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The Advanced Topographic Laser Altimetry System (ATLAS) is the sole instrument on the Ice, Cloud, and land Elevation Satellite 2 (ICESat-2). Without some method of reducing the transmitted data, the volume of ATLAS telemetry would far exceed the normal X-band downlink capability or require many more ground station contacts. The ATLAS Onboard Flight Science Receiver Algorithms (hereinafter Receiver Algorithms or Algorithms) control the amount of science data that is telemetered from the instrument, limiting the data volume by distinguishing surface echoes from background noise, and allowing the instrument to telemeter data from only a small vertical region about the signal. This is accomplished through the transfer of the spacecraft's location and attitude to the instrument every second, use of an onboard Digital Elevation Model, implementation of signal processing techniques, and use of onboard relief and surface type reference maps. Extensive ground testing verified the performance of the Algorithms. On-orbit analysis shows that the Algorithms are working as expected from the ground testing; they are performing well and meeting the mission requirements., (©2020. The Authors. Earth and Space Science published by Wiley Periodicals LLC on behalf of American Geophysical Union. This article has been contributed to by US Government employees and their work is in the public domain in the USA.)

The Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter (LRO) has collected nearly seven billion measurements of surface height on the Moon with an absolute accuracy of ∼1  m and a precision of ∼10  cm. Converting time-of-flight laser altimeter measurements to topographic elevations requires accurate knowledge of the laser pointing with respect to the spacecraft body-fixed coordinate system. To that end, we have utilized altimetric crossovers from LOLA, as well as bidirectional observations of the LOLA laser and receiver boresight via an Earth-based laser tracking ground station. Based on a sample of ∼780,000 globally distributed crossovers from the circular-orbit phase of LRO's mission (∼27 months), we derive corrections to the LOLA laser boresight. These corrections improve the cross-track and along-track agreement of the crossovers by 24% and 33%, respectively, yielding RMS residuals of ∼10  m. Since early in the LRO mission, the bidirectional laser tracking experiments have confirmed a pointing anomaly when the LOLA instrument is facing toward deep space or the night side of the Moon and have allowed the reconstruction of the laser far-field pattern and receiver telescope pointing. By conducting such experiments shortly after launch and nearly eight years later, we have directly measured changes in the laser characteristics and obtained critical data to understand the laser behavior and refine the instrument pointing model. The methods and results presented here are also relevant to the design, fabrication, and operation of future planetary laser altimeters and their long-term behavior in the space environment.

Laser communication and ranging experiments were successfully conducted from the satellite laser ranging (SLR) station at NASA Goddard Space Flight Center (GSFC) to the Lunar Reconnaissance Orbiter (LRO) in lunar orbit. The experiments used 4096-ary pulse position modulation (PPM) for the laser pulses during one-way LRO Laser Ranging (LR) operations. Reed-Solomon forward error correction codes were used to correct the PPM symbol errors due to atmosphere turbulence and pointing jitter. The signal fading was measured and the results were compared to the model.

Here we report timed observations with subnanosecond precision of short laser pulses at a distance of nearly 24 million kilometers between the Mercury Laser Altimeter (MLA) aboard the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft and the NASA Goddard Geophysical and Astronomical Observatory (GGAO). Forty MLA downlink observations and 90 uplink observations were obtained during observing sessions on 27 and 31 May 2005. Precise standard ground timing allowed a solution for spacecraft range, range rate, and acceleration, as well as clock bias. This experiment established a new distance record for laser detection and accomplished a two-way laser link at an interplanetary distance.

Fatty acyltransferase activity that catalyzes the transfer of palmitic acid from palmitoyl-CoA to the endogenous myelin proteolipid protein has been demonstrated in isolated rat brain myelin. Optimum enzyme activity for the acylation of proteolipid protein was obtained in 0.1% Triton X-100, 2 mM MgCl2, and 1 mM dithiothreitol at a pH of 7.5 and at 37 degrees C. Other detergents had little or no effect on the reaction whereas acylation was completely abolished by sodium dodecyl sulphate (0.1%). Pulse-chase experiments indicated that the reaction involves the net addition of fatty acid to the protein and not a rapid fatty acid exchange. The rate of acylation was linear up to 30 min, indicating that the concentration of endogenous protein acceptor was constant. Under these conditions and at short time periods, the enzyme activity versus acyl-CoA concentration showed a hyperbolic curve. The apparent Km and Vmax for palmitoyl-CoA was 41 microM and 115 pmol/mg protein/min. Similar values were obtained for stearoyl and oleoyl-CoA, whereas myristoyl-CoA showed a lower specificity for the enzyme. The acyl-CoA specificity was also studied in competition experiments using several saturated and unsaturated fatty acid-CoAs. The product of the reaction was identified as myelin proteolipid protein and the fatty acid was shown to be attached to the protein via an ester linkage. Limited proteolysis and peptide mapping showed that the same sites on the proteolipid protein were acylated when the reaction was carried out in isolated myelin preparations or in brain tissue slices, suggesting physiological importance for the in vitro acylation of endogenous myelin proteolipid protein.

The timing and correlation properties of pulsed laser backscatter from the ocean surface have been measured with a two-color short-pulse laser altimeter. The Nd: YAG laser transmitted 70-and 35-ps wide pulses simultaneously at 532 and 355 nm at nadir, and the time-resolved returns were recorded by a receiver with 800-ps response time. The time-resolved backscatter measured at both 330- and 1291-m altitudes showed little pulse broadening due to the submeter laser spot size. The differential delay of the 355- and 532-nm backscattered waveforms were measured with a rms error of ~75 ps. The change in aircraft altitudes also permitted the change in atmospheric pressure to be estimated by using the two-color technique.

Rat brain myelin proteolipid protein (PLP) is known to contain long chain, covalently bound fatty acids. In the course of characterizing the mechanism of acylation, we found that the isolated PLP, in the absence of any membrane fraction, was esterified after incubation with [3H]palmitoyl coenzyme A (CoA). This observation demonstrated that the protein acts as both an acylating enzyme and an acceptor. Thus, acylation occurs by an autocatalytic process. The possibility of a separate acyltransferase that copurifies with PLP was essentially excluded by adding brain subcellular fractions to the reaction mixtures and by changing the isolation procedure. After deacylation, the protein was acylated at a 4-fold greater rate, suggesting that the original sites were reacylated. The palmitoyl-CoA concentration followed Michaelis kinetics, confirming that spontaneous acylation was not occurring. Pulse-chase experiments indicated that the reaction entails net addition of acyl groups. Although fatty acids are bound via an O-ester linkage, free SH groups are required in the reaction. Denaturation of the protein by sodium dodecyl sulfate or heat inhibits the reaction, whereas cerulenin has little or no effect. PO, the major protein in peripheral nerve myelin, is also an acylated protein, but it was not labeled upon incubation of either peripheral myelin or the isolated protein with [3H]palmitoyl-CoA, demonstrating that it is acylated by a different route. Several synthetic peptides derived from PLP sequences with sites known to be acylated in vivo as well as a series of deacylated PLP tryptic peptides were not labeled, indicating that integrity of the protein is required for acylation. Limited proteolysis and peptide mapping showed that the same sites are acylated in vitro or in vivo, suggesting that the autocatalytic acylation reaction is physiological.