ICESat/GLAS Altimetry Measurements: Received Signal Dynamic Range and Saturation Correction.

NASA's Ice, Cloud, and land Elevation Satellite (ICESat), which operated between 2003 and 2009, made the first satellite-based global lidar measurement of Earth's ice sheet elevations, sea-ice thickness and vegetation canopy structure. The primary instrument on ICESat was the Geoscience Laser Altimeter System (GLAS), which measured the distance from the spacecraft to Earth's surface via the roundtrip travel time of individual laser pulses. GLAS utilized pulsed lasers and a direct detection receiver consisting of a silicon avalanche photodiode (Si APD) and a waveform digitizer. Early in the mission, the peak power of the received signal from snow and ice surfaces was found to span a wider dynamic range than planned, often exceeding the linear dynamic range of the GLAS 1064-nm detector assembly. The resulting saturation of the receiver distorted the recorded signal and resulted in range biases as large as ~50 cm for ice and snow-covered surfaces. We developed a correction for this "saturation range bias" based on laboratory tests using a spare flight detector, and refined the correction by comparing GLAS elevation estimates to those derived from Global Positioning System (GPS) surveys over the calibration site at the salar de Uyuni, Bolivia. Applying the saturation correction largely eliminated the range bias due to receiver saturation for affected ICESat measurements over Uyuni and significantly reduced the discrepancies at orbit crossovers located on flat regions of the Antarctic ice sheet.

Diffractive Achromat with Freeform Slope for Broadband Imaging over a Long Focal Depth.

We propose a method for designing a long-focal-depth diffractive achromat (LFDA). By applying rotational symmetric parameterization, an LFDA with a diameter of 10.89 mm is designed over three wavelengths at six focal planes. The smoothly changed slope designed by the binary variable slope search (BVSS) algorithm greatly reduces the discontinuity in depth, thus it is a fabrication-friendly process for grayscale laser direct writing lithography, involving less fabrication error and cost. The deviation between the designed and fabricated profiles amounts to 9.68%. The LFDA operates at multiple wavelengths (654 nm, 545 nm, and 467 nm) with a DOF of 500 mm~7.65λ × 105 (λ = 654 nm). The simulated and measured full-width at half-maximum (FWHM) of the focused beam is close to the diffraction limit. Experimental studies suggest that the LFDA possesses a superior capability to form high-quality chromatic images in a wide range of depths of field. The LFDA opens a new avenue to achieve compact achromatic systems for imaging, sensing, and 3D display.

Identification of ubiquitin nitration and oxidation using a liquid chromatography/mass selective detector system.

Ubiquitin is a member of the family of low-molecular-weight heat shock proteins that serve a vital role in physiological and pathological protein turnover. It appears to be one of the proteins involved in cell alterations during aging, degenerative disorders, and age-related cognitive decline. It is not known exactly how ubiquitin alterations are related to aging disorders; however, it is possible that ubiquitin is one of the target proteins for free-radical attack. In vivo, the free radical superoxide reacts with nitric oxide to form peroxynitrite, a powerful oxidant. Peroxynitrite may react directly with proteins, lipids, and other molecules to cause damage, with ubiquitin being a possible target. In vitro reaction of peroxynitrite with ubiquitin produces two modified forms of the protein, one oxidized at methionine and the other nitrated at tyrosine, which were characterized by electrospray ionization time-of-flight mass spectrometry. The exact location of the nitrated tyrosine residue was determined by in-source collision-induced dissociation using electrospray ionization time-of-flight mass spectrometry.

A reconciled estimate of ice-sheet mass balance.

We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods--especially in Greenland and West Antarctica--and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 ± 49, +14 ± 43, -65 ± 26, and -20 ± 14 gigatonnes year(-1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year(-1) to the rate of global sea-level rise.

An atmospheric pressure matrix-assisted laser desorption/ionization ion trap with enhanced sensitivity.

When atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) became commercially available, the technique generated a great deal of interest because ion production was decoupled from mass analysis. Mass accuracy and resolution were therefore dependent on parameters governing the mass analyzer rather than the matrix and sample preparation. Researchers have successfully used AP-MALDI sources with both orthogonal acceleration time-of-flight (oaTOFMS) and ion trap mass spectrometers. However, one limitation of the technique has been sensitivity, especially for mixtures of peptides generated from tryptic digests. In this work, data are presented documenting an increase in sensitivity of approximately two orders of magnitude as compared with results previously reported in the literature. The improvement in sensitivity is thought to derive primarily from the novel use of a countercurrent heated gas stream directed at the sample, although the target plate position and ion sampling configuration have also been optimized to reduce chemical noise from low molecular weight ions. A tryptic digest of BSA containing 125 attomoles on the plate was successfully identified in MS-only mode, while MS/MS analysis of 250 attomoles of the same digest provided product ion spectra with sufficient information to identify the protein. More complicated mixtures of standard proteins were used to model proteomics experiments, and preliminary data suggest a minimum working dynamic range of 20-fold for the analysis of mixtures of protein digests.