113 research outputs found
A realistic simulation framework to evaluate ionospheric tomography
Observation of the 3-dimensional (3-D) electron density of the ionosphere is useful to study large-scale physical processes in space weather events. Ionospheric data assimilation and ionospheric tomography are methods that can create an image of the 3-D electron density distribution. While multiple techniques have been developed over the past 30 years, there are relatively few studies that show the accuracy of the algorithms. This paper outlines a novel simulation approach to test the quality of an ionospheric tomographic inversion. The approach uses observations from incoherent scatter radar (ISR) scans and extrapolates them spatially to create a realistic ionospheric representation. A set of total electron content (TEC) measurements can then be simulated using real geometries from satellites and ground receivers. This data set, for which the ’truth’ ionosphere is known, is used as input for a tomographic inversion algorithm to estimate the spatial distribution of electron density. The reconstructed ionospheric maps are compared with the truth ionosphere to calculate the difference between the images and the truth. To demonstrate the effectiveness of this simulation framework, an inversion algorithm called MIDAS (Multi-Instrument Data Analysis Software) is evaluated for three geographic regions with differing receiver networks. The results show the importance of the distribution and density of GPS receivers and the use of a realistic prior conditioning of the vertical electron density profile. This paper demonstrates that when these requirements are met, MIDAS can reliably estimate the ionospheric electron density. When the region under study is well covered by GPS receivers, as in mainland Europe or North America, the errors in vertical total electron content (vTEC) are smaller than 1 TECu (2-4%) . In regions with fewer and more sparsely distributed receivers, the errors can be as high as 20-40%. This is caused by poor data coverage and poor spatial resolution of the reconstruction, which has an important effect on the calibration process of the algorithm
Agent swarms: cooperation and coordination under stringent communications constraint
Here we consider the communications tactics appropriate for a group of agents
that need to ``swarm'' together in a highly adversarial environment.
Specifically, whilst they need to exchange information with each other about
their location and their plans; at the same time they also need to keep such
communications to an absolute minimum. This might be due to a need for stealth,
or otherwise be relevant to situations where communications are significantly
restricted. Complicating this process is that we assume each agent has (a) no
means of passively locating others, (b) it must rely on being updated by
reception of appropriate messages; and if no such update messages arrive, (c)
then their own beliefs about other agents will gradually become out of date and
increasingly inaccurate. Here we use a geometry-free multi-agent model that is
capable of allowing for message-based information transfer between agents with
different intrinsic connectivities, as would be present in a spatial
arrangement of agents. We present agent-centric performance metrics that
require only minimal assumptions, and show how simulated outcome distributions,
risks, and connectivities depend on the ratio of information gain to loss. We
also show that checking for too-long round-trip-times can be an effective
minimal-information filter for determining which agents to no longer target
with messages.Comment: 11 pager, 7 figure
Evidence for the Magnetoionic Nature of Oblique VHF Reflections from Midlatitude Sporadic-E Layers
Mid-latitude sporadic-E (Es) is an intermittent phenomenon of the lower E region of the ionosphere. Es clouds are thin, transient, and patchy layers of intense ionization, with ionization densities which can be much higher than in the background ionosphere. Oblique reflection of radio signals in the very high frequency (VHF) range is regularly supported, but the mechanism for it has never been clearly established—specular reflection, scattering, and magnetoionic double refraction have all been suggested. This article proposes using the polarization behaviour of signals reflected from intense midlatitude sporadic-E clouds as an indicator of the true reflection mechanism. Results are presented from a measurement campaign in the summer of 2018, which gathered a large amount of data at a receiving station in the UK using 50 MHz amateur radio beacons as signal sources. In all cases the signals received were elliptically polarized, despite being transmitted with linear polarization; there were also indications that polarization behaviour varied systematically with the orientation of the path to the geomagnetic field. This represents, for all the examples recorded, clear evidence that signals were reflected from midlatitude Es by magnetoionic double refraction
Application of radio tomographic imaging to HF oblique incidence ray tracing
Radio tomography is a technique for generating images of the spatial structure of ionospheric electron density over a wide area. This paper assesses the potential use of radio tomography in HF oblique propagation and ray tracing applications. Synthetic ionograms produced by ray tracing through tomographic images and ionospheric models have been compared with experimental oblique ionograms from six paths lying close to the image plane in the United Kingdom. In particular, study has been made of the effects of various types of input information used to constrain the vertical electron density structure in the tomographic reconstructions. It was found that use of a fine height resolution (5 km) and incorporation of information from one vertical ionosonde in the reconstruction process makes significant improvements to the overall reliability of the tomographic image. As expected, E layer propagation is better defined using a climatological model than by tomography. However, in comparison with three ionospheric models, use of tomographic images can significantly reduce the RMS error in the determination of the F2 layer maximum usable frequency
Climatology of GPS scintillations over Antarctica under solar minimum conditions
We analyse GNSS ionospheric scintillation data recorded in Antarctica to investigate the conditions of the near-
Earth environment leading to scintillation scenarios, producing a “scintillation climatology” over a large geomagnetic
quiet period.Within this scope we realize maps of scintillation occurrence as a function of the magnetic local
time (MLT) and of the altitude adjusted corrected geomagnetic coordinates (AACGM). The maps are realized
merging observations of two GISTMs (GPS Ionospheric Scintillation and TEC Monitor) located at Mario Zucchelli
Station (74.7°S, 164.1°E) and Concordia Station (75.1°S, 123.2°E) in Antarctica during 2008. The results
highlight the possibility to investigate the impact of ionospheric irregularities on the phase and amplitude of GNSS
signals, evidencing the cusp/cap and auroral contributions. This works aims to contribute to the development of
nowcasting and forecasting tools for GNSS ionospheric scintillation
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