Journal of Atmospheric and Solar-terrestrial Physics, 2006
Imaging the equatorial ionosphere in three dimensions is challenging due to the high diurnal vari... more Imaging the equatorial ionosphere in three dimensions is challenging due to the high diurnal variations at low latitudes. Inversions of total electron content (TEC) together with model-derived TEC data as replacement of measurement data along satellite-to-ground paths allow reconstructing electron densities. The technique is tested using a realistic simulation study, where the ionosphere is modelled using the International Reference Ionosphere model and actual satellite and receiver locations are used to simulate the data that can be collected over the South American region. The multi-instrument data analysis system (MIDAS) algorithm is used, where reconstructions are based on defining the sample space of stochastic inversions by empirical orthonormal functions (EOFs), that need to be adjusted depending on region, time and geophysical conditions studied. Insight into information supporting vertical ionospheric reconstructions and suitable horizontal receiver constellations allows to create reliable images of low (mean and maximum) TEC errors. The technique is potentially of interest for physical studies of the anomaly, particularly under storm events when its morphology deviates from normal.
Transionospheric radio signals may experience fluctuations in their amplitude and phase due to ir... more Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity S 4 and the standard deviation of the received phase r / typically calculated over 1 min of data. Data from a GPS scintillation monitor based on 50 Hz measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard), in the frame of the ISACCO project are used to investigate possible adoption of an alternative parameter for the estimate of phase fluctuations: i.e., the standard deviation of the phase rate of change S / . This parameter is shown to better correlate with S 4 being much less detrending dependent than r / . The couple (S 4 , S / ) should be then considered a more physical proxy of radio scintillation than the couple (S 4 , r / ).
On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wa... more On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wall response while blue box indicates a flat high-energy reflection probably due to a buried floor. The white box indicates a room filled up with backfill.
Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data ... more Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data and products increase. The refractive index of the ionosphere is affected by its electron content and the magnetic field of the Earth, so the carrier phase of the GPS L1 and L2 signals is advanced and the modulated code delayed. Due to system design the polarisation is unaffected. Most of the effect is removed by expanding the refractive index as a series and eliminating the first term with a linear combination of the two signals. However, the higher order terms remain. Furthermore, transiting gradients in refractive index at a non-perpendicular angle causes signal bending. In addition to the initial geometric bending term, another term allows for the difference that the curvature makes in electron content along each signal. Varying approximations have been made for practical implementation, mainly to avoid the need for a vertical profile of electron density. The magnetic field may be modelled as a tilted co-centric dipole, or using more realistic models such as the International Geomagnetic Reference Field. The largest effect is from the second term in the expansion of the refractive index. Up to several cm on L2, it particularly affects z-translation, and satellite orbits and clocks in a global network of GPS stations. The third term is at the level of the errors in modelling the second order term, while the bending terms appear to be absorbed by tropospheric parameters. Modelling improvements are possible, and three frequency transmissions will allow new possibilities.
Faraday rotation observations carried out at the stations Neustrelitz, Graz, and Florence by rece... more Faraday rotation observations carried out at the stations Neustrelitz, Graz, and Florence by receiving the VHF beacon of the geostationary satellite SIRIO have been used to analyze more than hundred storm periods mainly on a statistical basis. Additionally, total electron content data obtained from differential Doppler measurements at NNSS satellite signals and vertical sounding data of the stations Dourbes and Rome are included in this analysis. The average response of the ionosphere shows an extended positive phase in winter, whereas a well pronounced negative phase is typical for summer conditions. It is evident that the positive phase in electron content occurs in all seasons on the first storm day. The more pronounced storm effects in electron content than in f0F2 indicate the important role of the upper ionosphere during storms. Immediately after SSC an eastward directed electric field is assumed to lift up the ionospheric plasma into regions of reduced loss. In general, the storm-associated percentage deviations of ionospheric electron content, F2 layer critical frequency, slab thickness, as well as F2 layer height increase with enhanced geomagnetic activity.
Transionospheric radio signals may experience fluctuations in their amplitude and phase due to ir... more Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the
Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data ... more Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data and products increase. The refractive index of the ionosphere is affected by its electron content and the magnetic field of the Earth, so the carrier phase of the GPS L1 and L2 signals is advanced and the modulated code delayed. Due to system design the polarisation is unaffected. Most of the effect is removed by expanding the refractive index as a series and eliminating the first term with a linear combination of the two signals. However, the higher order terms remain. Furthermore, transiting gradients in refractive index at a non-perpendicular angle causes signal bending. In addition to the initial geometric bending term, another term allows for the difference that the curvature makes in electron content along each signal. Varying approximations have been made for practical implementation, mainly to avoid the need for a vertical profile of electron density. The magnetic field may be modelled as a tilted co-centric dipole, or using more realistic models such as the International Geomagnetic Reference Field. The largest effect is from the second term in the expansion of the refractive index. Up to several cm on L2, it particularly affects z-translation, and satellite orbits and clocks in a global network of GPS stations. The third term is at the level of the errors in modelling the second order term, while the bending terms appear to be absorbed by tropospheric parameters. Modelling improvements are possible, and three frequency transmissions will allow new possibilities.
Journal of Atmospheric and Solar-terrestrial Physics
Certain modern radio systems that rely on trans-ionospheric propagation require knowledge of the ... more Certain modern radio systems that rely on trans-ionospheric propagation require knowledge of the changes in the ionosphere that can be characterised by changes in total electron content (TEC). An important cause of these TEC fluctuations is travelling ionosphere disturbances (TIDs). Temporal variations of European mid-latitude TEC measurements obtained from geostationary satellites across two solar cycle phases, i.e. solar minimum from 1975 to 1976 and solar maximum from 1989 to 1990, were studied. The frequencies and amplitudes of the TIDs were investigated using direct time series analysis, discrete wavelet analysis and Fourier analysis. Results show that the solar maximum TIDs are larger in absolute amplitude but comparable in percentage amplitude (relative to daily TEC maximum) in relation to the solar minimum TIDs. The largest daily amplitudes of the TIDs were distributed between 2.5% and 17% across the data set without a clear dependency on solar cycle phase. TIDs were found at all of the periods studied between roughly 20 min and 3 h, with the most common periods around 1.7-2 h. Throughout the solar cycle the TIDs had the largest amplitudes during winter, with a secondary peak in TID activity in summer at solar maximum.
Journal of Atmospheric and Solar-terrestrial Physics, 2006
ABSTRACT Imaging the equatorial ionosphere in three dimensions is challenging due to the high diu... more ABSTRACT Imaging the equatorial ionosphere in three dimensions is challenging due to the high diurnal variations at low latitudes. Inversions of total electron content (TEC) together with model-derived TEC data as replacement of measurement data along satellite-to-ground paths allow reconstructing electron densities. The technique is tested using a realistic simulation study, where the ionosphere is modelled using the International Reference Ionosphere model and actual satellite and receiver locations are used to simulate the data that can be collected over the South American region. The multi-instrument data analysis system (MIDAS) algorithm is used, where reconstructions are based on defining the sample space of stochastic inversions by empirical orthonormal functions (EOFs), that need to be adjusted depending on region, time and geophysical conditions studied. Insight into information supporting vertical ionospheric reconstructions and suitable horizontal receiver constellations allows to create reliable images of low (mean and maximum) TEC errors. The technique is potentially of interest for physical studies of the anomaly, particularly under storm events when its morphology deviates from normal.
About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satelli... more About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satellite System (NNSS) differential Doppler were compared with the corresponding TECs obtained from the Global Positioning System (GPS). The method by which the comparison is effected is also described. The resulting statistics are very effective in showing a good agreement between the two techniques. The GPS
On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wa... more On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wall response while blue box indicates a flat high-energy reflection probably due to a buried floor. The white box indicates a room filled up with backfill.
... Xiaoqing Pi (1), Michael Mendillo (1), Paolo Spalla (2), David N. Anderson (3 ... The morphol... more ... Xiaoqing Pi (1), Michael Mendillo (1), Paolo Spalla (2), David N. Anderson (3 ... The morphological features of the required electric field perturbations near drawn and dusk are compared with those at other times to examine the local time characteristics of magnetospheric influence ...
Transionospheric radio signals may experience fluctuations in their amplitude and phase due to ir... more Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity S 4 and the standard deviation of the received phase r / typically calculated over 1 min of data. Data from a GPS scintillation monitor based on 50 Hz measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard), in the frame of the ISACCO project are used to investigate possible adoption of an alternative parameter for the estimate of phase fluctuations: i.e., the standard deviation of the phase rate of change S / . This parameter is shown to better correlate with S 4 being much less detrending dependent than r / . The couple (S 4 , S / ) should be then considered a more physical proxy of radio scintillation than the couple (S 4 , r / ).
The results of 3 years of a comparison between daily curves of Total Electron Content (TEC) obtai... more The results of 3 years of a comparison between daily curves of Total Electron Content (TEC) obtained from GPS and from about 42000 NNSS passes during 1994, 1995 and 1996 are pmsented. A preliminary investigation on the latitudinal dependence of TEC at middle latitudes is also carried out. 01998 COSPAR. Published by Elsevier Science Ltd.
The study of amplitude scintillation on GPS radio links is usually done after detrending the time... more The study of amplitude scintillation on GPS radio links is usually done after detrending the time series of the transmitted power so to define scintillations as the chaotic fluctuation around a unitary value. In a sense, the choice of how to detrend the time series is part of the definition of scintillation.
Journal of Atmospheric and Solar-terrestrial Physics, 2006
Imaging the equatorial ionosphere in three dimensions is challenging due to the high diurnal vari... more Imaging the equatorial ionosphere in three dimensions is challenging due to the high diurnal variations at low latitudes. Inversions of total electron content (TEC) together with model-derived TEC data as replacement of measurement data along satellite-to-ground paths allow reconstructing electron densities. The technique is tested using a realistic simulation study, where the ionosphere is modelled using the International Reference Ionosphere model and actual satellite and receiver locations are used to simulate the data that can be collected over the South American region. The multi-instrument data analysis system (MIDAS) algorithm is used, where reconstructions are based on defining the sample space of stochastic inversions by empirical orthonormal functions (EOFs), that need to be adjusted depending on region, time and geophysical conditions studied. Insight into information supporting vertical ionospheric reconstructions and suitable horizontal receiver constellations allows to create reliable images of low (mean and maximum) TEC errors. The technique is potentially of interest for physical studies of the anomaly, particularly under storm events when its morphology deviates from normal.
Transionospheric radio signals may experience fluctuations in their amplitude and phase due to ir... more Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity S 4 and the standard deviation of the received phase r / typically calculated over 1 min of data. Data from a GPS scintillation monitor based on 50 Hz measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard), in the frame of the ISACCO project are used to investigate possible adoption of an alternative parameter for the estimate of phase fluctuations: i.e., the standard deviation of the phase rate of change S / . This parameter is shown to better correlate with S 4 being much less detrending dependent than r / . The couple (S 4 , S / ) should be then considered a more physical proxy of radio scintillation than the couple (S 4 , r / ).
On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wa... more On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wall response while blue box indicates a flat high-energy reflection probably due to a buried floor. The white box indicates a room filled up with backfill.
Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data ... more Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data and products increase. The refractive index of the ionosphere is affected by its electron content and the magnetic field of the Earth, so the carrier phase of the GPS L1 and L2 signals is advanced and the modulated code delayed. Due to system design the polarisation is unaffected. Most of the effect is removed by expanding the refractive index as a series and eliminating the first term with a linear combination of the two signals. However, the higher order terms remain. Furthermore, transiting gradients in refractive index at a non-perpendicular angle causes signal bending. In addition to the initial geometric bending term, another term allows for the difference that the curvature makes in electron content along each signal. Varying approximations have been made for practical implementation, mainly to avoid the need for a vertical profile of electron density. The magnetic field may be modelled as a tilted co-centric dipole, or using more realistic models such as the International Geomagnetic Reference Field. The largest effect is from the second term in the expansion of the refractive index. Up to several cm on L2, it particularly affects z-translation, and satellite orbits and clocks in a global network of GPS stations. The third term is at the level of the errors in modelling the second order term, while the bending terms appear to be absorbed by tropospheric parameters. Modelling improvements are possible, and three frequency transmissions will allow new possibilities.
Faraday rotation observations carried out at the stations Neustrelitz, Graz, and Florence by rece... more Faraday rotation observations carried out at the stations Neustrelitz, Graz, and Florence by receiving the VHF beacon of the geostationary satellite SIRIO have been used to analyze more than hundred storm periods mainly on a statistical basis. Additionally, total electron content data obtained from differential Doppler measurements at NNSS satellite signals and vertical sounding data of the stations Dourbes and Rome are included in this analysis. The average response of the ionosphere shows an extended positive phase in winter, whereas a well pronounced negative phase is typical for summer conditions. It is evident that the positive phase in electron content occurs in all seasons on the first storm day. The more pronounced storm effects in electron content than in f0F2 indicate the important role of the upper ionosphere during storms. Immediately after SSC an eastward directed electric field is assumed to lift up the ionospheric plasma into regions of reduced loss. In general, the storm-associated percentage deviations of ionospheric electron content, F2 layer critical frequency, slab thickness, as well as F2 layer height increase with enhanced geomagnetic activity.
Transionospheric radio signals may experience fluctuations in their amplitude and phase due to ir... more Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the
Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data ... more Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data and products increase. The refractive index of the ionosphere is affected by its electron content and the magnetic field of the Earth, so the carrier phase of the GPS L1 and L2 signals is advanced and the modulated code delayed. Due to system design the polarisation is unaffected. Most of the effect is removed by expanding the refractive index as a series and eliminating the first term with a linear combination of the two signals. However, the higher order terms remain. Furthermore, transiting gradients in refractive index at a non-perpendicular angle causes signal bending. In addition to the initial geometric bending term, another term allows for the difference that the curvature makes in electron content along each signal. Varying approximations have been made for practical implementation, mainly to avoid the need for a vertical profile of electron density. The magnetic field may be modelled as a tilted co-centric dipole, or using more realistic models such as the International Geomagnetic Reference Field. The largest effect is from the second term in the expansion of the refractive index. Up to several cm on L2, it particularly affects z-translation, and satellite orbits and clocks in a global network of GPS stations. The third term is at the level of the errors in modelling the second order term, while the bending terms appear to be absorbed by tropospheric parameters. Modelling improvements are possible, and three frequency transmissions will allow new possibilities.
Journal of Atmospheric and Solar-terrestrial Physics
Certain modern radio systems that rely on trans-ionospheric propagation require knowledge of the ... more Certain modern radio systems that rely on trans-ionospheric propagation require knowledge of the changes in the ionosphere that can be characterised by changes in total electron content (TEC). An important cause of these TEC fluctuations is travelling ionosphere disturbances (TIDs). Temporal variations of European mid-latitude TEC measurements obtained from geostationary satellites across two solar cycle phases, i.e. solar minimum from 1975 to 1976 and solar maximum from 1989 to 1990, were studied. The frequencies and amplitudes of the TIDs were investigated using direct time series analysis, discrete wavelet analysis and Fourier analysis. Results show that the solar maximum TIDs are larger in absolute amplitude but comparable in percentage amplitude (relative to daily TEC maximum) in relation to the solar minimum TIDs. The largest daily amplitudes of the TIDs were distributed between 2.5% and 17% across the data set without a clear dependency on solar cycle phase. TIDs were found at all of the periods studied between roughly 20 min and 3 h, with the most common periods around 1.7-2 h. Throughout the solar cycle the TIDs had the largest amplitudes during winter, with a secondary peak in TID activity in summer at solar maximum.
Journal of Atmospheric and Solar-terrestrial Physics, 2006
ABSTRACT Imaging the equatorial ionosphere in three dimensions is challenging due to the high diu... more ABSTRACT Imaging the equatorial ionosphere in three dimensions is challenging due to the high diurnal variations at low latitudes. Inversions of total electron content (TEC) together with model-derived TEC data as replacement of measurement data along satellite-to-ground paths allow reconstructing electron densities. The technique is tested using a realistic simulation study, where the ionosphere is modelled using the International Reference Ionosphere model and actual satellite and receiver locations are used to simulate the data that can be collected over the South American region. The multi-instrument data analysis system (MIDAS) algorithm is used, where reconstructions are based on defining the sample space of stochastic inversions by empirical orthonormal functions (EOFs), that need to be adjusted depending on region, time and geophysical conditions studied. Insight into information supporting vertical ionospheric reconstructions and suitable horizontal receiver constellations allows to create reliable images of low (mean and maximum) TEC errors. The technique is potentially of interest for physical studies of the anomaly, particularly under storm events when its morphology deviates from normal.
About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satelli... more About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satellite System (NNSS) differential Doppler were compared with the corresponding TECs obtained from the Global Positioning System (GPS). The method by which the comparison is effected is also described. The resulting statistics are very effective in showing a good agreement between the two techniques. The GPS
On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wa... more On the cover: Migrated and stacked GPR section Alb 4.8. Yellow boxes indicate anomalies due to wall response while blue box indicates a flat high-energy reflection probably due to a buried floor. The white box indicates a room filled up with backfill.
... Xiaoqing Pi (1), Michael Mendillo (1), Paolo Spalla (2), David N. Anderson (3 ... The morphol... more ... Xiaoqing Pi (1), Michael Mendillo (1), Paolo Spalla (2), David N. Anderson (3 ... The morphological features of the required electric field perturbations near drawn and dusk are compared with those at other times to examine the local time characteristics of magnetospheric influence ...
Transionospheric radio signals may experience fluctuations in their amplitude and phase due to ir... more Transionospheric radio signals may experience fluctuations in their amplitude and phase due to irregularity in the spatial electron density distribution, referred to as scintillation. Ionospheric scintillation is responsible for transionospheric signal degradation that can affect the performance of satellite based navigation systems. Usually, the scintillation activity is measured by means of indices such as the normalised standard deviation of the received intensity S 4 and the standard deviation of the received phase r / typically calculated over 1 min of data. Data from a GPS scintillation monitor based on 50 Hz measurements recorded at Dirigibile Italia Station (Ny-Alesund, Svalbard), in the frame of the ISACCO project are used to investigate possible adoption of an alternative parameter for the estimate of phase fluctuations: i.e., the standard deviation of the phase rate of change S / . This parameter is shown to better correlate with S 4 being much less detrending dependent than r / . The couple (S 4 , S / ) should be then considered a more physical proxy of radio scintillation than the couple (S 4 , r / ).
The results of 3 years of a comparison between daily curves of Total Electron Content (TEC) obtai... more The results of 3 years of a comparison between daily curves of Total Electron Content (TEC) obtained from GPS and from about 42000 NNSS passes during 1994, 1995 and 1996 are pmsented. A preliminary investigation on the latitudinal dependence of TEC at middle latitudes is also carried out. 01998 COSPAR. Published by Elsevier Science Ltd.
The study of amplitude scintillation on GPS radio links is usually done after detrending the time... more The study of amplitude scintillation on GPS radio links is usually done after detrending the time series of the transmitted power so to define scintillations as the chaotic fluctuation around a unitary value. In a sense, the choice of how to detrend the time series is part of the definition of scintillation.
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Papers by Paolo Spalla