1,721,142 research outputs found
GNSS Radio Occultation for Ionosphere Remote Sensing
No full textThe radio occultation (RO) measurements of the Global Navigation Satellite System’s (GNSS’s) signals onboard a Low Earth Orbiting (LEO) satellite enable the computation of the vertical elec-tron density profile from the LEO satellite’s orbit height down to the Earth’s surface. The iono-spheric extension experiment performed by the GNSS Receiver for Atmospheric Sounding (GRAS) receiver on board MetOp-A provides opportunities for ionospheric sounding but with the RO measurements only taken with an impact parameter height below 600 and 300 km within two different experiments, although MetOp-A was flying at an orbit height of about 800 km. Here, we present a model-assisted RO inversion technique for electron density retrieval from such kind of truncated data. The topside ionosphere and plasmasphere above the LEO orbit height are mod-elled by a Chapman layer function superposed with an exponential decay function representing the plasmasphere
A multi-satellite investigation for ionospheric scintillations observations over low-latitude African sector
No full textMitigation of higher order ionospheric effects on GNSS users in Europe
No full textCurrent dual-frequency GPS measurements can only eliminate the first-order ionospheric term and may cause a higher-order range bias of several centimeters. This research investigates the second-order ionospheric effect for GNSS users in Europe. In comparison to previous studies, the electron density profiles of the ionosphere/plasmasphere are modeled as the sum of three Chapman layers describing electron densities of the ionospheric F2, F1 and E layers and a superposed exponential decay function describing the plasmasphere. The International Geomagnetic Reference Field model is used to calculate the geomagnetic field vectors at numerous points along the incoming ray paths. Based on extended simulation studies, we derive a correction formula to compute the average value of the longitudinal component of the earth’s magnetic field along the line-of-sight as a function of geographic latitude and longitude, and geometrical parameters such as elevation and azimuth angles. Using our correction formula in conjunction with the total electron content along the line-of-sight, the second-order ionospheric term can be corrected to the millimeter level for a vertical total electron content level of 1018 electrons/m2
HIGHER ORDER IONOSPHERIC ERRORS IN MODERNIZED GPS AND FUTURE GALILEO SYSTEMS
No full textModernized GPS and future Galileo systems will transmit a third frequency allowing to cancel out the first and second order ionospheric terms in the refractive index by double differencing carrier phases or code pseudoranges. However, differential bending of the signal and third order ionospheric term are not fully removed in this approach. This work estimates the magnitude of various higher order ionospheric errors using a large number of ionospheric vertical profiles reconstructed from CHAMP-GPS radio occultation measurements. Our investigation shows that triple-frequency residual range errors become significant at low elevation angles (< 15°) and at high level of total electron content. The triple-frequency residual range error reaches up to 1 and 4 cm at 5° elevation angle in carrier-phase and pseudo-range measurements, respectively. Different approximation formulas have been discussed to mitigate higher order ionospheric errors in operating satellite positioning systems.
The Galileo system will allow four frequencies to be used for higher order ionospheric corrections. It has been found that ionospheric effects are successfully removed in quadruple-frequency measurements, i.e., no ionospheric correction is required in the range estimation. However, quadruple-frequency measurements for the ionospheric correction are not practically useful since the measurement noise exceeds the ionospheric correction
Higher order ionospheric propagation effects on GPS radio occultation signals
No full textWith the increasing number of remote sensing satellites using the GPS radio occultation technique for atmospheric sounding, the estimation of higher order ionospheric effects and their mitigation have become relevant and important. Due to long ionospheric limb paths, GPS signals are strongly affected by ionospheric refraction during radio occultation. Standard dual-frequency GPS measurements may be used to estimate the first order term of the refractive index. However, non linear terms such as the second and third order ionospheric terms and ray path bending effects are not considered in occultation measurements so far. Analysing selected CHAMP-GPS occultation events different higher order ionospheric terms are estimated and their effects on dual-frequency range estimation and total electron content (TEC) estimation are discussed. We have found that the separation between the GPS L1 and L2 ray paths exceeds the kilometer level during occultation for a vertical TEC level of more than 160 TEC units. Corresponding errors in the GPS dual-frequency range estimation and TEC estimation are found to exceed the meter and 10 TEC units level, respectively
Second order propagation delay effects in regional precise positioning
No full textWith the increasing number of precise navigation and positioning applications by using Global Navigation Satellite Systems (GNSS) such as GPS, the ionospheric higher order effects and their correction become more and more important. Whereas the first-order error is usually eliminated by a linear combination of dual frequency measurements, the second- and third-order residual effects remain uncorrected in this approach.
We discuss the effect of second order range errors as a function of the total electron content of the ionosphere and geomagnetic-geographic relationships for mid-European (Germany) users. Applying the corrections, the rest error can be reduced to the 2 mm level at a high vertical TEC level of 1018 el/m2 (100 TECU). Possibilities are discussed how second order refraction effects may be performed in precise positioning network
Estimate of higher order ionospheric errors in GNSS positioning
No full textPrecise navigation and positioning using GPS/GLONASS/Galileo require the ionospheric propagation errors to be accurately determined and corrected for. Current dual-frequency method of ionospheric correction ignores higher order ionospheric errors such as the second and third order ionospheric terms in the refractive index formula, and errors due to bending of the signal. The total electron content (TEC) is assumed to be same at two GPS frequencies. All these assumptions lead to erroneous estimations and corrections of the ionospheric errors. In this paper a rigorous treatment of these problems is presented. Different approximation formulas have been proposed to correct errors due to excess path length in addition to the free space path length, TEC difference at two GNSS frequencies and third order ionospheric term. The GPS dual-frequency residual range errors can be corrected within millimeter level accuracy using the proposed correction formulas
Ionospheric Refraction on GPS Signals Received Onboard LEO Satellites
No full textThe bending of the ray-path of GNSS signals is caused by the ionospheric refractivity which mainly depends on the electron density distribution along the ray path. Bending effects lead to a deviation of the curved optical path from the straight line of sight (LoS). Due to the dispersive nature of the ionosphere GNSS signals at L1 and L2 frequencies travel along different ray paths through the ionosphere. In both cases the ray path is longer than the length of the LoS, in particular at low elevation angles at ground based receivers. The corresponding excess path in addition to the LoS or true range may achieve several centimeters at low elevations under high solar activity conditions. Radio occultation measurements onboard Low Earth Orbiting (LEO) satellites indicate ionospheric bending effects of the excess path up to the meter level. The corresponding errors in TEC estimates used for retrieving vertical electron density profiles from GPS radio occultation measurements are discussed. The computed deviations of the curved optical path from the straight LoS may reach the kilometer level. Consequently, the tangential heights of the signal paths at the closest point of approach to the Earth surface deviate from that defined by the straight LoS propagation. This may cause an error in determining the reference height in the retrieved vertical refractivity profiles
Ionospheric scintillations studies using Spire and COSMIC-2 radio occultation and GOLD satellite data
Get PDFLow-earth orbits (LEO) satellites have been harnessing the concept of GNSS radio occultation (RO) for several atmospheric applications. With the advent of CubeSat technology, many space companies are now extending the GNSS-RO for ionospheric and space weather studies. This study demonstrates the capabilities of Spire’s constellation of CubeSats in detecting ionospheric scintillations. High rate 50 Hz GNSS measurements received by the STRATOS receivers onboard Spires’s CubeSats are used to detect scintillations over low latitude African sector. Spire’s GNSS-RO atmPhs files are accessed from University Corporation for Atmospheric Research (UCAR) data
repository along with COSMIC-2 conPhs files. The amplitude scintillation index (S4) is computed for each COSMIC-2 and Spire RO profiles. While COSMIC-2 conPhs files are restricted to tangent point altitudes up to 130 km, the scintillation detection algorithm onboard Spire receivers enable to downlink the associated 50 Hz phase and pseudorange data of the extended RO profiles (up to zenith). Spire’s extended RO profiles enable to detect F-layer amplitude scintillations often occurring in post-sunset hours. The occurrences of scintillations are corroborated by equatorial plasma bubble (EPB) structures observed from NASA’s Global‐scale Observations of the Limb and Disk (GOLD) satellite. This study indicates the potential of Spire GNSS-RO data in augmenting and complementing ionospheric scintillation studies available from COSMIC-2 and other similar RO missions. This apability can provide an important contribution to scintillation monitoring and can further be extended to space weather nowcasts and forecasts
Possibilities of Higher Order Range Error Correction in Real-time Precise Positioning
No full textWith the increasing number of precise navigation and positioning applications by using Global Navigation Satellite Systems (GNSS) such as GPS, the ionospheric higher order effects and their correction become more and more important. Whereas the first-order error can be completely eliminated by a linear combination of dual frequency measurements, the second- and third-order residual effects remain uncorrected in this approach. To quantify the second-order residual effect, a simple formula has been derived for GNSS users in Germany. Our proposed correction algorithm reduces the second order effects to a residual error of fractions of one millimeter up to 2 mm at a high vertical TEC level of 1018 electrons/m2 (100 TECU) depending on user azimuth and elevation angles. It is expected that the correction will enable a more accurate positioning by using carrier phase
measurements
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