Frequency GPS Research Articles

Higher-order ionospheric effects in GPS time and frequency transfer

In high-precision geodetic time and frequency transfer, which requires precise modeling of code and carrier phase GPS data, the ionosphere-free combinations P 3 and L 3 of the codes and carrier phases, made on the two GPS frequencies, are used to remove the first-order ionospheric effect. We quantify the impact of the residual second- and third-order ionospheric effects on geodetic time and frequency transfer solutions for continental and intercontinental baselines. All time transfer computations are done using the ATOMIUM software, developed at the Royal Observatory of Belgium. In order to avoid contamination by some imperfect modeling of the second- and third-order ionospheric effects in the satellite clock products, only single-difference, common-view processing is used, based on code and carrier phase measurements. The results are shown for weak and strong solar activity, as well as for particular epochs of ionospheric storms. Second-order ionospheric delays can lead to corrections up to about 10 ps in the common-view clock solution of intercontinental baselines with very different longitudes. However, realistic values of the geomagnetic field in the ionosphere are required to assess the amplitude of second-order ionospheric effects in time and frequency transfer during an ionospheric storm.

Influence of ionospheric perturbations in GPS time and frequency transfer

The stability of GPS time and frequency transfer is limited by the fact that GPS signals travel through the ionosphere. In high precision geodetic time transfer (i.e. based on precise modeling of code and carrier phase GPS data), the so-called ionosphere-free combination of the code and carrier phase measurements made on the two frequencies is used to remove the first-order ionospheric effect. In this paper, we investigate the impact of residual second- and third-order ionospheric effects on geodetic time transfer solutions i.e. remote atomic clock comparisons based on GPS measurements, using the ATOMIUM software developed at the Royal Observatory of Belgium (ROB). The impact of third-order ionospheric effects was shown to be negligible, while for second-order effects, the tests performed on different time links and at different epochs show a small impact of the order of some picoseconds, on a quiet day, and up to more than 10 picoseconds in case of high ionospheric activity. The geomagnetic storm of the 30th October 2003 is used to illustrate how space weather products are relevant to understand perturbations in geodetic time and frequency transfer.

Microstrip-Fed Circularly Polarized Square-Ring Patch Antenna for GPS Applications

A novel coupling technique for circularly polarized square-ring patch antenna is developed and discussed. The circular polarization (CP) radiation of the square-ring patch antenna is achieved by a simple microstrip feed line through the coupling of a square patch on the same plane of the antenna. Proper positioning of the coupling square patch excites two orthogonal resonant modes with 90deg phase difference, and a pure circular polarization is obtained. The dielectric material is a square block of ceramic with a permittivity of 58 and that reduces the size of the antenna. The prototype has been designed, fabricated and found to have an impedance bandwidth of 1.1% and a 3-dB axial-ratio bandwidth of about 0.03% at GPS frequency of 1573 MHz. The characteristics of the proposed antenna have been studied by simulation software HFSS and experiments. The measured and simulated results are in good agreement.

GPS/CAPS dual-mode software receiver

The positioning of the GPS or Chinese Area Positioning System (CAPS) software receiver was developed on a software receiver platform. The structure of the GPS/CAPS dual-mode software receiver was put forward after analyzing the differences in the satellite identification, ranging code, spread spectrum, coordinate system, time system, carrier band, and navigation data between GPS and CAPS. Based on Matlab software on a personal computer, baseband signal processing and positioning procedures were completed using real GPS and CAPS radio frequency signals received by two antennas. Three kinds of experiments including GPS positioning, CAPS positioning, and GPS/CAPS positioning were carried out. Stability and precision of the results were analyzed and compared. The experimental results show that the precision of CAPS is similar to that of GPS, while the positioning precision of the GPS/CAPS dual-mode software receiver is 1–2 m higher than that of CAPS or GPS. The smallest average variance of the positioning can be obtained by using the GPS/CAPS dual-mode software receiver.

Campaign Investigation of Ionospheric Plasma Irregularities in Sporadic E Region Using FORMOSAT-3/COSMIC Satellite and Chung-Li 30 MHz Coherent Radar

In this article, we present an electron density profile retrieved from total electron density estimated from the difference in phase path excess between GPS frequencies L1 and L2 measured by the FORMOSAT-3/COSMIC satellite, in which the radio occultation inversion technique is employed for retrieval. Except for a regular F layer peak located at a height of about 290 km and a minor peak centered at a height of 140 km, a pronounced sporadic E layer was observed at a height of about 105 km. This intense electron density layer with thickness of about 10 km has very sharp boundaries on the top and bottom sides with scale lengths of -22 and 13 km, respectively. At the time when COSMIC GPS radio occultation took place in the vicinity of Taiwan, the Chung-Li 30 MHz coherent radar detected strong backscatter from 5-meter plasma irregularities. The peak radar backscatter is situated at a height of about 110 km in the topside of the Es layer with a very steep electron density gradient. Interferometry measurement made by the four separate and independent receiving channels of the Chung-Li 30 MHz radar indicates that the configuration of the large scale plasma structure constituted by 5-meter scale field-aligned irregularities is patch-like, and a 2-minute oscillation in zonal displacement of the plasma structure was found. From the temporal displacement of the echo patterns from the plasma irregularities in the bottom side of the layer, the plasma structure in the bottom side of the Es layer was found to move westward at a trace velocity of about 6.2 m s^(-1). The exceedingly small drift velocity combined with the relatively large scale length of the electron density gradient seem to suggest that the 5-meter plasma irregularities are very unlikely generated through the non-linear cascade process of the large plasma structure at kilometer scale induced by gradient drift instability. Moreover, in light of the fact that both the observed drift velocity (less than 15 m s^(-1)) of the kilometer-meter scale plasma wave and the measured Doppler velocity (about 50 m s^(-1)) of the 5-m plasma irregularities are much smaller than the 280 m s^(-1) that is required to directly excite plasma waves through gradient drift instability, it suggests that the 5-m plasma irregularities observed by the Chung-Li 30 MHz radar are very unlikely the result of direct excitation through gradient drift instability.

Estimate of higher order ionospheric errors in GNSS positioning

Precise 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.

TIME TRANSFER BY LASER LINK – THE T2L2 EXPERIMENT ON JASON-2 AND FURTHER EXPERIMENTS

The new generation of optical time transfer will allow the synchronization of remote ultra stable clocks and the determination of their performances over intercontinental distances. The principle of T2L2 (Time Transfer by Laser Link) is based on the techniques of satellite laser ranging coupled with time-frequency metrology. It consists of synchronizing ground and space clocks using short laser pulses travelling between ground clocks and satellite equipment. The instrument will be integrated on the ocean altimetry satellite Jason-2 that is scheduled for launch in 2008. The experiment should enhance the performance of time transfer by one or two magnitudes compared to existing microwave techniques such as GPS and Two-Way Satellite Time and Frequency Transfer (TWSTFT).

A Compact Wideband Stacked Antenna for the Tri-Band GPS Applications

A compact wideband stacked patch antenna is presented for the applications of GPS systems. This antenna covers the L1, L2, and L5 GPS bands of operating frequencies 1.575, 1.227, and 1.176 GHz, respectively. High permittivity dielectric materials are used to minimize the antenna dimensions. The obtained antenna is of dimensions mm. To verify the design, the time dependence field distribution, the scattering parameters, and the radiation pattern are presented. The scattering parameters show that the antenna operates at the GPS frequencies with lower than dB. The finite difference time domain (FDTM) with the perfect matched layer (PML) is used in the present analysis.

Integrated Dual-Band Antenna System Design Incorporating Cell Phone Bezel

This letter describes a method to integrate a cell phone metal bezel in the design of dual-band antenna system. The proposed design includes a hybrid slot and inverted L antennas and covers both the GPS (1.575 GHz) and WLAN (2.4 ~ 4.484 GHZ) frequency bands. A prototype antenna was manufactured on a FR4 board and both S-parameters and radiation patterns are measured, thus validating the simulation results.

GPS Time and Frequency Transfer: PPP and Phase‐Only Analysis

To compute precise point positioning (PPP) and precise time transfer using GPS code and phase measurements, a new software named Atomium was developed by the Royal Observatory of Belgium. Atomium was also adapted to perform a phase‐only analysis with the goal to obtain a continuous clock solution which is independent of the GPS codes. In this paper, the analysis strategy used in Atomium is described and the clock solutions obtained through the phase‐only approach are compared to the results from the PPP mode. It is shown that the phase‐only solution improves the stability of the time link for averaging times smaller than 7 days and that the phase‐only solution is very sensitive to the station coordinates used. The method is, however, shown to perform better than the IGS clock solution in case of changes in the GPS receiver hardware delays which affects the code measurements.

GPS Composite Clock Analysis

The GPS composite clock defines GPS time, the timescale used today in GPS operations. GPS time is illuminated by examination of its role in the complete estimation and control problem relative to UTC/TAI. The phase of each GPS clock is unobservable from GPS pseudorange measurements, and the mean phase of the GPS clock ensemble (GPS time) is unobservable. A new and useful observability definition is presented, together with new observability theorems, to demonstrate explicitly that GPS time is unobservable. Simulated GPS clock phase and frequency deviations, and simulated GPS pseudorange measurements, are used to understand GPS time in terms of Kalman filter estimation errors.

On the Role of Glonass for the Development of the Russian Geodetic Reference Network

On the Role of Glonass for the Development of the Russian Geodetic Reference NetworkThe combined use of GPS and GLONASS satellite systems is a main technology for the development of the fundamental geodetic network in Russia and for crust movement studies along the North Eurasian tectonic plate. All permanent stations of the state fundamental geodetic network will be provided with the two frequency GPS and GLONASS receivers. A collocation of these stations with the existing Russian sites of the international GNSS (Global Navigation Satellite System) network is foreseen. The GLONASS state program foresees to come to 18 satellites in constellation in 2007-2008, and full operation capability (24 satellites) will be reached by 2009. The second generation of satellites: GLONASS-M have the L2 civil signal, extended lifetime and improved clock stability. GLONASS-K satellites are expected by 2009 with the L3 civil signal and Synthetic Aperture Radar function. A combination of GPS and GLONASS will benefit the scientific geodynamic research and practical users, especially in the urban, mountain and near polar areas. Results of the experimental GLONASS data analysis performed. Comparison of mean values of coordinates from only GLONASS solution, obtained at the Institute of Astronomy (Moscow) with the use of GIPSY-OASIS2

GLONASS as a key element of the Russian Positioning Service

The combined use of GPS and GLONASS satellite systems is a main technology for the development of the fundamental geodetic network in Russia and for crust movement studies along the North Eurasian tectonic plate. The GLONASS state program foresees to come to 18 satellites in constellation in 2007–2008, and full operation capability (24 satellites) will be reached by 2009. By this time, all permanent stations of the state fundamental geodetic network will be provided with the two frequency GPS and GLONASS receivers. A collocation of these stations with the existing Russian sites of the international GNSS (Global Navigation Satellite System) network is foreseen. The second generation of satellites: GLONASS-M have the L2 civil signal, extended lifetime and improved clock stability. GLONASS-K satellites are expected by 2009 with the L3 civil signal and Synthetic Aperture Radar function. The GLONASS ground control and monitoring network is planning to be extended up to 9–12 stations and cooperation with GNSS service will be useful. A combination of GPS and GLONASS will benefit the scientific geodynamic research and practical users, especially in the urban, mountain and near polar areas. Results of the experimental GLONASS data analysis performed at the Institute of Astronomy (Moscow) with the use of GIPSY-OASIS2 software for 16 globally distributed sites of the international global network for the time period 25. 11. 2005 to 03. 12. 2005 are considered.

Design of Frequency Selective Windows for Improved Indoor Outdoor Communication

The use of low emissivity windows degrades radio communication. This Communication presents design, manufacturing, and test measurements for an energy saving window that is transparent to GSM, GPS, and 3G radio wave frequencies. A frequency selective structure (FSS) is used in the metallic coating of the window to provide the desirable transparency in the frequency range from 900 MHz to 2 GHz. The periodic pattern used for the FSS is of the aperture type and the elements are hexagon loops. The FSS simulations are performed using the mode matching technique as well as the finite-difference time domain method. A frequency selective window was manufactured from a commercially available low emissivity glass. Measurements indicate that the frequency selective window has approximately 10 dB better transmission in the 900 MHz-2 GHz band than the original window.

The design synthesis of multiband artificial magnetic conductors using high impedance frequency selective surfaces

This paper introduces several different design methodologies for multiband artificial magnetic conducting (AMC) surfaces. The paper begins by investigating the multiband properties exhibited by a conventional electromagnetic bandgap (EBG) AMC that consists of a frequency selective surface (FSS) on top of a thin dielectric substrate with a PEC back plane. The higher-order resonances associated with these surfaces have not been discussed in detail to date, as previous research has been concerned only with exploiting the primary resonant frequency. However, it will be shown that by understanding and making appropriate use of these higher order resonances, it is possible to design multiband AMC surfaces that work for nearly any desired combination of operating frequencies. The first multiband AMC design approach that will be considered is based on the introduction of FSS screens that have fractal or nearly fractal unit cell geometries. This is followed by a more general and robust genetic algorithm (GA) technique for the synthesis of optimal multiband AMC surfaces. In this case, a GA is used to evolve multiband AMC surface designs by simultaneously optimizing the geometry and size of the FSS unit cell as well as the thickness and dielectric constant of the substrate material. Finally, several examples of multiband AMC surfaces are presented, including some practical dual-band and tri-band designs genetically evolved for operation at GPS and cellular frequencies, as well as an example illustrating the success in creating a multiband AMC surface with angular stability.

Encyclopedia of geomorphology

Aeolian Processes Anabranching channels and anastomosing channels Biogeomorphology Catastrophes and catastropism Chaos theory and geomorphology Complexity in geomorphology Computational fluid dynamics Cosmogenic dating Crustal deformation Debris flow Dynamic equilibrium Ergodic hypothesis Extraterrestrial geomorphology Fission track dating Floods Flow visualization Fractals and geomorphology GIS and geomorphology Global warming and geomorphology GPS and geomorphology Hillslope-channel coupling Landslides Magnitude and frequency concept Models in geomorphology Palaeofloods Remote sensing in geomorphology River restoration Seafloor spreading Self-organised criticality and geomorphology Submarine landslide geomorphology Terrain evaluation Urban geomorphology

An analysis of the frequency and distribution of ducting events in simulated radio occultation measurements based on ECMWF fields

A study of the occurrence of ducts valid at GPS frequencies in the European Center for Medium‐Range Weather Forecasts (ECMWF) atmospheric fields is presented. A radio occultation simulator has been applied to determine the mean location of the occultations for a 10 day period in May 2001. In total, 5348 occultations were found. Refractivity profiles have been calculated at the mean occultation location by extracting temperature and water vapor profiles from ECMWF data. The gradient of refractivity with respect to altitude was analyzed for ducting conditions, which occurs below about −160 km−1. About 10% of the simulated measurement profiles show ducts. Polar ducting events are mainly occurring over land, very close to the surface. Midlatitude ducting events are almost evenly distributed over land and sea, where land events are close to the surface too. Tropical ducting events are mainly located over the sea. Events over the sea show ducts within a vertical altitude interval of 0–2.5 km. Tropical events tend to occur at higher altitudes than midlatitudes ones do. The thickness of the layer over which ducting conditions occur increases with altitude and reaches about 120 m for higher altitudes. Near‐surface ducting events show the lowest refractivity gradients. Higher up in the atmosphere the gradient increases, where tropical events are generally lower. The found gradient does not support ducting conditions above about 2.5 km. On the basis of our study, about 4% of all occultations can be affected by ducts in the altitude range of 0.2–2.5 km. The number of affected occultations depends on the latitude band; none are found at polar latitudes, 2% are at midlatitudes, and about 10% are at tropical locations.

GPS radio holography as a tool for remote sensing of the atmosphere, mesosphere, and terrestrial surface from space

GPS radio occultation (RO) signals are highly coherent and precise, and thus sufficient for holographic investigation of the atmosphere, ionosphere, and the Earth's surface from space. In principle, three-dimensional radio-holographic remote sensing is possible by using new radio holographic equations to retrieve the radio field within the atmosphere from a radio field known at some interface outside the atmosphere. A simplified two-dimensional form of the radio-holographic equations which are developed under an assumption of local spherical symmetry can be used to obtain two-dimensional radio images of the atmosphere and terrestrial surface. To achieve this, radio holograms recorded by a GPS receiver onboard a low earth orbit (LEO) satellite at two GPS frequencies can be used and focused synthetic aperture principle applied. Analysis of GPS/MET RO data is presented to show the effectiveness of a radio-holographic approach. It is shown that the amplitude of GPS radio signals (in addition to phase data) can be used to obtain detailed altitude profiles of the vertical gradient of refractivity in the atmosphere and electron density in the mesosphere. The results demonstrate the applicability of GPS radio holography for a detailed global study of the natural processes in the atmosphere and mesosphere.

Relativistic Time Transformations in GPS

Since Selective Availability was permanently switched off on 7 May 2000, most of the GPS satellite clocks have been well behaved. During a 24-h period precise satellite clock solutions, corrected for GPS conventional relativistic corrections, follow straight lines within a few nanoseconds. The linear clock fit RMS for the best satellite clocks are well below the 1-ns level, which is consistent with the nominal stability of the GPS frequency standards. Typically, the GPS satellite clocks show an Allan variance at or below one part in 1011/100 s for the Cesium frequency standards and a few parts in 1012/100 s for the Rubidium frequency standards. These results correspond to clock RMSs for 15-min sampling at or below 3 and 0.3 ns, respectively. This already confirms experimentally that the conventional periodic relativity correction of the GPS system, also adopted for all the IGS clock solution products, is precise and correct to 0.6 ns or better. To establish the precision limits of the GPS conventional relativity treatment, the relativistic time transformations of GPS satellite frequency and clocks are critically reviewed, taking into account all the contributions larger than the 10−18 (or 0.001 ns). The conventional GPS relativity treatment was found to be accurate, i. e., correctly modeling the actual relativistic frequency (clock rate) effects of GPS satellites at about the 10−14 level. However, it is also affected by small periodic errors of the same magnitude. The integration of these small periodic frequency relativistic errors gives the approximation errors of the conventional periodic relativistic clock correction with amplitudes of about 0.1 ns and a predominant period equal to a half of the orbital period (∼ 6 h). These approximation errors of the conventional GPS relativistic clock correction are at about the same level as the current precision of the IGS clock solutions. © 2002 Wiley Periodicals, Inc.

A new “geometric” mapping function for the hydrostatic delay at GPS frequencies

The hydrostatic mapping function, m(ε), is a dimensionless factor which describes the elevation angle dependence of the hydrostatic path delay and relates the line of sight delay to the zenith delay. We have developed a simple “geometric” mapping function where the only free parameter (besides the elevation angle, ε) is the climatological pressure scale height. The value of m(ε) is given by the ratio of the straight-line ray path length within the first two scale heights above the surface and the “effective height” defined by these first two scale heights. We used simulated neutral delays at GPS frequencies derived from high resolution ECMWF (European Centre for Medium-Range Weather Forecasts) atmospheric analysis fields (T213L50, T213L60) at different latitudes to compare the new mapping function with others currently in use (we compare with the Niell and the Davis mapping function, respectively, most frequently encountered in literature).At elevations > 6° the geometric mapping function displays, without involving any meteorological data, comparable or better accuracy and precision than the other mapping functions.