Christos Pikridas

Local Geoid Computation and Evaluation

Abstract A local geoid solution for the northern part of Greece is presented based on a recent processing of newly available gravity data in the area 40.25 ≤ /o ≤ 41.00, 22.5 ≤λ ≤ 24.25. The derived gravimetric geoid heights are compared with geoid heights computed at recently measured GPS/ leveling benchmarks. A 4-parameter transformation model is applied to the differences between the two aforementioned geoid height sets, and a discussion is given on the current state of the leveling datum in the test area and the Greek territory. Regional and local transformation parameters are computed and some numerical tests are performed. A common adjustment of gravimetric geoid heights and corresponding GPS/leveling heights will be carried out in another study following an integrated procedure in order to study problems arising from the combination of different height data sets for geoid determination. Finally, some conclusions are drawn on the problems related to the optimization of a local geoid solution.

The Realization of a Semi-Kinematic Datum in Greece Including a New Velocity Model

The current geodetic network in Greece, which is realised by both the official Hellenic Geodetic Reference System of 1987 (HGRS87) and the GPS permanent network of the contemporary static Hellenic Terrestrial Reference System of 2007 (HTRS07), experience significant inhomogeneous ground displacements. As time passes, the distortion of both networks results in increasing degradation of positioning accuracy and datum stability. For these reasons the velocity field of the Earth’s crust in Greece has to be rigorously estimated and taken into account.

Combination Schemes for Local Orthometric Height Determination from GPS Measurements and Gravity Data

One of the most interesting and challenging tasks in the field of geodetic surveying is the accurate determination of orthometric heights from GPS measurements taking into account leveling data and additional gravity field information. This paper focuses on the presentation of the currently available various solution strategies which are then properly applied. The first method is based on the integrated geodetic model, where gravity field parameters are treated as signals. A second solution is based on a combination scheme employing least squares collocation as the optimal heterogeneous combination method for gravity and height data. Another method is the spectral domain equivalent of least squares collocation, namely the Multiple Input Multiple Output System Theory, where gravity and height data are treated as stochastic signals with full variance covariance information. The last method consists in a polynomial interpolation model of various orders expressing different geoid representations.

Prediction of zenith tropospheric delay by multi-layer perceptron

Abstract 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.

A comparative study of zenith tropospheric delay and precipitable water vapor estimates using scientific GPS processing software and web based automated PPP service

The zenith tropospheric delay (ZTD) is a significant error source which affects the GPS signal propagation time. ZTD time series can directly or indirectly reflect the weather variations. In geodetic studies their computation is important since it improves position accuracy. Sophisticated software packages using network data of ground-based GPS receivers are used for this purpose. During the last years, various web based precise point positioning (PPP) services can provide position solutions. Additional products, such as ZTD estimates, can be derived both from PPP as well as from network solutions. Research institutes and laboratories like JPL provide similar services. This study’s objective is twofold. In a first instance we used the Bernese and GAMIT software packages and the PPP service via the JPL webpage to estimate ZTD values every hour for a period of one month. We selected a geographical area in Greece, where seasonal weather variations are frequent and GPS permanent station infrastructure is available. The estimated ZTD values derived from GPS processing for a single station were compared with the ZTD estimates derived directly from the Saastamoinen model using meteorological data from a co-located meteorological sensor as input to this model. The results show an rms agreement of about 45 mm. The second scope of this study is to compare precipitable water (PW) values between different processing schemes. For this purpose, we used the derived zenith wet delay estimates of each processing scheme and a global formula for the computation of the weighted mean temperature of the atmosphere for our area of study. The rms differences between the web PPP solution and the Bernese derived estimates were 0.35 mm. The rms of the differences between and the web PPP solution and the GAMIT derived estimates of PW were 1.67 mm. We conclude that this difference follows from the zenith hydrostatic delay component that needs to be properly modeled when high accuracy results are required.

A novel microclimate forecasting system architecture integrating GPS measurements and meteorological-sensor data

It is apparent that local climatic conditions have clear implications for agriculture. For this reason accurate information on local climate dynamics is essential for agricultural planning. This study introduces a methodology that aims to enhance microclimate monitoring through GPS networks. This paper describes this novel approach, its implications and presents some preliminary results. Authors propose a novel implementation of a weather monitoring system comprised of existing geodetic and meteorological sensors cooperative efforts, coordinated by roaming microclimate sensors. Such system shall be utilized for the real-time prediction of crucial and emergent phenomena for the agricultural industry. This system shall offer more accurate predictions with the use of spatial microclimate profiles and water vapor content estimates attained from GPS measures. Furthermore, a new session protocol based on SNMP protocol shall be used for data acquisition and a caster station for post processing and visualization purposes.

A New Velocity Field of Greece Based on Seven Years (2008–2014) Continuously Operating GPS Station Data

Greece is characterized by complex and intense geodynamics, because it is located between the collision boundaries of two tectonic plates (Africa-Nubia and Eurasia), with major active tectonic features such as the Hellenic Arc, the Anatolian fault in North Aegean Trough and the Kefalonia fault in the Ionian Sea. GPS is a well-established tool for geophysical research purposes, because it is able to provide continuous measurements for monitoring displacements of the Earth’s crust. The aim of the present study is to create a modern and improved geodetic velocity field for Greece using GPS observations from continuously operating reference stations. The new set of geodetic velocities is derived from the processing of 7 years (2008–2014) of daily GPS data, using 155 stations distributed in the broader Greek territory and 30 IGS-EPN GPS stations. The GAMIT/GLOBK software package was used to process the GPS measurements. The results are expressed in the ITRF2008 reference frame. The analysis showed that the northern region of Greece is the most stable and has identical movement with the Eurasian plate in contrast with the region of the southern part and the Aegean Sea. According to the results, the estimated horizontal geodetic velocities show completely different pattern between northern and southern Greece with significant differences both in magnitude and direction. The derived site values were used for a velocity grid creation in order to predict velocities within the Greek area and to enforce proper realization of GNSS reference systems in Greece.

Assessing a new velocity field in Greece towards a new semi-kinematic datum

Greece has always been an interesting study area for the geodesists due to its intense geodynamic behaviour. Especially, the exploitation of a modern geodetic velocity field using GNSS becomes a crucial requirement for geodetic purposes. The main aim of the present study is the assessment of a modern geodetic velocity model. This procedure is based on the unification of various reference frames, implemented from individual extended GNSS campaigns in different epochs with the effective involvement of the Hellenic Military Geographic Service. The implementation of geodetic velocities in the inhomogeneous Greek velocity field improves significantly the reliability of transformations, more than 60% with respect to the horizontal component, applying a vice versa time shift to a common epoch. We also emphasise the necessity to adopt a velocity model in order to process regional GNSS networks and therefore to establish a modern semi-kinematic reference frame in Greece.

The Role of GNSS Vertical Velocities to Correct Estimates of Sea Level Rise from Tide Gauge Measurements in Greece

ABSTRACT In this study, we show how the Global Navigation Satellite System (GNSS)-derived vertical velocities contribute to the correction of tide gauge (TG) measurements used for the sea level rise estimation in Greece. Twelve sites with records of local sea level heights are processed in order to estimate their trend. Certain error sources related to TGs, e.g. equipment changes, data noise, may lead to biased or erroneous estimations of the sea level height. Therefore, it would be preferred to follow a robust estimation technique in order to detect and reduce outlier effects. The geocentric sea level rise is estimated by taking into account the land vertical motion of co-located GNSS permanent stations at the Hellenic area. TGs measure the height of the water relative to a monitored geodetic benchmark on land. On the other hand, using GNSS-based methods the vertical land motion can be derived. By means of extended models fitted to the GNSS time-series position, obtained from seven years of continuous data analysis, periodic signals are well described. The synergy of the two co-located techniques results in the correction of TG relative sea level heights taking into account the GNSS vertical velocities and consequently obtaining the conversion to absolute (geocentric) sea level trend.

Analysis of Precipitable Water Estimates Using Permanent GPS Station Data During the Athens Heavy Rainfall on February 22th 2013

The Global Positioning System (GPS) has been used in the remote sensing of the atmosphere. A significant component of the atmosphere that affects the GPS signals is the zenith tropospheric delay (ZTD). The computation of ZTD estimates can directly or indirectly reflect weather variations. Through the analysis of ZTD values the hydrostatic and wet component of the total delay can be determined. For example, the wet tropospheric delay could be derived by subtracting the hydrostatic from the total delay. Hydrostatic delay can be estimated from surface or other meteorological data. The wet tropospheric delay can then be used in the derivation of the amount of precipitable water. Precipitable water plays a significant role in the physical and chemical processes of the atmosphere. It also greatly contributes to studies of weather forecasting and climate change. In this study GPS data from 12 permanent stations covering the broader area of the city of Athens, between February 18th and 24th, were used. This period was selected because of a heavy rainfall event on February 22nd. Data were processed using the GAMIT software and precipitable water (PW) estimates with 1 h time interval were derived. The PW values were analyzed in combination with meteorological data such as cloudiness, wind direction and precipitation obtained from the Hellenic Center for Marine Research and the Hydrological Observatory of Athens. The results indicate consistency between the estimated PW values and the related meteorological observations. This study suggests that a continuous record of PW estimates and meteorological variables is highly recommended for further studies on the behavior of the atmospheric water vapor and its contribution to the climate monitoring.