C. Brunini

A New Ionosphere Monitoring Technology Based on GPS

Although global positioning system (GPS) was originally planned as a satellite-based radio-navigation system for military purposes, civilian users have significantly increased their access to the system for both, commercial and scientific applications. Almost 400 permanent GPS tracking stations have been stablished around the globe with the main purpose of supporting scientific research. In addition, several GPS receivers on board of low Earth orbit satellites fitted with special antennas that focus on Earths horizon, are tracking the radio signals broadcasted by the high-orbiting GPS satellites, as they rise and set on Earth horizon. The data of these ground and space-born GPS receivers, readily accessible through Internet in a ‘virtual observatory’ managed by the International GPS Service, are extensively used for many researches and might possibly ignite a revolution in Earth remote sensing.By measuring the changes in the time it takes for the GPS signals to arrive at the receiver as they travel through Earths atmosphere, scientists can derive a surprising amount of information about the Earths ionosphere, a turbulent shroud of charged particles that, when stimulated by solar flares, can disrupt communications around the world. This contribution presents a methodology to obtain high temporal resolution images of the ionospheric electron content that lead to two-dimensional vertical total electron content maps and three-dimensional electron density distribution. Some exemplifying results are shown at the end of the paper.

Interferometric meteor head echo observations using the Southern Argentina Agile Meteor Radar

A radar meteor echo is the radar scattering signature from the free electrons generated by the entry of extraterrestrial particles into the atmosphere. Three categories of scattering mechanisms exist: specular, nonspecular trails, and head echoes. Generally, there are two types of radars utilized to detect meteors. Traditional VHF all-sky meteor radars primarily detect the specular trails, while high-power, large-aperture (HPLA) radars efficiently detect meteor head echoes and, in some cases, nonspecular trails. The fact that head echo measurements can be performed only with HPLA radars limits these studies in several ways. HPLA radars are sensitive instruments constraining the studies to the lower masses, and these observations cannot be performed continuously because they take place at national observatories with limited allocated observing time. These drawbacks can be addressed by developing head echo observing techniques with modified all-sky meteor radars. Such systems would also permit simultaneous detection of all different scattering mechanisms using the same instrument, rather than requiring assorted different classes of radars, which can help clarify observed differences between the different methodologies. In this study, we demonstrate that such concurrent observations are now possible, enabled by the enhanced design of the Southern Argentina Agile Meteor Radar (SAAMER). The results presented here are derived from observations performed over a period of 12 days in August 2011 and include meteoroid dynamical parameter distributions, radiants, and estimated masses. Overall, the SAAMERs head echo detections appear to be produced by larger particles than those which have been studied thus far using this technique.

Improved Analysis Strategy and Accessibility of the SIRGAS Reference Frame

The SIRGAS reference system is at present realized by the SIRGAS Continuously Operating Network (SIRGAS-CON) composed by about 200 stations distributed over Latin America and the Caribbean. SIRGAS member countries are improving their national reference frames by installing continuously operating GPS stations, which have to be consistently integrated into the continental network. As the number of these stations is rapidly increasing, the analysis strategy of the SIRGAS-CON network is based on two hierarchy levels: a) A core network with homogeneous continental coverage and stable site locations ensures the long-term stability of the reference frame. This network is processed by DGFI (Germany) as the IGS RNAAC SIR. b) Several densification sub-networks (corresponding to the national reference networks) improve the accessibility to the reference frame in the individual countries. Currently, the SIRGAS-CON stations are classified in three densification sub-networks (a southern, a middle, and a northern one), which are processed by the SIRGAS Local Processing Centres CIMA (Argentina), IBGE (Brazil), and IGAC (Colombia). These four Processing Centres deliver loosely constrained weekly solutions for the assigned sub-networks, which are integrated in a unified solution by the SIRGAS Combination Centres (DGFI and IBGE). The main SIRGAS products are: loosely constrained weekly solutions in SINEX format for further combinations of the network, weekly positions aligned to the ITRF as reference for GPS positioning in Latin America; and multi-year solutions (positions + velocities) for practical and scientific applications requiring time-dependent coordinates. This paper describes the analysis of the SIRGAS-CON network as the current realization of the SIRGAS reference system, its quality and consistency, as well as the planned activities to continue improving this reference frame.

Comparing vertical total electron content from GPS, Bent and IRI models with TOPEX-Poseidon

Abstract The Global Positioning System (GPS) has become a powerful and mature geodetic tool widely used for a broad range of technological and scientific applications. The observations of permanent GPS tracking stations, under the management of the International GPS Service (IGS), seem suitable for ionospheric research, providing continuous and quite good world-wide coverage at low cost for the users. TOPEX-Poseidon surveys sea-level heights by measuring the time required for pulses generated by the onboard radar altimeters to bounce back to the satellite from the sea surface. Free electrons in Earths ionosphere can delay the return of the radar pulses to the satellite, interfering with the accuracy of sea-level measurements. To correct this delay, the satellites altimeter makes measurements in two channels. The difference between the two measurements provides a measure of the integrated total electron content, between satellite and sea surface. To analyse the quality of different vertical total electron content (VTEC) maps, we compare our empirical model of the VTEC obtained using GPS data (hereafter called La Plata model) with the VTEC measurements of TOPEX-Poseidon and with the VTEC obtained using the Bent and IRI models. The Bent and IRI models provide monthly averages in the non—auroral ionosphere for magnetically quiet conditions and they are ones of the classical global ionospheric models used as referent in many ionospheric researches. On other hand we have that La Plata model provides a numerical ionospheric model at “any time” using GPS measurements; in disturbed magnetic activity it provides also a mean representation of ionosphere that are very different that in quiet conditions. We make comparison between the different models in quiet geomagnetic conditions because Bent and IRI models were developed to work in these conditions. So in this study we chose some selected quiet geomagnetic days during the year 1997. In our analysis we can see that the GPS model has the best global VTEC representation at any latitude and longitude even without modelling the ionospheric anomaly.

Global behaviour of the ionosphere electron density using GPS observations

Abstract A large number of permanent GPS tracking stations have been established during the past years, most of them under the administration of the International GPS Service (IGS) (Beutler et al., 1999). On April 3, 1995, the MicroLab I (MLI) (Melbourne et al., 1994) satellite with a GPS receiver on board, was launched into a circular orbit of about 775 km of altitude and 60° of inclination. This experiment is named GPS-MET. Apart of other specific applications, the data of the terrestrial and spatial stations can also be used to study the ionosphere. The main goal of this kind of ionosphere research is to exploit the capability of these observations to continuously and routinely monitor the ionosphere at global scale. In particular, we will focus on the estimation of parameters describing the distribution of free electrons in the ionosphere. The electron density profiles introduced in this work are a new input to the ionosphere analysis with GPS observations. The possibility of having a space receiver that gives us information about the total electron content at different heights allows us to bring forward a model for the variation of the electron density with height. This model reflects the average and global behaviour of the ionosphere during the measured interval. We compare our profile with the IRI ones and find that there is better agreement at middle northern latitudes than low and middle southern latitudes.

Toward a SIRGAS Service for Mapping the Ionosphere’s Electron Density Distribution

SIRGAS is responsible of the terrestrial reference frame of Latin America and the Caribbean. To fulfil this commitment it manages a continuously operational GNSS network with more than 200 receivers. Although that network was not planed for ionospheric studies, SIRGAS attempted to exploit it by establishing, in early 2008, a regular service for computing regional maps of the vertical Total Electron Content. This paper describes an effort for developing a new SIRGAS product, concretely, a 4-dimensional (space and time) representation of the free electron distribution in the ionosphere. The working methodology is based on the ingestion of dual-frequency GNSS observations into a global electron density model in order to update its parameters. Preliminary results are presented and their quality is assessed by comparing the electron density computed with the methodology here described and the one estimated from totally independent observations. A preliminary analysis reveals that the performance of the electron density model improves by a factor greater than 2 after data ingestion.

Improving SIRGAS Ionospheric Model

The IAG Sub-Commission 1.3b, SIRGAS (Sistema de Referencia Geocentrico para las Americas), operates a service for computing regional ionospheric maps based on GNSS observations from its Continuously Operating Network (SIRGAS-CON). The ionospheric model used by SIRGAS (named La Plata Ionopsheric Model, LPIM), has continuously evolved from a “thin layer” simplification for computing the vTEC distribution to a formulation that approximates the electron density (ED) distributions of the E, F1, F2 and top-side ionospheric layers.

Semi-annual Anomaly and Annual Asymmetry on TOPEX TEC During a Full Solar Cycle

During the first decades of ionospheric research, the physical description of the ionospheric free electron vertical density was mainly given by the Chapman theory in which the main driving parameters were the solar irradiance level and the solar zenith distance from the observation point. Any new observed phenomenon that could not be explained by the Chapman theory was considered an ‘anomaly’. After more than 50 years of continuous aeronomic research, many of these phenomena then called ‘anomalies’ were physically explained but some of them are still open to discussions, like the so-called Semi-annual Anomaly that produces global mean TEC values larger for equinoxes than for solstices; and the Annual Asymmetry that causes larger mean global TEC during the December than the June solstice (far larger than the 7% that would be expected from the change on the Sun–Earth relative distance). Using the high-precision TEC 13-year data series provided by the TOPEX/Poseidon mission, the main finding of this work is the characterization of the annual variation of the ionospheric daily mean TEC that reflects the combined effects of the both mentioned anomalies. The analysis of this annual pattern allows a precise quantification of the level of the effects of both anomalies, and suggests that the semi-annual anomaly does not have a half-year period, instead might be considered as another annual anomaly with two maxima separated by 220 days.

Improvements in the Ellipsoidal Heights of the Argentine Reference Frame

In 1997, the classical argentine geodetic system Campo Inchauspe 69 was replaced by POSGAR 94 (POSiciones Geodesicas ARgentinas), a realization of WGS84 through GPS observations. After the SIRGAS reference frame was available, POSGAR was recomputed following the guidelines given by the SIRGAS working group II. The resulting new frame, termed POSGAR 98, realizes the International Terrestrial Reference System (ITRS). The main scope of this paper is to assess the precision and accuracy of the vertical component for both the 94 and 98 frames. The investigation was carried out using several independent GPS data sets. The results show almost randomly distributed errors of up to 1 m for POSGAR 94. The improvement in the heights is more than ten times when POSGAR 98 coordinates are considered.

SIRGAS core network stability

The main objective of SIRGAS (Sistema de Referencia Geocentrico para las Americas) is to provide an accurate spatial and time-referenced infrastructure as a basis for Earth System research and to support scientific and practical applications based on high-precise positioning. Following this purpose, significant achievements related to the extension, analysis, and maintenance of this reference frame have been reached during the last years. However, there are still unresolved problems hindering the attainment of the best possible precision. In particular, the assimilation of seismic-related deformations and non-lineal station movements is very difficult and its omission considerably reduces the reliability of SIRGAS as a high precision reference frame. To advance in the solution of these inconveniences, this paper presents the first kinematic model of the SIRGAS reference frame computed after the strong earthquake occurred in the Chilean region of Maule in February 2010. This model is based on the combination of weekly free normal equations covering the time span from April 18, 2010 to June 15, 2013. Computed station positions and velocities refer to the IGb08 reference frame (the IGS realisation of the ITRF2008), epoch 2012.0. The averaged rms precision is ±1.4 mm horizontally and ±2.5 mm vertically for the station positions at the reference epoch, and ±0.8 mm/year horizontally and ±1.2 mm/year vertically for the constant velocities. Comparisons with reference frames based on measurements before the earthquake (like ITRF2008 or former SIRGAS solutions) make evident the strong deformation caused by this earthquake and the necessity of updating accordingly the reference frames in the affected region.