12,396 research outputs found
Sentinel-1 Imaging Performance Verification with TerraSAR-X
This paper presents dedicated analyses of TerraSAR-X data with respect to the Sentinel-1 TOPS imaging mode.
First, the analysis of Doppler centroid behaviour for high azimuth steering angles, as occurs in TOPS imaging, is
investigated followed by the analysis and compensation of residual scalloping. Finally, the Flexible-Dynamic
BAQ (FD-BAQ) raw data compression algorithm is investigated for the first time with real TerraSAR-X data
and its performance is compared to state-of-the-art BAQ algorithms. The presented analyses demonstrate the
improvements of the new TOPS imaging mode as well as the new FD-BAQ data compression algorithm for
SAR image quality in general and in particular for Sentinel-1
Common pulse retrieval algorithm: a fast and universal method to retrieve ultrashort pulses
We present a common pulse retrieval algorithm (COPRA) that can be used for a
broad category of ultrashort laser pulse measurement schemes including
frequency-resolved optical gating (FROG), interferometric FROG, dispersion
scan, time domain ptychography, and pulse shaper assisted techniques such as
multiphoton intrapulse interference phase scan (MIIPS). We demonstrate its
properties in comprehensive numerical tests and show that it is fast, reliable
and accurate in the presence of Gaussian noise. For FROG it outperforms
retrieval algorithms based on generalized projections and ptychography.
Furthermore, we discuss the pulse retrieval problem as a nonlinear
least-squares problem and demonstrate the importance of obtaining a
least-squares solution for noisy data. These results improve and extend the
possibilities of numerical pulse retrieval. COPRA is faster and provides more
accurate results in comparison to existing retrieval algorithms. Furthermore,
it enables full pulse retrieval from measurements for which no retrieval
algorithm was known before, e.g., MIIPS measurements
The effect of signal digitisation in CMB experiments
Signal digitisation may produce significant effects in balloon - borne or
space CMB experiments, since the limited bandwidth for downlink of data
requires imposes a large quantisation step q applied on board by the instrument
acquisition chain. In this paper we present a study of the impact of the
quantization error in CMB experiments using, as a working case, simulated data
from the Planck/LFI. At TOD level, the effect of the quantization can be
approximated as a source of nearly normally distributed noise. At map level,
the data quantization alters the noise distribution and the expectation of some
higher order moments. Finally, at the levell of power spectra, the quantization
introduces a power excess, that, although related to the instrument and mission
parameters, is weakly dependent on the multipole l at middle and large l and
can be quite accurately subtracted, leaving a residual uncertainty of few % of
the RMS uncertainty. Only for l<30 the quantization removal is less accurate.Comment: 15 pages, 5 figures, LaTeX2e, A&A style (aa.cls). Release 1, april
1st 2003. Submitted to A&A for the pubblication, april 1st 2003. Contact
author: [email protected]
Distributed top-k aggregation queries at large
Top-k query processing is a fundamental building block for efficient ranking in a large number of applications. Efficiency is a central issue, especially for distributed settings, when the data is spread across different nodes in a network. This paper introduces novel optimization methods for top-k aggregation queries in such distributed environments. The optimizations can be applied to all algorithms that fall into the frameworks of the prior TPUT and KLEE methods. The optimizations address three degrees of freedom: 1) hierarchically grouping input lists into top-k operator trees and optimizing the tree structure, 2) computing data-adaptive scan depths for different input sources, and 3) data-adaptive sampling of a small subset of input sources in scenarios with hundreds or thousands of query-relevant network nodes. All optimizations are based on a statistical cost model that utilizes local synopses, e.g., in the form of histograms, efficiently computed convolutions, and estimators based on order statistics. The paper presents comprehensive experiments, with three different real-life datasets and using the ns-2 network simulator for a packet-level simulation of a large Internet-style network
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