Biagio Forte

System Design for Geosynchronous Synthetic Aperture Radar Missions

Geosynchronous synthetic aperture radar (GEO SAR) has been studied for several decades but has not yet been implemented. This paper provides an overview of mission design, describing significant constraints (atmosphere, orbit, temporal stability of the surface and atmosphere, measurement physics, and radar performance) and then uses these to propose an approach to initial system design. The methodology encompasses all GEO SAR mission concepts proposed to date. Important classifications of missions are: 1) those that require atmospheric phase compensation to achieve their design spatial resolution; and 2) those that achieve full spatial resolution without phase compensation. Means of estimating the atmospheric phase screen are noted, including a novel measurement of the mean rate of change of the atmospheric phase delay, which GEO SAR enables. Candidate mission concepts are described. It seems likely that GEO SAR will be feasible in a wide range of situations, although extreme weather and unstable surfaces (e.g., water, tall vegetation) prevent 100% coverage. GEO SAR offers an exciting imaging capability that powerfully complements existing systems.

Scintillations observed at Tucuman as observed from GPS signals. First results

The occurrence of ionospheric irregularities was studied using Global Positioning System (GPS) measurements at Tucuman (26.9° S, 294.6° E; geomag. lat.: −15.5), a station placed near the souther peak of the equatorial anomaly. Radio scintillations at the GPS L1 frequency (1.575 GHz) were monitored using the GPS receiver installed at the mentioned station. The considered measurements were obtained during September 1998 – April 1999 period. The results show that amplitude scintillations frequently occur between 21 and 02 LT. Scintillations are more frequent when the satellites are in the northwest — northeast sector of Tucumans sky. Strong scintillations were observed in about 40% of the nights.

Identification of scintillation signatures on GPS signals originating from plasma structures detected with EISCAT incoherent scatter radar along the same line of sight

Abstract Ionospheric scintillation originates from the scattering of electromagnetic waves through spatial gradients in the plasma density distribution, drifting across a given propagation direction. Ionospheric scintillation represents a disruptive manifestation of adverse space weather conditions through degradation of the reliability and continuity of satellite telecommunication and navigation systems and services (e.g., European Geostationary Navigation Overlay Service, EGNOS). The purpose of the experiment presented here was to determine the contribution of auroral ionization structures to GPS scintillation. European Incoherent Scatter (EISCAT) measurements were obtained along the same line of sight of a given GPS satellite observed from Tromso and followed by means of the EISCAT UHF radar to causally identify plasma structures that give rise to scintillation on the co‐aligned GPS radio link. Large‐scale structures associated with the poleward edge of the ionospheric trough, with auroral arcs in the nightside auroral oval and with particle precipitation at the onset of a substorm were indeed identified as responsible for enhanced phase scintillation at L band. For the first time it was observed that the observed large‐scale structures did not cascade into smaller‐scale structures, leading to enhanced phase scintillation without amplitude scintillation. More measurements and theory are necessary to understand the mechanism responsible for the inhibition of large‐scale to small‐scale energy cascade and to reproduce the observations. This aspect is fundamental to model the scattering of radio waves propagating through these ionization structures. New insights from this experiment allow a better characterization of the impact that space weather can have on satellite telecommunications and navigation services.

Three‐dimensional modeling of high‐latitude scintillation observations

Global Navigation Satellite System (GNSS) signals exhibit rapid fluctuations at high and low latitudes as a consequence of propagation through drifting ionospheric irregularities. We focus on the high latitude scintillation problem, taking advantage of a conjunction of EISCAT Incoherent Scatter Radar (ISR) observations and a GPS scintillation monitor viewing the same line-of-sight. Just after 20:00 UT on 17 October 2013, an auroral E-region ionization enhancement occurred with associated phase scintillations. This investigation uses the scintillation observations to estimate the ionospheric electron density distribution beyond the spatial resolution of the ISR (5 - 15 km along the line-of-sight in this case). Following the approach of Deshpande et al. [2014], signal propagation is modeled through a specified density distribution. A multiple phase screen propagation algorithm is applied to irregularities conforming to the description of Costa and Kelley [1977] and constrained to match the macroscopic conditions observed by the ISR. A 50-member ensemble of modeled outputs is approximately consistent with the observations according to the standard deviation of the phase (σp). The observations have σp = 0.23 radians, while the ensemble of modeled realizations has σp = 0.23 + 0.04 -0.04. By comparison of the model output with the scintillation observations, we show that the density fluctuations cannot be a constant fraction of the mean density. The model indicates that E-region density fluctuations whose standard deviation varies temporally between 5 - 25% of the mean (ISR-observed) density are required to explain the observed phase scintillations.

A virtual engine laboratory for teaching powertrain engineering

A virtual engine laboratory application for use in automotive engineering education is proposed to allow the practical teaching of powertrain calibration. The laboratory software is built as a flexible Matlab tool that can easily be transferred for applications in other disciplines and promotes the link between teaching and research.

On the estimate and assessment of the ionospheric effects affecting low frequency radio astronomy measurements

The development of the LOw Frequency telescopes ARray (LOFAR) has posed a serious issue on the calibration of those measurements in the presence of the Earths ionosphere. The purpose of measuring at radio frequencies as low as VHF exposes LOFAR to a number of ionospheric phenomena, capable of deteriorating the accuracy of the measurements and subsequently of the sky imaging. The ionosphere is normally treated at signal processing level, where various efforts attempt to remove possible errors introduced by it. Here, a close look at particular ionospheric features and their possible consequence to radio astronomy measurements is given from a point of view of the ionospheric radio wave propagation. It seems the radio astronomy and ionosphere communities will need to work closely together in order to achieve a satisfactory solution to the problem.

GPS availability and positioning issues when the signal paths are aligned with ionospheric plasma bubbles

The propagation paths of signals through equatorial ionospheric irregularities are analyzed by evaluating their effects on Global Navigation Satellite System (GNSS) positioning and availability. Based on observations during 32 days by a scintillation monitor at São José dos Campos, Brazil, it was noted that there is a dominance of enhanced scintillation events for Global Positioning System (GPS) ray paths aligned with the azimuth angle of 345° (geographic northwest). This azimuth corresponds to the magnetic meridian that has a large westward declination angle in the region (21.4ºW). Such results suggest that the enhanced scintillation events were associated with GPS signals that propagated through plasma bubbles aligned along the direction of the magnetic field. It will be shown that, under this alignment condition, the longer propagation path length through plasma bubbles can result in more severe scintillation cases and more losses of signal lock, as supported by proposed statistics of bit error probability and mean time between cycle slips. Additionally, large precise positioning errors are also related to these events, as demonstrated by precise point positioning experiments.

Separating Nightside Interplanetary and Ionospheric Scintillation with LOFAR

Observation of interplanetary scintillation (IPS) beyond Earth-orbit can be challenging due to the necessity to use low radio frequencies at which scintillation due to the ionosphere could confuse the interplanetary contribution. A recent paper by Kaplan {\it et al} (2015) presenting observations using the Murchison Widefield Array (MWA) reports evidence of night-side IPS on two radio sources within their field of view. However, the low time cadence of 2\,s used might be expected to average out the IPS signal, resulting in the reasonable assumption that the scintillation is more likely to be ionospheric in origin. To verify or otherwise this assumption, this letter uses observations of IPS taken at a high time cadence using the Low Frequency Array (LOFAR). Averaging these to the same as the MWA observations, we demonstrate that the MWA result is consistent with IPS, although some contribution from the ionosphere cannot be ruled out. These LOFAR observations represent the first of night-side IPS using LOFAR, with solar wind speeds consistent with a slow solar wind stream in one observation and a CME expecting to be observed in another.

Analysis of the Regional Ionosphere at Low Latitudes in Support of the Biomass ESA Mission

Biomass is a spaceborn polarimetric P-band (435 MHz) synthetic aperture radar (SAR) in a dawn–dusk low Earth orbit. Its principal objective is to measure biomass content and change in all the Earth’s forests. The ionosphere introduces the Faraday rotation on every pulse emitted by low-frequency SAR and scintillations when the pulse traverses a region of plasma irregularities, consequently impacting the quality of the imaging. Some of these effects are due to total electron content (TEC) and its gradients along the propagation path. Therefore, an accurate assessment of the ionospheric morphology and dynamics is necessary to properly understand the impact on image quality, especially in the equatorial and tropical regions. To this scope, we have conducted an in-depth investigation of the significant noise budget introduced by the two crests of the equatorial ionospheric anomaly (EIA) over Brazil and Southeast Asia. This paper is characterized by a novel approach to conceive a SAR-oriented ionospheric assessment, aimed at detecting and identifying spatial and temporal TEC gradients, including scintillation effects and traveling ionospheric disturbances, by means of Global Navigation Satellite Systems ground-based monitoring stations. The novelty of this approach resides in the customization of the information about the impact of the ionosphere on SAR imaging as derived by local dense networks of ground instruments operating during the passes of Biomass spacecraft. The results identify the EIA crests as the regions hosting the bulk of irregularities potentially causing degradation on SAR imaging. Interesting insights about the local characteristics of low-latitudes ionosphere are also highlighted.

On the residual ionospheric error in radio occultation measurements

The residual ionospheric error (RIE) in radio occultation (RO) measurement analysis is investigated through both analytic and numerical ray tracing. The study has shown that the difference between the propagation paths at different frequencies is an extremely important factor in determining the level of the RIE. Further, that horizontal ionospheric structure, such as the equatorial anomaly, can have a substantial impact. The results confirm the conclusion of previous studies that asymmetry in the ionosphere between the upward and downward legs of the propagation is a significant factor. The study has also considered the effect of travelling ionospheric disturbances upon the RIE. It has been shown that there can be dramatic changes in RIE through the full period of a TID. Consequently, because of their frequency and unpredictability, TIDs have the potential to set a limit on the achievable accuracy of RO measurement analysis.