50 research outputs found
Comparison of ionospheric scintillation models with experimental data for satellite navigation applications
A comparison between two of the most used scintillation models and experimental data is presented. The experimental data have been derived from a GPS scintillation monitor developed at Cornell University and placed in Tucuman (Argentina), under the peak of the anomaly. The models used (GISM and WBMOD) have been run for the geophysical conditions corresponding to the measurements. The comparison is done by subdividing the information on the basis of an ionospheric grid of 5°×5° surface square boxes. The comparison has been performed for several local times, from 18 LT until 04 LT. Here, only a few cases of particular interest are shown. The goal is to understand if the models are able to forecast actual scintillation morphology (from the satellite navigation systems point of view) and if they could be used to yield an estimate of scintillation effects on satellite navigation systems
Methodology to estimate ionospheric scintillation risk maps and their contribution to position dilution of precision on the ground
Satellite-based communications, navigation systems and many scientific
instruments rely on observations of trans-ionospheric signals. The quality of
these signals can be deteriorated by ionospheric scintillation which can have
detrimental effects on the mentioned applications. Therefore, monitoring of
ionospheric scintillation and quantifying its effect on the ground are of
significant interest. In this work, we develop a methodology which estimates
the scintillation induced ionospheric uncertainties in the sky and translates
their impact to the end-users on the ground. First, by using the risk concept
from decision theory and by exploiting the intensity and duration of
scintillation events (as measured by the S4 index), we estimate ionospheric
risk maps that could readily give an initial impression on the effects of
scintillation on the satellite-receiver communication. However, to better
understand the influence of scintillation on the positioning accuracy on the
ground, we formulate a new weighted dilution of precision (WPDOP) measure that
incorporates the ionospheric scintillation risks as weighting factors for the
given satellite-receiver constellations. These weights depend implicitly on
scintillation intensity and duration thresholds which can be specified by the
end-user based on the sensitivity of the application, for example. We
demonstrate our methodology by using scintillation data from South America, and
produce ionospheric risk maps which illustrate broad scintillation activity,
especially at the equatorial anomaly. Moreover, we construct ground maps of
WPDOP over a grid of hypothetical receivers which reveal that ionospheric
scintillation can also affect such regions of the continent that are not
exactly under the observed ionospheric scintillation structures. Particularly,
this is evident in cases when only the Global Positioning System (GPS) is
available.Comment: Keywords: Ionospheric scintillation risk, dilution of precision,
statistics error covariances, weights, South America, S4 index, GNSS
positioning uncertaint
Impact of Swarm GPS receiver updates on POD performance
The Swarm satellites are equipped with state-of-the-art Global Positioning System (GPS) receivers, which are used for
the precise geolocation of the magnetic and electric field instruments, as well as for the determination of the Earth’s
gravity field, the total electron content and low-frequency thermospheric neutral densities. The onboard GPS receivers
deliver high-quality data with an almost continuous data rate. However, the receivers show a slightly degraded
performance when flying over the geomagnetic poles and the geomagnetic equator, due to ionospheric scintillation.
Furthermore, with only eight channels available for dual-frequency tracking, the amount of collected GPS tracking
data is relatively low compared with various other missions. Therefore, several modifications have been implemented
to the Swarm GPS receivers. To optimise the amount of collected GPS data, the GPS antenna elevation mask has
slowly been reduced from 10° to 2°. To improve the robustness against ionospheric scintillation, the bandwidths of
the GPS receiver tracking loops have been widened. Because these modifications were first implemented on Swarm-
C, their impact can be assessed by a comparison with the close flying Swarm-A satellite. This shows that both modifications
have a positive impact on the GPS receiver performance. The reduced elevation mask increases the amount of
GPS tracking data by more than 3 %, while the updated tracking loops lead to around 1.3 % more observations and a
significant reduction in tracking losses due to severe equatorial scintillation. The additional observations at low elevation
angles increase the average noise of the carrier phase observations, but nonetheless slightly improve the resulting
reduced-dynamic and kinematic orbit accuracy as shown by independent satellite laser ranging (SLR) validation.
The more robust tracking loops significantly reduce the large carrier phase observation errors at the geomagnetic
poles and along the geomagnetic equator and do not degrade the observations at midlatitudes. SLR validation indicates
that the updated tracking loops also improve the reduced-dynamic and kinematic orbit accuracy. It is expected
that the Swarm gravity field recovery will benefit from the improved kinematic orbit quality and potentially also from
the expected improvement of the kinematic baseline determination and the anticipated reduction in the systematic
gravity field errors along the geomagnetic equator. Finally, other satellites that carry GPS receivers that encounter
similar disturbances might also benefit from this analysis
A different approach to the analysis of GPS scintillation data
Amplitude scintillation data from GPS were analyzed. The objective is to estimate the impact of ionospheric scintillations at Satellite Based Augmentation Systems Ranging and Integrity Monitoring Station (SBAS RIMS) level and at GPS user level. For this purpose, a new approach to the problem was considered. Data were studied from the point of view of the impact of scintillations on the calculation of VTEC at pierce points and ionospheric grid points. An ionospheric grid of 5° 5° surface squares was assumed. From geometrical considerations and taking into account the basic principle to compute VTEC at grid points, with the data analyzed it is shown that scintillations very seldom affect the calculation of a grid point VTEC. Data from all the RIMS and for the entire GPS satellites network must be analyzed simultaneously to describe a realistic scenario for the impact of scintillations on SBAS. Finally, GPS scintillation data were analyzed at user level: service availability problems were encountered
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Interpretation of Radio Wave Scintillation Observed through LOFAR Radio Telescopes
Radio waves propagating through a medium containing irregularities in the spatial distribution of the electron density develop fluctuations in their intensities and phases. In the case of radio waves emitted from astronomical objects, they propagate through electron density irregularities in the interstellar medium, the interplanetary medium, and Earth’s ionosphere. The LOFAR radio telescope, with stations across Europe, can measure intensity across the VHF radio band and thus intensity scintillation on the signals received from compact astronomical objects. Modeling intensity scintillation allows the estimate of various parameters of the propagation medium, for example, its drift velocity and its turbulent power spectrum. However, these estimates are based on the assumptions of ergodicity of the observed intensity fluctuations and, typically, of weak scattering. A case study of single-station LOFAR observations of the strong astronomical source Cassiopeia A in the VHF range is utilized to illustrate deviations from ergodicity, as well as the presence of both weak and strong scattering. Here it is demonstrated how these aspects can lead to misleading estimates of the propagation medium properties, for example, in the solar wind. This analysis provides a method to model errors in these estimates, which can be used in the characterization of both the interplanetary medium and Earth’s ionosphere. Although the discussion is limited to the case of the interplanetary medium and Earth’s ionosphere, its ideas are also applicable to the case of the interstellar medium
Identification of scintillation signatures on GPS signals originating from plasma structures detected with EISCAT incoherent scatter radar along the same line of sight
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. EGNOS). The purpose of the experiment presented here was to determine the contribution of auroral ionisation structures to GPS scintillation. EISCAT measurements were obtained along the same line of sight of a given GPS satellite observed from Tromso and followed by means of the ESCAT 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 northern 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-to-small scale energy cascade and to reproduce the observations. This aspect is fundamental to model the scattering of radio waves propagating through these ionisation structures. New insights from this experiment allow a better characterisation of the impact that space weather can have on satellite telecommunications and navigation services
A novel approach to improve GNSS Precise Point Positioning during strong ionospheric scintillation: theory and demonstration
At equatorial latitudes, ionospheric scintillation is the major limitation in achieving high-accuracy GNSS positioning. This is because scintillation affects the tracking ability of GNSS receivers causing losses of lock and degradation on code pseudorange and carrier phase measurements, thus degrading accuracy. During strong ionospheric scintillation, such effects are more severe and GNSS users cannot rely on the integrity, reliability, and availability required for safety-critical applications. In this paper, we propose a novel approach able to greatly reduce these effects of scintillation on precise point positioning (PPP). Our new approach consists of three steps: 1) a new functional model that corrects the effects of range errors in the observables; 2) a new stochastic model that uses these corrections to generate more accurate positioning; and 3) a new strategy to attenuate the effects of losses of lock and consequent ambiguities re-initializations that are caused by the need to re-initialize the tracking. We demonstrate the effectiveness of our method in an experiment using a 30-day static dataset affected by different levels of scintillation in the Brazilian southeastern region. Even with limitations imposed by data gaps, our results demonstrate improvements of up to 80% in the positioning accuracy. We show that, in the best cases, our method can completely negate the effects of ionospheric scintillation and can recover the original PPP accuracy that would have existed without any scintillation. The significance of this paper lies in the improvement it offers in the integrity, reliability, and availability of GNSS services and applications.</p