11 research outputs found
Broadband Meter-Wavelength Observations of Ionospheric Scintillation
Intensity scintillations of cosmic radio sources are used to study
astrophysical plasmas like the ionosphere, the solar wind, and the interstellar
medium. Normally these observations are relatively narrow band. With Low
Frequency Array (LOFAR) technology at the Kilpisj\"arvi Atmospheric Imaging
Receiver Array (KAIRA) station in northern Finland we have observed
scintillations over a 3 octave bandwidth. ``Parabolic arcs'', which were
discovered in interstellar scintillations of pulsars, can provide precise
estimates of the distance and velocity of the scattering plasma. Here we report
the first observations of such arcs in the ionosphere and the first broad-band
observations of arcs anywhere, raising hopes that study of the phenomenon may
similarly improve the analysis of ionospheric scintillations. These
observations were made of the strong natural radio source Cygnus-A and covered
the entire 30-250\,MHz band of KAIRA. Well-defined parabolic arcs were seen
early in the observations, before transit, and disappeared after transit
although scintillations continued to be obvious during the entire observation.
We show that this can be attributed to the structure of Cygnus-A. Initial
results from modeling these scintillation arcs are consistent with simultaneous
ionospheric soundings taken with other instruments, and indicate that
scattering is most likely to be associated more with the topside ionosphere
than the F-region peak altitude. Further modeling and possible extension to
interferometric observations, using international LOFAR stations, are
discussed.Comment: 11 pages, 17 figure
On point sources and near field measurements in inverse acoustic obstacle scattering
Abstract
The dissertation considers an inverse acoustic obstacle scattering
problem in which the incident field is generated by a point source and the
measurements are made in the near field region.
Three methods to solve the problem of reconstructing the support of
an unknown sound-soft or sound-hard scatterer from the near field
measurements are presented. Methods are modifications of Kirsch
factorization and modified Kirsch factorization methods. Numerical
examples are given to show the practicality of one of the methods
EISCAT_3D final report of work package 11
Deliverable D11.2 of Work Package 11:
Software Theory and Implementation of the European Commission 7th Framework Programme project
EISCAT_3D: A European three-dimensional imaging radar for atmospheric and geospace research (Preparatory Phase)
Project number: 26196
Transmission code optimization method for incoherent scatter radar
When statistical inversion of a lag profile is used to determine an
incoherent scatter target, the posterior variance of the estimated
target can be used to determine how well a set of transmission codes
perform. In this work we present an incoherent scatter radar
transmission code optimization search method suitable for different
modulation types, including binary phase, polyphase and amplitude
modulation. We found that the combination of amplitude and phase
modulation provides better performance than traditional binary phase
coding, in some cases giving better accuracy than alternating codes
Optimal true time delay filter with application to FPGA firmware-based phased array radar signal processing
Abstract
The European Incoherent Scatterâ3D phased array radar system will be largely based on fieldâprogrammable gate array (FPGA) firmware electronics that carry out the signal processing by using different digital filters. In this paper we have presented a method of designing an optimal true time delay finite impulse response filter with applications to an FPGA firmwareâbased multichannel signal processing system. The method provides an optimal true time delay finite impulse response filter with the desired responses at both band pass and stopband. This is possible by finding a mathematical minimization solution for the total power of all filter coefficients longer than a prespecified halfâlength. The analysis is based on freely choosing the responses in the transition band until userâspecified desired responses are achieved. We have investigated the performance of these optimal digital filters in terms of the required digital signal processing (DSP) resources in GMACs (giga multiply accumulates per second) by considering both allâinâone stage filtering and cascaded solutions for ion and plasma line incoherent scatter radar measurements. We have shown that the cascaded solution provides more efficient utilization of DSP resources and hence represents the optimal choice for processing the proposed European Incoherent Scatterâ3D phased array radar signals. An example is demonstrated in which 906.88 GMACs are required to process 208 ion line beams with 2Ă4âbit resolution in allâinâone stage processing, as compared to the 79.16 GMACs needed for a similar task in a cascaded solution
Lag profile inversion method for EISCAT data analysis
The present standard EISCAT incoherent scatter experiments are based on
alternating codes that are decoded in power domain by simple summation and
subtraction operations. The signal is first digitised and then different
lagged products are calculated and decoded in real time. Only the decoded
lagged products are saved for further analysis so that both the original data
samples and the undecoded lagged products are lost. A fit of plasma
parameters can be later performed using the recorded lagged products. In this
paper we describe a different analysis method, which makes use of statistical
inversion in removing range ambiguities from the lag profiles. An analysis
program carrying out both the lag profile inversion and the fit of the plasma
parameters has been constructed. Because recording the received signal itself
instead of the lagged products allows very flexible data analysis, the
program is constructed to use raw data, i.e. IQ-sampled signal recorded from
an IF stage of the radar. The program is now capable of analysing standard
alternating-coded EISCAT experiments as well as experiments with any other
kind of radar modulation if raw data is available. The program calculates the
ambiguous lag profiles and is capable of inverting them as such but, for
analysis in real time, time integration is needed before inversion. We
demonstrate the method using alternating code experiments in the EISCAT UHF
radar and specific hardware connected to the second IF stage of the receiver.
This method produces a data stream of complex samples, which are stored for
later processing. The raw data is analysed with lag profile inversion and the
results are compared to those given by the standard method
Gaussian Markov random field priors in ionospheric 3D multi-instrument tomography
In ionospheric tomography, the atmospheric electron density is reconstructed from different electron density related measurements, most often from ground-based measurements of satellite signals. Typically, ionospheric tomography suffers from two major complications. First, the information provided by measurements is insufficient and additional information is required to obtain a unique solution. Second, with necessary spatial and temporal resolutions, the problem becomes very high dimensional, and hence, computationally infeasible. With Bayesian framework, the required additional information can be given with prior probability distributions. The approach then provides physically quantifiable probabilistic interpretation for all model variables. Here, Gaussian Markov random fields (GMRFs) are used for constructing the prior electron density distribution. The use of GMRF introduces sparsity to the linear system, making the problem computationally feasible. The method is demonstrated over Fennoscandia with measurements from global navigation satellite system (GNSS) and low Earth orbit (LEO) satellite receiver networks, GNSS occultation receivers, LEO satellite Langmuir probes, and ionosonde and incoherent scatter radar measurements
Gaussian Markov random field priors in ionospheric 3-D multi-instrument tomography
Abstract
In ionospheric tomography, the atmospheric electron density is reconstructed from different electron density related measurements, most often from ground-based measurements of satellite signals. Typically, ionospheric tomography suffers from two major complications. First, the information provided by measurements is insufficient and additional information is required to obtain a unique solution. Second, with necessary spatial and temporal resolutions, the problem becomes very high dimensional, and hence, computationally infeasible. With Bayesian framework, the required additional information can be given with prior probability distributions. The approach then provides physically quantifiable probabilistic interpretation for all model variables. Here, Gaussian Markov random fields (GMRFs) are used for constructing the prior electron density distribution. The use of GMRF introduces sparsity to the linear system, making the problem computationally feasible. The method is demonstrated over Fennoscandia with measurements from global navigation satellite system (GNSS) and low Earth orbit (LEO) satellite receiver networks, GNSS occultation receivers, LEO satellite Langmuir probes, and ionosonde and incoherent scatter radar measurements