Another serious difficulty which had to be faced consisted in 
determining a rational frequency of the network 
measurements: on the one hand, it was possible to detect 
secular variations using the most accurate observation 
techniques available at that time but only after a relatively 
long period of time. On the other hand, the need to detect 
unexpected sudden changes called for more frequent 
measurements. The major shortcoming of this kind procedure 
was the fact that results obtained from analysis reflected only 
generalised secular changes and they did not reflect time 
changes that are periodical in their nature, which are 
sometimes very important (e.g. in the case of analysis for 
areas of strong seismic activity). 
The dense network of permanent GPS stations which was 
established over the past few years, and which is still being 
extended, provides us with new possibilities as regards 
geodynamic research, not only global and regional but also 
research of local character. It is evident that GPS observations 
conducted permanently at many international stations ("core" 
or "fiducial" ones) and processed on the day-to-day basis 
supply us with currently up-dated material which is extremely 
valuable for geodynamic analyses. Compared to classical 
techniques, analyses of mutual position of points located 
hundreds and thousands of kilometers from each other have 
become relatively easy. It has also become possible to monitor 
movements of tectonic units and continents accurately and 
permanently as well as to verify many geological geodynamic 
hypotheses. The growing network of permanent GPS stations 
makes it possible for us to conduct similar research both in 
regional as well as sometimes in a local scale. The mentioned 
above geodynamic network established in Japan, which 
consists of 900 permanent GPS stations located at a distance 
of 20-30 km from each other, can serve as an example. It is 
true enough that the territory of Japan is particularly active in 
terms of seismology and for this reason a great number of 
permanent GPS monitoring stations was established in order 
to register and monitor any short and long-term as well as 
secular changes in their positions, which are of prime 
importance to the forecast of seismic disasters in this region. 
The growing accuracy of the satellite observation techniques, 
especially of the GPS ones, which is already significantly 
much higher than that of the classical observation techniques, 
lays down particularly high requirements as regards 
stabilization of geodynamic points. It appears to be necessary 
that we should not only consider unambiguous submillimetre 
centering of antennas of satellite receivers and take account of 
many other factors that affect the accuracy and univocal 
nature of observations (e.g. examination of the ground water 
level, periodical variations in gravity values, atmospheric 
effects, etc.), but there is also a growing need to use deep 
stabilization of points that are of prime importance. This 
stabilization is not a stabilization just "below the ground 
freezing level", it is a stabilization based on poles - 
foundations that are approximately 25-30 metres deep. It had 
been found out that the 30 m deep subsurface ground layer is 
subject to periodical displacements depending on the time of 
the day, the season of the year, the humidity level, etc. The 
aforementioned phenomena can be investigated only by using 
permanent observations conducted at a geodynamic station. 
There is one more issue that needs discussing in our 
considerations on geodynamic analysis, namely the question 
of using traditional techniques along with modern (satellite) 
observation techniques. It is obvious that when fast, more 
precise and extremely economical satellite measurement 
technology came into use, triangulation methods (with 
angular and linear measurements) almost completely ceased 
to be used and also linear measurements have been 
significantly limited since that time. Also the role which 
should be played by precise levelling in modern geodynamic 
research requires a special discussion. Precise levelling used 
to be one of the basic observation techniques in traditional 
geodynamic research. As confirmed by research performed by 
e.g. the British school from Nottingham, it appears that 
determination of heights between points located at significant 
distances by means of precise levelling is not so accurate; 
when the distances between points are equal to hundreds of 
kilometres better results can be obtained today by using 
satellite and space methods (SLR, VLBI or even GPS 
levelling). This is related to the need of determining the so- 
called "centimetre" geoid. The conclusion to be drawn from 
the above is that precise levelling can and should be still used 
today in investigations of displacements and movements in 
local networks within an area limited to several score 
kilometres. 
The following conclusions can be drawn from what has been 
said above: 
• The function of the classical geodynamic networks should 
be taken over by permanent GPS stations, at which 
permanent GPS observations are carried out and subject 
to currently up-to date processing and analysis. 
• Up-to-date analysis of permanent GPS observations helps 
to detect and monitor both secular and short- and long 
term phenomena occurring at the station. 
• The high accuracy of modern observation techniques 
allows us to examine individual phenomena 
(components) and not only their summary combined 
impact. 
• The purposefulness of conducting measurements by 
classical measurement techniques should be subject to 
special careful considerations. This refers particularly to 
linear measurements by means of distancemeters and to 
precise levelling measurements. These can be used in 
local geodynamic networks of a limited size. 
• Today, in the face of the fact that precise GPS 
technologies are commonly accessible and easy to use, 
the establishment of so-called "national" geodynamic 
networks of a classical type seems to be unjustified both 
from the scientific and economic viewpoints. The 
function of the geodynamic networks should be taken 
over by permanent GPS stations located in properly 
selected areas. However, it is purposeful to maintain 
local geodynamic test networks, but these networks 
should be connected to the networks of permanent GPS 
stations and periodically controlled. 
• For economic reasons, it is advisable that permanent GPS 
stations should also constitute at the same time a 
network of permanent reference stations of the national 
DGPS navigation system emitting differential corrections 
for the land, air and marine users and various economic 
services of the country. 
• Taking into account what has been said before it is to be 
stressed that in the scientific programme CERGOP-2 the 
permanent GPS sites should be considered as main 
stations of the Extended Central European GPS 
Reference Network ECEGRN. This reference network 
can be regarded as subnetwork of IGS and/or EUREF. 
Extensive observation data collected at these stations 
should be carefully processed and analysed within the 
scientific programme of CERGOP-2.