Volume 3, Issue 4

OmniSTAR is one of the world's leading suppliers of satellite-based augmentation services for onshore and offshore GNSS applications. OmniSTAR currently offers three services: VBS, HP and XP. OmniSTAR VBS is the code-based service, suitable for sub-metre positioning accuracy. The HP and XP services provide sub-decimetre accuracy, with the HP service based on a precise differential methodology and the XP service uses precise absolute positioning. The sub-decimetre HP and XP services both have distinctive convergence behaviour, and the positioning task is essentially a time-dependent process during which the accuracy of the estimated coordinates continuously improves over time. To validate the capabilities of the OmniSTAR services, and in particular the HP (High Performance) service, globally distributed measurement campaigns were performed. The results of these investigations confirm that the HP service satisfies its high accuracy specification, but only after a sufficient initialisation phase. Two kinds of disturbances can handicap HP operation: lack of GNSS observations and outages of the augmentation signal. The most serious kind of disturbance is the former. Within a few seconds the achieved convergence level is completely lost. Outages in the reception of augmentation data merely affect the relevant period of the outage – the accuracy during the outage is degraded. Only longer interruptions lead to a loss of the HP solution. When HP convergence is lost, the HP process has to be re-initialized. If there are known points (so-called “seed points”) available, a shortened “kick-start”-initialization is possible. With the aid of seed points it only takes a few minutes to restore convergence.

The increasing use of GPS and data integration technologies in agricultural operations has led to large gains in operational efficiencies. This paper presents a new method to bridge periods when the number of visible GPS satellites falls below the minimum required to provide a navigation solution by utilising an external positioning system. This can be an additional dead reckoning INS system or any other system that can provide an approximate position solution for the vehicle which can be improved through the processing of data from the available GPS satellites.

The aim of the present study was to use the artificial neural network approach and specifically the multi-layer perceptron algorithm in order to predict total zenith tropospheric delay (ZTD) for various time spans of 1, 3 and 6 hours. The test data was ZTD values derived from the analysis centers of the EUREF Permanent tracking Network. The prediction process was applied to six EUREF permanent GPS stations for using period data of 2006 and 2007. The results obtained show an agreement at the order of few centimetres (2–3 cm) with those derived from EPN. Comparisons were also made with ZTD values calculated by other methods like the radiosonde observations and Saastamoinen model using ground measurements in order to confirm the final results and the feasibility of the neural network methodology.

This study compares the latest Unified Lunar Control Network, ULCN 2005, solution with the earlier ULCN 1994 solution at global and local scales. At the global scale, the relative rotation, translation, and deformation (normal strains and shears) parameters between the two networks are estimated as a whole using their colocated station Cartesian coordinate differences. At the local scale, the network station coordinate differences are examined in local topocentric coordinate systems whose origins are located at the geometric center of quadrangles and tetrahedrons. This study identified that the omission of the topography in the old ULCN solutions shifted the geometric center of the lunar figure up to 5 km in the lunar equatorial plane and induced a few hundred-meter level global rotations of the ULCN 1994 reference frame with respect to ULCN 2005. The displacements between the old and new control networks are less than ± 2 km on the average at the local scale, which behave like translations, caused by the omission of lunar topography in the earlier solution. The contribution of local rigid body rotations and dilatational and compressional components to the local displacements are approximately ± 100 m for a quadrangle/tetrahedron of an average side length of 10 km.

A free-form surface expressed by NURBS (nonuniform rational B-splines) is fitted to the measured coordinates of points by the lofting method. The unknown control points of the free-form surface are therefore not simultaneously estimated but determined by cross-sectional curve fits. This uses much less computer time than the simultaneous estimation and gives identical results. The free-form surface should be determined with an uncertainty which does not considerably surpass the uncertainty of the measurements. This is investigated here for the example of a free-form surface for a pothole in a road determined by the measurements of a laserscanner. The uncertainties are expressed by standard deviations and confidence intervals. They are computed using Monte Carlo simulations for the positioning of a point by the measured coordinates and by fitting a free-form surface. The resulting uncertainties agree. In addition, the uncertainties of quantities characterizing the shape and the slope of the surface are determined by Monte Carlo simulations. It turns out that the uncertainties resulting from the measurements and from the free-form surface fit are approximately identical.

This paper presents a comparative study of the displacement of the Brazilian Funil Shell Dam centre arch crown as derived from two measurement campaigns using three independent procedures: GPS satellite positioning, terrestrial geodetic techniques and coupled optical plumb measurements with an inverted pendulum. For the GPS measurement, four dual frequency GX1220 GPS receivers from Leica Geosystems were installed in the crest of the dam and in three concrete pillars used as control points. For geodetic measurement, a DI2002 Electronic Distancemeter fitted on the telescope of a T2 theodolite was used. The optical plumb measurement was made using a Nadiral Optical Plumb manufactured by Wild Heerbrugg and calibrated by Furnas Metrology Laboratory. Results showed that GPS technology can be used successfully for horizontal displacement monitoring in areas with high level of multi-path such as water dam surrounding area. The GPS accuracy was also estimated based on the Gauss-Markov homoscedastic model using the historical nadiral optical plumb observations as standard observation.