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  • 1
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    Greenland annual accumulation along the EGIG line, 1959--2004, from ASIRAS airborne radar and neutron-probe density measurements (2016)
    COPERNICUS GESELLSCHAFT MBH
    In:  EPIC3The Cryosphere, COPERNICUS GESELLSCHAFT MBH, 10(4), pp. 1679-1694, ISSN: 1994-0416
    Details
    Publication Date: 2017-10-17
    Description: We report annual snow accumulation rates from 1959 to 2004 along a 250 km segment of the Expéditions Glaciologiques Internationales au Groenland (EGIG) line across central Greenland using Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) radar layers and high resolution neutron-probe (NP) density profiles. ASIRAS-NP-derived accumulation rates are not statistically different (95 % confidence interval) from in situ EGIG accumulation measurements from 1985 to 2004. ASIRAS-NP-derived accumulation increases by 20 % below 3000 m elevation, and increases by 13 % above 3000 m elevation for the period 1995 to 2004 compared to 1985 to 1994. Three Regional Climate Models (PolarMM5, RACMO2.3, MAR) underestimate snow accumulation below 3000 m by 16–20 % compared to ASIRAS-NP from 1985 to 2004. We test radar-derived accumulation rates sensitivity to density using modeled density profiles in place of NP densities. ASIRAS radar layers combined with Herron and Langway (1980) model density profiles (ASIRAS-HL) produce accumulation rates within 3.5 % of ASIRAS-NP estimates in the dry snow region. We suggest using Herron and Langway (1980) density profiles to calibrate radar layers detected in dry snow regions of ice sheets lacking detailed in situ density measurements, such as those observed by the Operation IceBridge campaign.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 2
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    Interferometric swath processing of Cryosat data for glacial ice topography (2013)
    In:  EPIC3The Cryosphere, 7(6), pp. 1857-1867, ISSN: 1994-0416
    Details
    Publication Date: 2017-10-17
    Description: We have derived digital elevation models (DEMs) over the western part of the Devon Ice Cap in Nunavut, Canada, using "swath processing" of interferometric data collected by Cryosat between February 2011 and January 2012. With the standard ESA (European Space Agency) SARIn (synthetic aperture radar interferometry) level 2 (L2) data product, the interferometric mode is used to map the cross-track position and elevation of the "point-of-closest-approach" (POCA) in sloping glacial terrain. However, in this work we explore the extent to which the phase of the returns in the intermediate L1b product can also be used to map the heights of time-delayed footprints beyond the POCA. We show that there is a range of average cross-track slopes (~ 0.5 to ~ 2°) for which the returns will be dominated by those beneath the satellite in the main beam of the antenna so that the resulting interferometric phase allows mapping of heights in the delayed range window beyond the POCA. In this way a swath of elevation data is mapped, allowing the creation of DEMs from a sequence of L1b SARIn Cryosat data takes. Comparison of the Devon results with airborne scanning laser data showed a mean difference of order 1 m with a standard deviation of about 1 m. The limitations of swath processing, which generates almost 2 orders of magnitude more data than traditional radar altimetry, are explored through simulation, and the strengths and weaknesses of the technique are discussed.
    Repository Name: EPIC Alfred Wegener Institut
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  • 3
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    ASIRAS airborne radar resolves internal annual layers in the dry-snow zone of Greenland (2006)
    In:  EPIC3Geophysical research letters, 33, L04502
    Details
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
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  • 4
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    ESAs new range of radar altimeters for the extraction of geophysical parameters from land, sea ice and ocean surfaces (2006)
    In:  EPIC3Proceedings of the Symposium on 15 Years of Progress in Radar Altimetry, 13-18 March 2006, Venice, Italy, Paper No. 507, ESA SP-614, July 2006.
    Details
    Publication Date: 2017-10-20
    Description: Despite the loss of CryoSat, ESAs first Earthopportunity mission, during its launch sequence in Oct2005 ESA have been fortunate enough to have acquired,processed to Level 1b and analysed a significant amountof campaign data from ESAs demonstration AirborneSAR/Interferometric Radar Altimeter System (ASIRAS)designed to have similar functionality to CryoSatsSynthetic Interferometric Radar Altimeter (SIRAL).This data acquisition took place for the original purposeof validating CryoSat retrievals.Our initial analyses of the level 1b data have revealedsome very interesting results both over land and sea icetest sites from respective campaigns conducted in theArctic during the land-ice spring/autumn campaigns of2004 and for sea-ice in the Bay of Bothnia duringMarch 2005. Since a further ASIRAS campaign in theArctic is guaranteed for April/May 2006 we look at howthis data can be exploited in view of future ESA Earthobservation radar altimeter missions. Verification of theASIRAS data with coincident laser altimeter and in-situdata collected at test sites is also presented.The paper also provides a review of key space and airborne radar altimeters dating from 1957 including thoseusing the synthetic aperture radar (SAR) technique asapplied to radar altimeters leading to the development ofCryoSat.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 5
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    CryoSat cal/val activities on Austfonna – Part II: Ground-based measurements using a Ku-band radar, for validation of the CryoSat-2 SIRAL data (2012)
    In:  EPIC3Earth Observation and Cryosphere Science Conference, 2012-11-13-2012-11-16ESA-ESRIN, Frascati
    Details
    Publication Date: 2017-10-17
    Description: We show first results of a combined ground based and airborne validation campaign for CryoSat-2 synthetic aperture interferometric radar altimeter (SIRAL) measurements over Austfonna, Svalbard. A decade time series of glacier facies for this ice cap interpreted from 800 MHz ground-penetrating radar data clearly show there to be considerable variation within the firn pack, which needs to be captured by CryoSat-2 if we are to correctly interpret the satellite observed surface changes. The ground based radar data were obtained with a CRESIS Ku-band FMCW radar. The radar was operated to cover the frequency band of the CryoSat-2 SIRAL and the airborne version, Airborne Synthetic Aperture and Interferometric Radar Altimeter System (ASIRAS) carried by the Danish geophysics plane. Simultaneous profiles were obtained within half a day of each other with all three radars during the spring calibration/validation campaign on the Austfonna ice cap, Svalbard. The profiles, totaling approximately 200 km, span an elevation change of 400 m from the summit down to the ablation area, and cover a range of glacier facies and surface snow conditions. The ground based KU-band radar, which clearly images the near surface layering, was supported by manual snow depth sounding, 800 MHz radar, snow pits, firn cores, and borehole videos, all of which are used to validate interpretation of the CryoSat-2 data.
    Repository Name: EPIC Alfred Wegener Institut
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  • 6
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    Ice-sheet elevations from across-track processing of airborne interferometric radar altimetry (2009)
    In:  EPIC3Geophysical Research Letters., 36(L22501)
    Details
    Publication Date: 2017-10-20
    Description: Interferometric Radar Altimeters (IRA's) use dual receive antennas to overcome one of the spatial limitations of pulse-limited altimeters. In a conventional IRA measurement, the range and across-track direction of a scatterer are determined using the phase difference between the antennas. We demonstrate a method of determining multiple elevation points across a swath orthogonal to the instrument ground track in regions of steep terrain, such as ice-sheet margins. We use data from an airborne IRA (a prototype of the CryoSat-2 instrument), and compare the results to simultaneous Airborne Laser Scanner (ALS) observations. This application results in a 75-fold increase in measurement density compared to conventional radar altimetry. Along a ∼2.5 km ground track, the RMS departure between the IRA- and ALS-derived measurements was 1.67 m. Based on our result, although our approach is limited to areas of relatively steep slope, a 25- to 75-fold increase in elevation measurements could be achieved in coastal regions of Antarctica and Greenland with similar processing of CryoSat-2 data.
    Repository Name: EPIC Alfred Wegener Institut
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  • 7
    Electronic Resource
    Electronic Resource
    Analysis of Chlorides (1947)
    s.l. : American Chemical Society
    Analytical chemistry 19 (1947), S. 841-842 
    Details
    ISSN: 1520-6882
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ac60011a007
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  • 8
    Electronic Resource
    Electronic Resource
    mei-41 is required for precocious anaphase in Drosophila females (2000)
    Springer
    Chromosoma 109 (2000), S. 44-49 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract. This paper reports on a new role for mei-41 in cell cycle control during meiosis. This function is revealed by the requirement of mei-41 for the precocious anaphase observed in crossover-defective mutants. Normally in Drosophila oocytes, tension on the meiotic spindle causes a metaphase I arrest. This tension results because crossovers, and the resulting chiasmata, hold homologs together that are being pulled by kinetochore microtobules toward opposite spindle poles. In the absence of tension, such as in a recombination-defective mutant, metaphase arrest is not observed and meiosis proceeds through the two divisions. Here we show that in some recombination-defective mutants, the precocious anaphase requires the mei-41 gene product. For example, metaphase arrest is not observed in mei-218 mutants because of the severe reduction in crossing over. In mei-41 mei-218 double mutants, however, metaphase arrest was restored. The effect of mei-41 is dependent on double-strand break formation. Thus, in mutants that fail to initiate meiotic recombination the absence of mei-41 has no effect.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004120050411
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  • 9
    Electronic Resource
    Electronic Resource
    Hanging on to your homolog: the roles of pairing, synapsis and recombination in the maintenance of homolog adhesion (2000)
    Springer
    Chromosoma 109 (2000), S. 3-9 
    Details
    ISSN: 1432-0886
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract. Homologous chromosomes initially undergo weak alignments that bring homologous sequences into register during meiosis. These alignments can be facilitated by two types of mechanisms: interstitial homology searches and telomere-telomere alignments. As prophase (and chromatin compaction) proceeds, these initial pairings or alignments need to be stabilized. In at least some organisms, such as Saccharomyces cerevisiae and S. pombe, these pairings can apparently be maintained by the creation of recombination intermediates. In contrast, synapsis during zygotene may be able to facilitate and/or maintain chromosome pairing even in the absence of exchange in several higher organisms. It thus seems possible that the synaptonemal complex plays a role both in maintaining homolog adhesion during meiotic prophase and, more speculatively, in facilitating meiotic exchange.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004120050407
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  • 10
    Electronic Resource
    Electronic Resource
    Possible Transition in the Initiating Mechanism Leading to Electrical Breakdown in Vacuum (1963)
    [s.l.] : Nature Publishing Group
    Nature 199 (1963), S. 978-980 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] MANY experiments have been carried out to determine the initiating event leading to electrical breakdown between metallic electrodes separated by high vacuum1'2. While the mechanism causing breakdown in certain cases, such as when using sharply pointed cathodes in extremely clean conditions, is now ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/199978a0
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