Local GPS Network Research Articles

Calibration of Envisat radar altimeter over Lake Issykkul

This study presents the results of calibration/validation (C/V) of Envisat satellite radar altimeter over Lake Issykkul located in Kyrgyzstan, which was chosen as a dedicated radar altimetry C/V site in 2004. The objectives are to estimate the absolute altimeter bias of Envisat and its orbit based on cross-over analysis with TOPEX/Poseidon (T/P), Jason-1 and Jason-2 over the ocean. We have used a new method of GPS data processing in a kinematic mode, developed at the Groupe de Recherche de Geodesie Spatiale (GRGS), which allows us to calculate the position of the GPS antenna without needing a GPS reference station. The C/V is conducted using various equipments: a local GPS network, a moving GPS antenna along the satellites tracks over Lake Issykkul, In Situ level gauges and weather stations. The absolute bias obtained for Envisat from field campaigns conducted in 2009 and 2010 is between 62.1 and 63.4±3.7cm, using the Ice-1 retracking algorithm, and between 46.9 and 51.2cm with the ocean retracking algorithm. These results differ by about 10cm from previous studies, principally due to improvement of the C/V procedure. Apart from the new algorithm for GPS data processing and the orbit error reduction, more attention has been paid to the GPS antenna height calculation, and we have reduced the errors induced by seiche over Lake Issykkul. This has been assured using cruise data along the Envisat satellite track at the exact date of the pass of the satellite for the two campaigns. The calculation of the Envisat radar altimeter bias with respect to the GPS levelling is essential to allow the continuity of multi-mission data on the same orbit, with the expected launch of SARAL/Altika mission in 2012. Implications for hydrology in particular, will be to produce long term homogeneous and reliable time series of lake levels worldwide.

On Constraining Zenith Tropospheric Delays in Processing of Local GPS Networks with Bernese Software

The aim of this research was to develop the best strategy for the mitigation of the tropospheric delays in processing of precise local GPS networks. With the requirement of sub-centimetre accuracy and the availability of precise IGS products, one of the ultimate accuracy limiting factors in GPS positioning is the tropospheric delay. This is especially true for the accuracy of the height component. In many precise GPS applications, e.g. ground deformation and displacement analyses, volcano monitoring, the vertical accuracy is of crucial importance.Several processing strategies for the troposphere modelling available in the Bernese software were applied and tested. The results from our research show that in the case of small networks (with baselines <10 km and point height differences < 100m)the best strategy is to use of a troposphere model in order to derive zenith tropospheric delays that are fixed in the adjustment. This allows to achieve mm-level accuracies of both horizontal and vertical coordinates. The estimation of the tropospheric delays from the GPS data does not provide satisfactory results, even in the case of a relative troposphere estimation.

GPS Landslide Monitoring: Single Base vs. Network Solutions — A case study based on the Puerto Rico and Virgin Islands Permanent GPS Network

GPS Landslide Monitoring: Single Base vs. Network Solutions — A case study based on the Puerto Rico and Virgin Islands Permanent GPS Network This study demonstrated an approach to using permanent GPS stations from a local continuous GPS network as no-cost references in conducting long-term millimeter-level landslide monitoring. Accuracy and outliers from a series of single-base and network GPS measurements of a creeping landslide were studied. The criterion for accuracy was the weighted root-mean-square (RMS) of residuals of GPS measurements with respect to true landslide displacements over a period of 14 months. This investigation indicated that the current Puerto Rico and Virgin Islands GPS network, as a reference frame, can provide accuracy of 1 to 2 mm horizontally and 6 mm vertically for local 24-hour continuous landslide monitoring with few outliers (&lt;1%). The accuracy degraded by a factor of two for 6-hour sessions, and more for shorter sessions. This study indicated that adding a few reference stations to GPS data processing can reduce the number of outliers and increase the accuracy and robustness of landslide surveying, even if these references are far from the study site. This improvement was particularly significant for short sessions and vertical components. The accuracy of network solutions depended slightly on the number of reference stations, but the dependence on the distance and geometric distribution of the references was weak. For long-term landslide monitoring, accuracy under 5 mm horizontally and 15 mm vertically are often expected. Accuracy at this level can be stably achieved in the Puerto Rico and Virgin Islands region by performing field observations for 4 hours or longer, and applying 3 or more reference stations for solving a network solution. This study also indicated that rainfall events can play a crucial rule in high-precision GPS measurements. GPS data collected during heavy rainfall events should be cautiously analyzed in landslide studies.

Crustal deformation in the Central Ionian Islands (Greece): Results from DGPS and DInSAR analyses (1995–2006)

Ground deformation studies based on Differential GPS (DGPS) and Differential Interferometric SAR (DInSAR) analyses have been conducted in the seismically active area of the Central Ionian Islands. Local GPS networks were installed in Cephallonia (2001) and Zakynthos (2005). The Cephallonian network has been remeasured five times and Zakynthos' once as of July 2006. The studies have yielded detailed information regarding both local and regional deformations that are occurring in the area. For Lefkas Island, DInSAR analysis (March to September 2003) revealed 56 mm of uplift in the central and western parts and is attributed to the August 2003 earthquake ( M w = 6.3) that occurred offshore to the west. Synthetic DInSAR modelling of the magnitude and extent of deformation is consistent with the seismologically deduced parameters for the ruptured segment along the Lefkas Transform Fault. Subsidence (< 28 mm) along the northern part of the island is attributed to local conditions unrelated to the earthquake. For Zakynthos Island, large-magnitude earthquakes that occurred offshore to the south in October 2005 and April 2006 most likely contributed to the observed deformation as deduced from DGPS measurements for an encompassing period (August 2005 to July 2006). The largest amount of horizontal deformation occurred in the south, where its western part moved in a W–NW direction, while the eastern part moved towards the NE, with magnitudes ranging from 15 to 26 mm. The southern part of the island uplifted a maximum of 65 mm whereas the north subsided from 12 to 28 mm. For Cephallonia Island, DInSAR analysis (1995 to 1998) indicated ground deformation up to 28 mm located in small sections of the island. Further interferometric analysis for the period 2003 to 2004, encompassing the occurrence of the Lefkas earthquake in August 2003, indicated 28 mm of uplift in the northern part, while during the next two years (2004 to 2005), further uplift of at least 56 mm had taken place at the western and northern part of the island. DGPS measurements for the period 2001 to 2006 revealed a clockwise rotation of the island with respect to a centrally located station on Aenos Mt. The horizontal component of deformation generally ranged from 6 to 34 mm, with the largest values at the western and northern parts of the island. Considering the vertical deformation, two periods are distinguished. The first one (2001 to 2003) is consistent with anticipated motions associated with the main geological and tectonic features of the island. The second one (2003 to 2006) has been tentatively attributed to dilatancy in which relatively small uplift (20–40 mm) occurred along the southern and southeastern parts of the island, while larger magnitudes (> 50 mm) happened at the western part (Paliki Peninsula). These large magnitudes of uplift over an extended area (> 50 km), in conjunction with an accelerated Benioff strain determined from the analysis of the seismicity in the broader region, are consistent with dilatancy. This effect commenced some time after 2003 and is probably centered in the area between Zakynthos and Cephallonia. If this interpretation is correct, it may foreshadow the occurrence of a very strong earthquake(s) sometime during 2007 to 2008 in the above designated region.

Effects of ionospheric disturbances on GPS observation in low latitude area

In this paper, ionospheric disturbance data from a local GPS network in Hong Kong (low latitude region) are studied in the solar maximum period (2001–2003). The spatial and temporal distributions of the disturbances in Hong Kong are investigated. It is found that strong ionospheric disturbances occur frequently during the solar maximum period, particularly around March and September, and concentrate at the region around geographic latitude 22°N (geomagnetic latitude 12°N). The effects of the disturbances on GPS geodetic receivers, such as loss of lock and measurement noise level, are also analyzed for the 3-year period. It shows that the measurement noise level and the number of losses of lock in GPS data increase dramatically during ionospheric disturbance periods. The behaviors of different types of GPS receivers during the disturbances are also compared.

Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector

This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August, 2003 (SSC 14:21 UT on 17/08; ΣKp = 52+; Ap = 108; ∣Dst∣ max = 168 at 1600 UT on 18/08). Simultaneous ionospheric sounding measurements from two stations, viz., Palmas (10.2°S, 48.2°W; dip latitude 5.7°S) and Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), Brazil, are presented for the nights of 16–17, 17–18 and 18–19 August, 2003 (quiet, disturbed and recovery phases). Both stations are equipped with the Canadian Advanced Digital Ionosonde (CADI). Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations. The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics, and generation of ionospheric irregularities are discussed. On the disturbed nights (17–18 and 18–19 August), the low latitude station S. J. Campos showed strong enhancements in the F-region critical frequency (foF2), whereas the near equatorial station Palmas showed strong uplifting of the F-layer about 1 h earlier. Normally during the June solstice months (May–August) in the Brazilian sector, large-scale ionospheric irregularities in form of plasma bubbles are rarely observed. On the night of 17–18 August, ionsospheric sounding observations at Palmas showed the presence of bottomside spread-F, whereas on the night of 18–19 August, the observations at Palmas and S. J. Campos showed the presence of plasma bubbles when the storm recovery phase had just started. The complementary GPS data available from several stations in the “Rede Brasileira de Monitoramento Continuo de GPS (Brazilian Network for Continuous GPS Monitoring)” are used to obtain the vertical total electron content (VTEC) and the rate of change of TEC per minute on UT days 18 and 19 August, 2003 and presented. Also, several global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread latitudinal and longitudinal TEC changes during the different phases of the storm. All the observations (local ionospheric sounding and GPS network measurements, and global GPS measurements) presented in this investigation related to the geomagnetic storm on 18 August indicate that the equatorial and low latitude region in the Brazilian sector had much stronger effect during the recovery phase compared with the main phase. A comparison of the observed disturbance drifts with the Fejer–Scherliess storm-time model drifts indicate that the modeled drifts are not consistent with the present observations.

Crustal deformation monitoring around the High and old Aswan dams, Egypt

Many countries are using GPS for monitoring large engineering structures such as dams. Monitoring deformation pattern is often one of the most effective ways to be informed the safety status of such structures. Thus, the safety control of dams lies on the analysis of its structural behaviour, based on monitoring a large set of variables, which contains information about earthquakes, crustal movement, temperature variations etc. and about corresponding stresses. For the purpose of monitoring the deformations of the High and old Aswan dams, a number of methods have been developed. For monitoring the height changes along the High Dam body, eight levelling lines were established at different levels during its construction. The levelling measurements were carried out each month since 1970 and continue till present. Mohamed et al. (2003) have presented and analysed the levelling measurements from 1970 to 2000. The results reveal different trends for different levels of the High Dam body. The rates of setting of the High Dam body tend to be stable. During the last five years the rates of the subsidence range 1-2 mm/a at most benchmarks of the levelling lines along the High Dam body. During the last decade no significant correlation has been found between the rates of setting and the water level changes at the up- and down-stream sides. For monitoring crustal deformations at the High and old Aswan dams region, a local GPS network consisting of 9 geodetic points was established. Three campaigns from December 2001 to April 2003 were performed. The study results indicate a definite correlation between the amount of water in the lake and the deformation in the area. The reservoir pushes the land towards the north where the two dams are. The paper throws light upon the present state of the crustal deformations in the study area.

Monitoring of Small‐Scale Ionospheric Irregularity Using Local GPS Network

AbstractBased on the high spatio‐temporal resolution GPS dual‐frequency data collected by the Shanghai Comprehensive Application GPS Network, an ionospheric irregularity and its process of arising and disappearing were monitored and studied. First, Kalman filter is used to improve the accuracy of ΔP2P1, and the hardware delay of GPS System is estimated by the Least Square method. Then according to the Single Layer Model (SLM) of the ionosphere and the altitude of the SLM shell, the accurate positions of the GPS satellites and receivers, and the dual‐frequency observations, the location of the ionosphere pierce points (IPPs) and their TEC are obtained, then the TEC Contour Maps are derived using the interpolating technique. The TEC variations with time reflected by these maps indicate that due to the turbulence flow of the atmosphere, an ionospheric irregularity located at (38°N, 118°E), whose scale is about 50km and the existence time 5 minutes, came to being at 2003‐09‐08‐01:40 UTC. It diffused northeastward, which was caused by the upper air winds and geomagnetic field. After analysis with the diffusion equation under 1976 U. S. Standard Atmosphere, we find that the irregularity appeared at the height of 350km above the ground.

First experience with near real-time water vapor estimation in a German GPS network

The GeoForschungsZentrum (GFZ) has started together with three other research centers of the German Helmholtz Association the “GPS Atmosphere Sounding” Project (GASP) on using ground-based (Subproject 1) and space-based (Subproject 2) GPS observations for applications in the numerical weather predictions, climate research and space weather monitoring, which this paper focusses on. The Subproject 1 comprises all necessary components – data links and handling, path delay estimation, conversion to water vapor. The GPS and meteorological data are transmitted in hourly batches to the GFZ since December 1998 and processed with GFZ EPOS software. First experiences with near real-time analysis have been obtained and the operational precision determination of water vapor within local GPS networks has been demonstrated with an accuracy of better than 2 mm with a standard deviation at the level of better than ±1 mm . Densification of the test network is progressing from 10 to 100 sites, requiring new analysis strategies.

Estimated pressure source on Kozu Island volcano, South Central Japan, from GPS measurements (July 1996–August 1999)

Although the Kozu Island Volcano, one of the Izu Islands Volcanoes in the south part Central Japan, is an active volcano, there is no record of the eruption for about 1100 years since the last eruption in 833 A.D. Since 1988, frequent earthquake swarms are observed around the Kozu Island, and the uplift of 2–4 cm/yr is observed on the island by tidal observations. Station velocities detected by GPS measurements since 1989 show velocities that differ from the convergent velocity of the Philippine Sea plate calculated from plate motion models. A local GPS network with 12 stations is occupied around the volcano, and the GPS measurements are repeated every about six month since July 1996. Inflated deformation of 2–4 cm/yr are detected from the GPS measurements and the pressure source is estimated to be located in the northeastern part of the island at a depth of 2.1 km using Mogi solution. Negative gravity changes of more than 30 microgal are also measured above the pressure source in the period November 1998 to July 1999, consistent with uplift.

Earth tide effects on kinematic/static GPS positioning in Denmark and Greenland

A detailed study of the Earth tide effects on the GPS kinematic/static positioning is presented in this paper by using theoretical Earth tide computation and practical GPS data processing. Tidal effects could reach up to 30 cm in Denmark and Greenland depending on the measuring time and the position of reference station. With a baseline less than 80 km, the difference of the Earth tide effects could reach more than 5 mm. So, in precise applications of GPS positioning, the Earth tide effect has to be taken into account even for a relative small local GPS network. Several examples are given for demonstrating that the Earth tide effects can be viewed by GPS surveying. They are given through static GPS data static processing, static GPS data kinematic processing, and airborne kinematic GPS data processing. In these cases, the Earth tide effects can be subtracted from the GPS results. The determination of tidal parameter through static GPS data kinematic processing has also been tested.

A simple adjustment procedure for local GPS networks

Abstract The paper describes a method of transforming observed GPS co-ordinate differences in the WGS84 system into positions in a local national co-ordinate system. The method requires the inclusion of at least three fixed and heighted points in the system and, for optimum results, datum shifts from the local system to WGS84 together with geoid heights on WGS84. The scale and rotation transformation parameters between the two system are not required.

A Simple Adjustment Procedure for Local GPS Networks

A Simple Adjustment Procedure for Local GPS Networks

A new, high-resolution geoid for Canada and part of the U.S. by the 1D-FFT method

A new, high-resolution and high-precision geoid has been computed for the whole of Canada and part of the U.S., ranging from 35°N to about 90°N in latitude and 210°E to 320°E in longitude. The OSU91A geopotential model complete to degree and order 360 was combined with a 5′ × 5′ mean gravity anomaly grid and 1km × 1km topographical information to generate the geoid file. The remove-restore technique was adopted for the computation of terrain effects by Helmert's condensation reduction. The contribution of the local gravity data to the geoid was computed strictly by the 1D-FFT technique, which allows for the evaluation of the discrete spherical Stokes integral without any approximation, parallel by parallel. The indirect effects of up to second order were considered. The internal precision of the geoid, i.e. the contribution of the gravity data and the model coefficients noise, was also evaluated through error propagation by FFT. In a relative sense, these errors seem to agree quite well with the external errors and show clearly the weak areas of the geoid which are mostly due to insufficient gravity data coverage. Comparison of the gravimetric geoid with the GPS/levelling-derived geoidal heights of eight local GPS networks with a total of about 900 stations shows that the absolute agreement with respect to the GPS/levelling datum is generally better than 10 cm RMS and the relative agreement ranges, in most cases, from 4 to 1 ppm over short distances of about 20 to 100km, 1 to 0.5 ppm over distances of about 100 to 200 km, and 0.5 to 0.1 ppm for baselines of 200 to over 1000 km. Other existing geoids, such as UNB90, GEOID90 and GSD91, were also included in the comparison, showing that the new geoid achieves the best agreement with the GPS/levelling data.