Precise point positioning (PPP) involves observations from a single global navigation satellite system (GNSS) receiver and benefits 4 of satellite orbit and clock products obtained from the global infrastructure of permanent stations. PPP avoids the expense and logistic diffi-5 culties of deploying a network of GNSS receivers around survey areas in isolated places, such as the arctic or less populated areas. Potential 6 accuracies are at the centimeter level for static applications and at the subdecimeter level for kinematic applications. Static and kinematic PPP 7 based on the processing of global positioning system (GPS) observations is limited by the number of visible satellites, which is often insufficient 8 for urban or mountain applications, or it can be partially obstructed or present multipath effects. Even if a number of GPS satellites are available, 9 the accuracy and reliability can still be affected by poor satellite geometry. One possible way of increasing satellite signal availability and po-10 sitioning reliability is to integrate GPS and global navigation satellite system (GLONASS) observations. This case study deals with the pos-
The microgravimetric surveying technique is applicable to the detection of shallow subsurface structures if a lateral density contrast is presented, and thus, it is a valid technique for archaeological prospection. In this paper, this technique has been revealed to be an efficient tool for archaeological studies, such as those performed inside Don Church (18th century), located in the urban area of Alfafar town, Valencia (Spain), where a buried crypt, suggested by different boreholes drilled during the second restoration process in 1993, is expected. Details of the site"s characteristics, topographic survey procedures, microgravimetric field operations, data collection and gravity reduction operations (where the inner building effect of walls, pillars and the altar is confirmed as one of the most important) are also presented. Finally, the results confirm the buried crypt.
Magnetic data consists of a sequence of collected points with spatial coordinates and magnetic information. The spatial location of these points needs to be as exact as possible in order to develop a precise interpretation of magnetic anomalies. GPS is a valuable tool for accomplishing this objective, especially if the RTK approach is used. In this paper the VRS (Virtual Reference Station) technique is introduced as a new approach for real-time positioning of magnetic sensors. The main advantages of the VRS approach are, firstly, that only a single GPS receiver is needed (no base station is necessary), reducing field work and equipment costs. Secondly, VRS can operate at distances separated 50–70 km from the reference stations without degrading accuracy. A compact integration of a GSM-19 magnetometer sensor with a geodetic GPS antenna is presented; this integration does not diminish the operational flexibility of the original magnetometer and can work with the VRS approach. The coupled devices were tested in marshlands around Gandia, a city located approximately 100 km South of Valencia (Spain), thought to be the site of a Roman cemetery. The results obtained show adequate geometry and high-precision positioning for the structures to be studied (a comparison with the original low precision GPS of the magnetometer is presented). Finally, the results of the magnetic survey are of great interest for archaeological purposes.
A new generation of global geopotential models (GGM) is being developed. These solutions offer a file with fully-normalized spherical harmonic coefficients of the Earth's gravitational potential up to a degree greater than 2000 with very low commission errors. This paper analyses the recent Earth Gravitational Model EGM2008, developed up to degree and order 2159 with additional coefficients to degree 2190 and order 2159, which means recovering the gravitational field up to approximately 20 km wavelengths. 223 GPS/levelling points of the new Spanish High Precision Levelling Network in the Valencia region (Eastern Spain) are used as external tool for evaluation in that particular region. The same evaluation has been performed to other different global (EGM96 and EIGEN-CG03C), continental (EGG97), regional (IGG2005 and IBERGEO2006) and local (GCV07) geoid models for comparison purposes only. These comparisons show that EGM2008 is the geoid model that best fits to the GPS/levelling data in that region.Based on these results, EGM2008 GGM is used to determine a new local geoid model in the region of Valencia by means of the remove-restore technique in the scenario proposed by least-squares collocation, in order to check the ability of the EGM2008, as a very high-degree GGM, to calculate a local geoid model in the studied area. The determination is presented step by step in this article, comparing the results of each step with those obtained using the same process but with the global model EIGEN-CG03C, complete up to degree and order 360, that is, a high-degree GGM. These two new geoid models have been analyzed using the 223 GPS/levelling points. The results show that local geoid determination based on EGM2008model gives significantly better fit to GPS/levelling points than any other geoid model in the studied area. However the improvement is not significant with respect to the direct use of EGM2008 without any additional local gravity data. Hence we strongly recommend the use of EGM2008 without applying least-squares collocation in the areas where good ground data were available for the computation of EGM2008.K e y w o r d s : geopotential theory, global geopotential model, EGM2008, local geoid determination A. Martin et al. 348 Stud. Geophys. Geod., 54 (2010)
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