Tackling ionospheric scintillation threat to GNSS in Latin America

Scintillations are rapid fluctuations in the phase and amplitude of transionospheric radio signals which are caused by small-scale plasma density irregularities in the ionosphere. In the case of the Global Navigation Satellite System (GNSS) receivers, scintillation can cause cycle slips, degrade the positioning accuracy and, when severe enough, can even lead to a complete loss of signal lock. Thus, the required levels of availability, accuracy, integrity and reliability for the GNSS applications may not be met during scintillation occurrence; this poses a major threat to a large number of modern-day GNSS-based applications. The whole of Latin America, Brazil in particular, is located in one of the regions most affected by scintillations. These effects will be exacerbated during solar maxima, the next predicted for 2013. This paper presents initial results from a research work aimed to tackle ionospheric scintillation effects for GNSS users in Latin America. This research is a part of the CIGALA (Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America) project, co-funded by the EC Seventh Framework Program and supervised by the GNSS Supervisory Authority (GSA), which aims to develop and test ionospheric scintillation countermeasures to be implemented in multi-frequency, multi-constellation GNSS receivers.

Análise da cintilação ionosférica no Brasil empregando GNSS e técnicas de mineração e visualização de dados

Pós-graduação em Ciências Cartográficas - FCT

Tomografia da ionosfera a partir do GNSS e técnicas algébricas: estudo de caso para a região brasileira

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Monitoramento das irregularidades ionosféricas e cintilação dos sinais GNSS: em tempo real e pós-processado

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Strain rates estimated by geodetic observations in the Borborema Province, Brazil

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Desempenho do RTK em rede sob efeito da cintilação ionosférica

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Análise de variância WAVELET de séries temporais com dados faltantes: avaliação do índice de cintilação ionosférica S4 em sinais de satélite GPS

Pós-graduação em Matematica Aplicada e Computacional - FCT

Análise de variância WAVELET de séries temporais com dados faltantes: avaliação do índice de cintilação ionosférica S4 em sinais de satélite GPS

Pós-graduação em Matematica Aplicada e Computacional - FCT

An Integrated Transmission-Media Noise Calibration Software For Deep-Space Radio Science Experiments

The thesis describes the implementation of a calibration, format-translation and data conditioning software for radiometric tracking data of deep-space spacecraft. All of the available propagation-media noise rejection techniques available as features in the code are covered in their mathematical formulations, performance and software implementations. Some techniques are retrieved from literature and current state of the art, while other algorithms have been conceived ex novo. All of the three typical deep-space refractive environments (solar plasma, ionosphere, troposphere) are dealt with by employing specific subroutines. Specific attention has been reserved to the GNSS-based tropospheric path delay calibration subroutine, since it is the most bulky module of the software suite, in terms of both the sheer number of lines of code, and development time. The software is currently in its final stage of development and once completed will serve as a pre-processing stage for orbit determination codes. Calibration of transmission-media noise sources in radiometric observables proved to be an essential operation to be performed of radiometric data in order to meet the more and more demanding error budget requirements of modern deep-space missions. A completely autonomous and all-around propagation-media calibration software is a novelty in orbit determination, although standalone codes are currently employed by ESA and NASA. The described S/W is planned to be compatible with the current standards for tropospheric noise calibration used by both these agencies like the AMC, TSAC and ESA IFMS weather data, and it natively works with the Tracking Data Message file format (TDM) adopted by CCSDS as standard aimed to promote and simplify inter-agency collaboration.

Oblique ionograms automatic scaling and eikonal based ray tracing

A method for automatic scaling of oblique ionograms has been introduced. This method also provides a rejection procedure for ionograms that are considered to lack sufficient information, depicting a very good success rate. Observing the Kp index of each autoscaled ionogram, can be noticed that the behavior of the autoscaling program does not depend on geomagnetic conditions. The comparison between the values of the MUF provided by the presented software and those obtained by an experienced operator indicate that the procedure developed for detecting the nose of oblique ionogram traces is sufficiently efficient and becomes much more efficient as the quality of the ionograms improves. These results demonstrate the program allows the real-time evaluation of MUF values associated with a particular radio link through an oblique radio sounding. The automatic recognition of a part of the trace allows determine for certain frequencies, the time taken by the radio wave to travel the path between the transmitter and receiver. The reconstruction of the ionogram traces, suggests the possibility of estimating the electron density between the transmitter and the receiver, from an oblique ionogram. The showed results have been obtained with a ray-tracing procedure based on the integration of the eikonal equation and using an analytical ionospheric model with free parameters. This indicates the possibility of applying an adaptive model and a ray-tracing algorithm to estimate the electron density in the ionosphere between the transmitter and the receiver An additional study has been conducted on a high quality ionospheric soundings data set and another algorithm has been designed for the conversion of an oblique ionogram into a vertical one, using Martyn's theorem. This allows a further analysis of oblique soundings, throw the use of the INGV Autoscala program for the automatic scaling of vertical ionograms.

A Near-Real-Time Automatic Orbit Determination System for COSMIC and Its Follow-On Satellite Mission: Analysis of Orbit and Clock Errors on Radio Occultation

The COSMIC-2 mission is a follow-on mission of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) with an upgraded payload for improved radio occultation (RO) applications. The objective of this paper is to develop a near-real-time (NRT) orbit determination system, called NRT National Chiao Tung University (NCTU) system, to support COSMIC-2 in atmospheric applications and verify the orbit product of COSMIC. The system is capable of automatic determinations of the NRT GPS clocks and LEO orbit and clock. To assess the NRT (NCTU) system, we use eight days of COSMIC data (March 24-31, 2011), which contain a total of 331 GPS observation sessions and 12 393 RO observable files. The parallel scheduling for independent GPS and LEO estimations and automatic time matching improves the computational efficiency by 64% compared to the sequential scheduling. Orbit difference analyses suggest a 10-cm accuracy for the COSMIC orbits from the NRT (NCTU) system, and it is consistent as the NRT University Corporation for Atmospheric Research (URCA) system. The mean velocity accuracy from the NRT orbits of COSMIC is 0.168 mm/s, corresponding to an error of about 0.051 μrad in the bending angle. The rms differences in the NRT COSMIC clock and in GPS clocks between the NRT (NCTU) and the postprocessing products are 3.742 and 1.427 ns. The GPS clocks determined from a partial ground GPS network [from NRT (NCTU)] and a full one [from NRT (UCAR)] result in mean rms frequency stabilities of 6.1E-12 and 2.7E-12, respectively, corresponding to range fluctuations of 5.5 and 2.4 cm and bending angle errors of 3.75 and 1.66 μrad .

GOCE: assessment of GPS-only gravity field determination

The GOCE satellite was orbiting the Earth in a Sun-synchronous orbit at a very low altitude for more than 4 years. This low orbit and the availability of high-quality data make it worthwhile to assess the contribution of GOCE GPS data to the recovery of both the static and time-variable gravity fields. We use the kinematic positions of the official GOCE precise science orbit (PSO) product to perform gravity field determination using the Celestial Mechanics Approach. The generated gravity field solutions reveal severe systematic errors centered along the geomagnetic equator. Their size is significantly coupled with the ionospheric density and thus generally increasing over the mission period. The systematic errors may be traced back to the kinematic positions of the PSO product and eventually to the ionosphere-free GPS carrier phase observations used for orbit determination. As they cannot be explained by the current higher order ionospheric correction model recommended by the IERS Conventions 2010, an empirical approach is presented by discarding GPS data affected by large ionospheric changes. Such a measure yields a strong reduction of the systematic errors along the geomagnetic equator in the gravity field recovery, and only marginally reduces the set of useable kinematic positions by at maximum 6 % for severe ionosphere conditions. Eventually it is shown that GOCE gravity field solutions based on kinematic positions have a limited sensitivity to the largest annual signal related to land hydrology.

(Table 2) Atmospheric electric measurements during the Atlantic Expedition 1965 of METEOR

During the Atlantic expedition potential gradient, small ion density and space charge density have been recorded. Laborious efforts have been taken for receiving an exact estimation of the reduction factor for the field measurements. The mean value of the potential gradient on the free Atlantic Ocean was 105 V/m. The mean daily course is in very good agreement with the results of the Carnegie Institution. Even records taken on individual days near the quator show this course. For the first time it has been attempted to correlate the potential gradient at sea and the voltage between ionosphere and earth measured over land. A narrow relation has been found in 10 cases of balloon ascents with radiosondes. A further remarkable result is, that the short periodical fluctuations of the air electric field at sea with periods of 2 to 20 minutes have amplitudes of the magnitude of the mean field strength and exist all over the oceans. Recordings of the space charge density show, that positively charged air parcels drift in the first hectometer of the air near the sea surface and produce the fluctuation of the potential gradient. A period analysis did not indicate a recognizable relation to the wind velocity up to now, although an effect of air turbulence must be involved. The concentration of small ions also has been measured occasionally. With this and mean values of the potential gradient the air earth curent density has been computed. With n+ = 310 cm**-3, n- = 220 cm**-3 the air conductivity would be Lambda = 1,14 * 10**-14 Ohm**-1 m**-1. These values are smaller than values of other authors by a factor of 2 or 3. Therefore the computed air earth current density is also smaller. The discrepancy could not be explained yet.

Atmospheric electric measurement in the troposphere and stratosphere on the Atlantic Ocean during 1965 and 1969

One main point of the air electric investigations at the atlantic 1965 and 1969 was the record of the potential gradient in the troposphere with free and captive balloon ascents. The course of the field vs. altitude above the sea differs from that over land. A remarkable enlargement of the field strength occurs at the altitude of the passat inversion. The electric voltage between ionosphere and earth could be obtained by integrating the potential gradient over the altitude. Such computations have been made by balloon ascents simultaneous over the ocean and at Weissenau (South Germany), From 15 simultaneous measurements the average value of the potential of the ionosphere over the ocean is 214 kV and over South Germany 216 kV, that means very close together. Because of the small differences also between the single values it can be concluded that in generally the ionosphere potential has an equal value over these both places at one moment. From the potential of the ionosphere VI, the field strength E0 and the conductivity lamda o, both measured at the sea surface, the columnar resistance R could be derived to 2.4 x 10**17 Ohm x m**2. By correlation of the single values of the ionosphere potential with the potential gradient measured simultaneously at the surface of the sea a linear proportional relationship exists; it follows from this result, that R is nearly constant. The mean value of the air-earth current density over the ocean could be calculated by using the measured values of the small ion density with respect to the electrode effect prooved at the equator station. The current density was only 0.9 x 10**-12 A/m**2, which means, a three and a half times smaller value than estimated by Carnegie and accepted up to now. Therefore it seems to be necessary to correct the former calculations of the global current balance.

Energy Analysis of Bare Electrodynamic Tethers

The design of an electrodynamic tether is a complex task that involves the control of dynamic instabilities, optimization of the generated power (or the descent time in deorbiting missions), and minimization of the tether mass. The electrodynamic forces on an electrodynamic tether are responsible for variations in the mechanical energy of the tethered system and can also drive the system to dynamic instability. Energy sources and sinks in this system include the following: 1) ionospheric impedance, 2) the potential drop at the cathodic contactor, 3) ohmic losses in the tether, 4) the corotational plasma electric field, and 5) generated power and/or 6) input power. The analysis of each of these energy components, or bricks, establishes parameters that are useful tools for tether design. In this study, the nondimensional parameters that govern the orbital energy variation, dynamic instability, and power generation were characterized, and their mutual interdependence was established. A space-debris mitigation mission was taken as an example of this approach for the assessment of tether performance. Numerical simulations using a dumbbell model for tether dynamics, the International Geomagnetic Reference Field for the geomagnetic field, and the International Reference Ionosphere for the ionosphere were performed to test the analytical approach. The results obtained herein stress the close relationships that exist among the velocity of descent, dynamic stability, and generated power. An optimal tether design requires a detailed tradeoff among these performances in a real-world scenario.