International GPS Service Network Research Articles

Response of the Total Electron Content of Terrestrial Ionosphere to GRB041227

At 21:30 UT on 2004 December 27 an extremely strong gamma-ray burst swept across the earth and caused the part of the terrestrial upper atmosphere exposed to it to produce extra ionization. Sudden ionospheric disturbance (SID) events were simultaneously observed at many of the very low frequency (VLF) electric wave observing stations. Analyses of the X-ray data of the GOES satellite as well as the solar wind and interplanetary data of the ACE satellite with the relevant theories show that the observed SID event observed was indeed caused by GRB041227. We calculated the response of the total electron content (TEC) of the terrestrial ionosphere to this γ-ray burst using the observed data provided by the international GPS service network (IGS) and the data processing method of coherent summation. The result indicates that the GRB041227 produced by the SGR1806-20 had an evident effect on the terrestrial ionosphere: in the course of the burst the average ionospheric TEC increased, to a maximum size of about 0.04 TECU (1 TECU = 10 16 el/m 2), equivalent to a solar flare with importance of C or lower. The calculated result demonstrates once again that a remote celestial body can also affect the terrestrial space environment to some extent.

Study of ionospheric response to the 4B flare on 28 October 2003 using International GPS Service network data

Using the GPS data from as many as 114 GPS stations of the International GPS Service for Geodynamics (IGS), the morphological features of the ionospheric total electron content (TEC) variations on the sunlit hemisphere during the 4B solar flare on 28 October 2003 is studied. It is found that the strongest sudden increase of TEC (SITEC) happened during the flare, and the magnitudes of SITEC vary at regions with different local solar zenith angle (SZA). In the northern hemisphere, the TEC enhancement is approximately symmetrical to the local noon, and its value is usually greater than 14 TECU (1 TECU = 1016/m2) if the SZA is less than 60°. On the whole, as the SZA increases, the value of TEC enhancement in the northern hemisphere decreases. It is worth mentioning that even in the regions of SZA between 90° and 100°, the SITEC was still seen from the temporal TEC curves. Using a photochemical model, the electron production rate over the sunlit boundary region is calculated and some obvious features of SITEC over this region are analyzed. In the polar region, the effect of this flare on the ionosphere exceeds the effect of the ionospheric scintillations and it seems that the ionosphere in the northern polar region responses more sensitively to this flare. In the end, superimposed on the curves of the rate of TEC change, there are some small disturbances (spikes) synchronously appearing on all curves and thus indicating an existence of similar structures in the EUV band of the flare.

Evaluation of GPS Precipitable Water over Canada and the IGS Network

Abstract Precipitable water (PW) derived from the GPS zenith tropospheric delay (ZTD) is evaluated (as a first step toward variational data assimilation) through comparison with that of collocated radiosondes (RS_PW), operational analyses, and 6-h forecasts (from the Canadian Global Environmental Multiscale model) of the Canadian Meteorological Centre. Two sources of ZTD data are considered: 1) final ZTD (over Canada), computed by the Geodetic Survey Division (GSD) of Natural Resources Canada, and 2) final ZTD (distributed globally), obtained from the International GPS Service (IGS). The mean GSD GPS–derived PW (GPS_PW) is 14.9 mm (reflecting the relatively cold Canadian climate), whereas that of the IGS dataset is 20.8 mm. Intercomparison statistics [correlation, standard deviation (SD), and bias] between GPS_PW and RS_PW are, respectively, 0.97, 2.04 mm, and 1.35 mm for the GSD data and 0.98, 2.6 mm, and 0.67 mm for the IGS data. Comparisons of GPS_PW with 6-h forecast PW (TRIAL_PW) show slightly lower correlations and a higher SD. The increase in SD is greater for the IGS data, which is not surprising, because in regions such as the Tropics and subtropics, moisture forecasts are of a lower quality and the RS observation network is sparse. From a three-way intercomparison (IGS GPS_PW, RS_PW, and TRIAL_PW) of the SD statistics, it is found that GPS_PW has the lowest estimated PW error (≈1 mm) for PW in the 5–30-mm range. For PW greater than 30 mm, the RS_PW estimated error is ≈2 mm, and that of GPS_PW is ≈2.5 mm. The TRIAL_PW estimated error increases with PW, reaching 5.5 mm in the 40–55-mm PW range. These intercomparison results indicate that GPS_PW should be a useful source of humidity information for NWP applications.

Towards the determination of displacements in the Upper Rhine Graben area using GPS measurements and precise antenna modelling

The Upper Rhine Graben is a north-northeast trending rift system, which belongs to the European Cenozoic Rift System. Today, the southern part of the graben is seismically still active. Earthquakes of magnitude 5 have a recurrence time of approximately 30 years. The EUCOR-URGENT project focuses on the seismic hazards and neotectonics in the Upper Rhine Graben and the surrounding areas. Within the framework of this project five institutions have created and have been maintaining a GPS network in the Upper Rhine Graben and the surrounding area in order to monitor horizontal and vertical displacements in the study area. Two GPS campaigns have been carried out in the network to monitor and to determine the horizontal and vertical displacements in the study area. The first one was observed in 1999 and the second one in 2000. In 2002, 2 weeks of data of the available permanent GPS reference stations in the area have also been included in the investigations. These permanent stations belong to the European Reference Frame Permanent Network, International GPS Service network, Satellite Positioning Service of Baden-Württemberg, Réseau GPS Permanent-France (RGP), Réseau GPS Permanent dans les Alpes (REGAL) and the AGNES (operated by the Swiss Positioning Service). Due to the small size of the displacements expected, high accuracy requirements must be met in the GPS processing. In order to achieve this, the results of the available absolute antenna calibrations have been considered in the data processing, too. The results have shown that the application of absolute antenna calibration models is crucial for the derivation of a highly accurate displacement field. Without these models large discrepancies (on the level of few centimetres) can be observed at those stations, where the observations were carried out by different antennas in the various campaigns. As expectedly, due to the short time span, no significant displacement could be detected in the study area. However, the deformation analysis shows that the horizontal displacements do not exceed the level of 1 mm/yr. This value agrees with the results of other investigations.

Study of the ionospheric total electron content response to the great flare on 15 April 2001 using the International GPS Service network for the whole sunlit hemisphere

The characteristics of the ionospheric response to the solar flare on Apr. 15, 2001 were studied using the total electron content (TEC) obtained at GPS observational stations in the whole sunlit hemisphere under International GPS Service for Geodynamics. It was found that the largest enhancement of the sudden increase of total electron content during this flare is ∼2.6 TECU (1 TECU = 1016 m−2). The effects of solar flare radiation on the ionosphere can be recognized even in the region at 0600 or 1800 LT. Owing to ionospheric scintillation, the TEC enhancement could not be derived from temporal TEC variation curves in high latitudes. On the other hand, the synoptic picture of the sunlit ionospheric response to the flare was obtained, and the results showed the relationship of TEC enhancements with solar zenith angles. The larger the solar zenith angle, the smaller the TEC enhancement. Even minor fast changes in TEC during this flare are revealed in the changing rate of temporal TEC variations in midlatitudes and low latitudes. These small globally synchronous disturbances of TEC were compared with the solar X‐ray fluxes of this flare observed by satellites, and a close correlation between those small ionospheric disturbances and the hard X‐ray flux fluctuations was found.

The activities of the Ionosphere Working Group of the International GPS Service (IGS)

This article is based on a position paper presented at the IGS Network, Data and Analysis Center Workshop 2002 in Ottawa, Canada, 8–11 April 2002, and introduces the IGS Ionosphere Working Group (Iono_WG). Detailed information about the IGS in general can be found on the IGS Central Bureau Web page: http://igscb.jpl.nasa.gov. The Iono_WG commenced working in June 1998. The working group's main activity currently is the routine production of ionosphere Total Electron Content (TEC) maps with a 2-h time resolution and daily sets of GPS satellite and receiver hardware differential code bias (DCB) values. The TEC maps and DCB sets are derived from GPS dual-frequency tracking data recorded with the global IGS tracking network.

Ionospheric Correction for GPS Tracking of LEO Satellites

This paper describes an ionospheric correction technique for single frequency GPS measurements from satellites in low Earth orbit. The fractional total electron content (TEC) above the receiver altitude is obtained from global TEC maps of the International GPS Service network and an altitude dependent scale factor. By choosing a suitable effective height of the residual ionosphere, the resulting path delay for positive elevations is then computed from a thin layer approximation. The scale factor can be predicted from the assumption of a Chapman profile for the altitude variation of the electron density or adjusted as a free parameter in the processing of an extended set of single frequency measurements. The suitability of the proposed model is assessed by comparison with flight data from the Champ satellite that orbits the Earth at an altitude of 450 km. For the given test case, a 90% correction of the ionospheric error is achieved in a reduced dynamic orbit determination based on single frequency C/A-code measurements.

Progress in Carrier Phase Time Transfer

The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network. This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time). (2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will be presented publicly in the spring 2001. © 2001 John Wiley & Sons, Inc.

Determining parameters of large-scale traveling ionospheric disturbances of auroral origin using GPS-arrays

The intention in this paper is to investigate the form and dynamics of large-scale traveling ionospheric disturbances (LS TIDs) of auroral origin. We have devised a technique for determining LS TID parameters using GPS-arrays whose elements can be selected from a large set of GPS stations forming part of the International GPS Service network. The method was used to determine LS TID parameters during a strong magnetic storm of September 25, 1998. The North-American sector where many GPS stations are available, and also the time interval 00:00–06:00 UT characterized by a maximum value of the derivative Dst were used in the analysis. The study revealed that this period of time was concurrent with the formation of the main ionospheric trough with a conspicuous southward wall in the range of geographic latitudes 50–60° and the front width of no less than 7500 km. The auroral disturbance-induced large-scale solitary wave with a duration of about 1 h and the front width of at least 3700 km propagated in the equatorward direction to a distance of no less than 2000–3000 km with the mean velocity of about 300 m/s. The wave front behaved as if it ‘curled’ to the west in longitude where the local time was around afternoon. Going toward the local nighttime, the propagation direction progressively approximated an equatorward direction.