land cover data. Overall, we show that the MODIS 3 km product adds new information to the existing set of satellite derived aerosol products and validates well over the region, but due to noise and problems in urban areas, should be treated with some degree of caution.

MODIS 3 km aerosol product: applications over land in an urban/suburban region

MODerate resolution Imaging Spectroradiometer (MODIS) instruments aboard the Terra and Aqua satellites have provided a rich dataset of aerosol information at a 10 km spatial scale. Although originally intended for climate applications, the air quality community quickly became interested in using the MODIS aerosol data. However, 10 km resolution is not sufficient to resolve local scale aerosol features. With this in mind, MODIS Collection 6 is including a global aerosol product with a 3 km resolution. Here, we evaluate the 3 km product over the Baltimore/Washington D.C., USA, corridor during the summer of 2011, by comparing with spatially dense data collected as part of the DISCOVER-AQ campaign; these data were measured by the NASA Langley Research Center airborne High Spectral Resolution Lidar (HSRL) and a network of 44 sun photometers (SP) spaced approximately 10 km apart. The HSRL instrument shows that AOD can vary by up to 0.2 within a single 10 km MODIS pixel, meaning that higher resolution satellite retrievals may help to characterize aerosol spatial distributions in this region. Different techniques for validating a high-resolution aerosol product against SP measurements are considered. Although the 10 km product is more statistically reliable than the 3 km product, the 3 km product still performs acceptably, with more than two-thirds of MODIS/SP collocations falling within the expected error envelope with high correlation (R > 0.90). The 3 km product can better resolve aerosol gradients and retrieve closer to clouds and shorelines than the 10 km product, but tends to show more significant noise especially in urban areas. This urban degradation is quantified using ancillary land cover data. Overall, we show that the MODIS 3 km product adds new information to the existing set of satellite derived aerosol products and validates well over the region, but due to noise and problems in urban areas, should be treated with some degree of caution.

A Method to Assess Robustness of GPS C/A Code in Presence of CW Interferences

Navigation/positioning platforms integrated with wireless communication systems are being used in a rapidly growing number of new applications. The mutual benefits they can obtain from each other are intrinsically related to the interoperability level and to a properly designed coexistence. In this paper a new family of curves, called Interference Error Envelope (IEE), is used to assess the impact of possible interference due to other systems (e.g., communications) transmitting in close bandwidths to Global Navigation Satellite System (GNSS) signals. The focus is on the analysis of the GPS C/A code robustness against Continuous Wave (CW) interference.

Precision of Calibrated Radiocarbon Ages of Historic Earthquakes in the Dead Sea Basin

The precise determination of the age of historical and geological events by radiocarbon dating is often hampered by the long intersection ranges of the measured data with the calibration curve. In this study we examine the possibility of narrowing the calibrated range of the 14C ages of earthquake-disturbed sediments (seismites) from the Late Holocene lacustrine section in the Dead Sea Basin. The calibrated ranges of samples collected from seismites were refined by applying stratigraphic constraints and tuning the calibrated ranges to known historical earthquakes. Most of the earthquakes fall well within the 1σ error envelope of the 14C age. This refinement demonstrates that the lag period due to transport and deposition of vegetation debris is very short in this arid environment, probably not more than a few decades. This assessment of seismite 14C ages attests to the validity of 14C ages in Holocene sediments of the arid area of the Dead Sea. Furthermore, it demonstrates our ability to achieve highly precise (correct to within several decades) 14C ages.

A New Concept in Continuous-Flow Gas-Lift Design

The importance of the differential pressure at the point of injection in continuous-flow gas-lift design is discussed. The role played by differential pressure in the selection of optimal flow in gas lift is also explained. It is shown that good wells with high productivity have continued increase in production as the differential pressure decreases. Weaker wells with low productivity, however, are less sensitive to the change in differential pressure. Also, a concept of error envelope surrounding the point of gas injection is presented. Suitable valve spacing in this error envelope is shown to offset any errors in locating the depth of injection caused by errors in the multiphase flow correlations or in the well productivity. The maximum valve spacing within the error envelope is shown to be directly proportional to the differential pressure. The smaller this differential pressure, the smaller the valve spacing. Introduction The theory behind continuous-flow gas-lift design is quite simple. It allows injection of gas in the production string to aerate the producing fluids which in turn lowers the bottomhole flowing pressure (BHFP). Any reduction in BHFP causes the reservoir to respond with increased flow rate. Consequently, once the piping system is fixed, the extent of reduction in the BHFP depends on two parameters-the amount of gas injected and the depth of injection. Although the increased volume of gas injected should yield higher production, there is an upper limit to the volume of gas injected. This upper limit can be an economic limit of gas injection beyond which the cost of gas injection supersedes the price of extra oil produced as discussed by Kanu et al. The economic limit is beyond the scope of this discussion. There is a physical limit of gas injection too, which results in the reversal of the tubing gradients caused by the increased irreversible pressure losses in the tubing. Consequently, a sensitivity analysis on the volume of gas injected should always be carried out before any decision is made regarding this parameter. The second parameter that significantly affects the efficiency of continuous-flow gas-lift design is the depth of injection. The maximum depth of injection achievable in a gas-lift design is function of surface injection pressure and rate, if all other variables remain constant. Once the surface injection pressure is fixed, the depth of injection can be controlled by altering the differential pressure at the point of infection. The lower this differential pressure, the lower the point of injection will be before bottomhole injection starts (see Fig. 6). However, the computed depths of injection may be inaccurate because of errors associated with the use of pressure gradient correlations. As a result, an error envelope surrounding the point of injection is created to define the upper and lower limit of the point of injection caused by calculation errors resulting from pressure loss correlations or well productivity. Considerations such as declining productivity with depletion can also be accounted for in the selection of error envelopes. Judgments based on the closeness of valve spacings, valve interference, and costs must be exercised in making the final selection of the differential pressure at the point of gas injection. SPEJ P. 885^

Selective Precision Synthesis of the Four-Bar Motion Generator With Prescribed Input Timing

The recently developed Selective Precision Synthesis technique has been extended to the four-bar motion generating mechanism with prescribed input timing. A designer using this method can determine several mechanisms whose coupler triangle positions and orientations will be coordinated with the input crank rotations. The unique feature in the Selective Precision Synthesis formulation is that the path, orientations and rotations are specified along with allowable limits of accuracy creating an error envelope for each of these parameters. This modification removes the limiting conditions imposed by the precision point approach so that standard nonlinear programming techniques can be used to determine several mechanism solutions. It was found that the method yields fundamentally stable solutions rarely encountered in closed-form methods of mechanism synthesis. The problem of dyadic construction error in the original SPS technique is eliminated and the method developed here is well suited to batch or interactive computer-aided design. The computer program of this method is being made available to interested readers.