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A standard network to control GPS receivers

Lambrou E., Pantazis G.

Reports on Geodesy

2004

nr 2/69

261--269

This work deals with the establishment of a close-range standard 3-dimensional geodetic network in order to certify the GPS receivers' proper function. The standard network is located at the area of the University Campus of NTUA at Zografos Athens, Greece. The first aim is the determination of the X and Y coordinates and the orthometric height H of the network's points as well as the determination of their uncertainties by accurate terrestrial measurements. The final target is to check any pair of GPS receivers. The above standard network consists of five points, marked by special pillars, which ensure the stability of their place and the single setting of both a Total Station and a GPS receiver. The terrestrial observations consist of horizontal and vertical angles and distance measurements between the points. The measurements were carried out by using a digital total station Leica TCA 1800 and the indispensable and appropriate accessories such as tribraches, prisms, targets, heavy forced centering bases, prism holders and adapters, which lead to the maximum possible measuring accuracy. The adjustment of the terrestrial measured data was carried out in the Greek Geodetic Reference System 1987 (GGRS '87) by the least square method. The standard network was used in order to check a pair of GPS receivers Trimble 4600LS. Ten baselines were measured and determined the coordinates of the five points and the corresponding uncertainties in the same reference system. The comparison of the measured elements such as distances, horizontal angles and height differences as well as the calculated coordinates of both solutions, leads with credibility to the certification of the proper or improper function of GPS receivers and to the checking of their industrial prescribed accuracy.

Accurate determination of the geoidal undulation N

Lambrou E., Pantazis G., Balodimos D.

Reports on Geodesy

2003

z. 1/64

165-172

This work, re!ated to the activities of the CERGOP Study Group Geodynamics or the Ba!kan Peninsula, presents a method for the determination or the variations AN and, indirectly, of the geoida! undulation N with an accuracy of a few millimetres. It is based on the determination of the components of the deviation of the vertical using modern geodetic instruments (digital total station and GPS receiver). An analysis of the method is given. Accuracy of the order of O.Olarcsec in the estimated values of the astronomical coordinates and lambda is achieved. The result of applying the proposed method in an area around Athens is presented. In this test application, a system is used which takes advantage of the capabilities of modern geodetic instruments. The GPS receiver permits the determination of the geodetic coordinates in a chosen reference system and, in addition, provides accurate time information. The astronomical observations are performed through a digital total station with electronic registering of angles and time. The required accuracy of the values of the coordinates is achieved in about four hours of fieldwork. In addition, the instrumentation is lightweight, easily transportable and can be setup in the field very quickly. Combined with a stream-lined data reduction procedure and the use of up-to-date astronomic data, the values of the components of the deviation of the vertical and, eventually, the changes deltaN of he geoidal undulations are determined easily and accurately. In conclusion, this work demonstrates that it is quite feasible to create an accurate map of the geoid undulation, especially in areas that present large geoid variations and other methods are not capable to give accurate and reliable results.