You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
european region is one of the most intensively studied areas of the earth from the point of view of regional geodynamics .
there are more than 100 permanently operating gps receivers , about 10 permanent vlbi stations and more than 10 permanent slr stations .
lately much attention has been devoted to comparison and combination of results obtained using different space geodesy techniques .
this work is devoted to comparison of baseline length variations derived from gps and vlbi observations , continuing the cycle of works on this problem , see e.g. .
it should be mentioned here that observed changes of the baseline length on the one hand are resulted by insufficient corrections for observational effects such as thermal deformations of vlbi antennas , for example , or errors in modeling of tropospheric refraction , but on the other hand they are subjected to a number of insufficiently studied or not taken into account properly geophysical effects that can result in the real changes of the baseline length , these effects may be atmospheric and snow loading , tides , postglacial rebound , and so on .
it is important that the majority of these effects has both seasonal and secular components .    in this study
we have analyzed vlbi and gps observations at 6 european stations carrying out both vlbi and gps regular observations and having long enough observational history .
vlbi baseline lengths were computed with the occam package using all available 24h sessions for the period of 1983.92001.5 .
details of the method used can be found in @xcite .
linear trend in the baseline lengths was computed for the whole period of observations and , for more accurate comparison with gps data , for the period of 1996.0 - 2001.0 .
only later results are presented in this study . in @xcite
we compared linear trend in variations of baseline length derived from the observations over the period 1996.02001.0 with ones computed using all available vlbi sessions over a period 1990.02001.4 .
differences of estimated rates are inside one sigma interval for all baselines analyzed here .    for more strict account for variation in baseline lengths due to thermal antenna deformations we used advanced model of this effect @xcite which allow to correct observed station position not only for vertical but also for horizontal displacement .
at this stage of the research we used zero time delay between change of air and telescope construction temperature , because of lack of such a data for most of antennas .
however , this mismodelling will effect only intra - day displacement of the telesope reference point , but not seasonal variations .
it should be mentioned here that account for horizontal displacement is especially important for processing regional networks , whereas vertical displacement due to thermal deformations prevails in variations of global baselines length . in particular , errors in modelling of this effect
may be a possible reason of seasonal baseline length variations found e.g. in @xcite .
variation of baseline lengths obtained from vlbi data are shown in figure  [ fig : vlbi ] .
unfortunately , stations crimea and yebes are not equipped with gps receiver .
=      for computation of baseline lengths between european gps stations we used weekly epn solutions distributed in sinex files . however , this solutions are not suitable for immediate use in geodynamical analysis because they can not provide homogeneous long - time coordinate time series due to periodic changes in reference coordinate system and set of fiducial stations .
for this reason , direct use of the euref solutions shows jumps in baseline length variations @xcite . besides , method of computation of station
coordinates used in epn is based on using tight constrains to fiducial stations which cause a distortion of the network , i.e. fictive variations in baseline lengths ( see e.g. @xcite ) .
so , variations of baseline lengths from gps data were obtained from analysis of coordinate time series for epn stations computed by the method described in @xcite .
this computation is based on de - constraining of the official epn solutions with further transformation to itrf2000 .
for this study we used 6-parameter helmert transformation to avoid loss of seasonal geophysical signal in baseline length . using our independent coordinate time series allows us to obtain realistic station displacement practically free of network distortion for all epn stations over the period 1996.02001.0 .
variation of baseline lengths obtained from gps data are shown in figure  [ fig : gps ] .
unfortunately , madr coordinate time series is too short ( less than two years ) that does not allow to get reliable results .
it should be mentioned that errors in gps baseline length significantly decrease during a period under investigation .
e.g. , one can see abnormal trend in mate displacement in 1996 .
however , that does not influence result very much due to relatively small weight of these data .
=      one of the most important factor affecting variations of station coordinates derived from space geodesy observations is atmospheric loading .
we investigated influence of this effect using 3-dimensional atmospheric loading time series provided by h .-
scherneck @xcite .
the data were averaged over a week interval corresponding to every gps week and variations of baseline lengths were computed from these weekly values .
variation of baseline lengths obtained from analysis of atmospheric data are shown in figure  [ fig : atmosph ] .
=    it is interesting that baseline length variations contain not only seasonal but also secular component , even for short baselines , especially for continental - coastal ones , in particular baselines including wettzell stations which are often used in studies on european geodynamics , e.g. @xcite .
the reason of that may be long - periodic or progressive weather and climate changes , but period of our investigation is too short to separate them .    since variations in height component of station displacement due to atmospheric loading prevail , this effect is especially significant for global baselines . for regional networks
horizontal displacements yield main contribution to variation of baseline lengths .
results of computation of variations in baseline lengths are presented in [ tab : result ] .
one can see that values of rates obtained from vlbi and gps observations are in good agreement for most baselines .
unfortunately , it is not the case for seasonal variations .
obviously , interval of investigation is too short and number of used vlbi observations is too small for many baselines .
indeed , it would be important to verify our results using data obtained from other space geodesy techniques , but only mate and wett stations are equipped with slr units , and only nyal station is equipped with doris beacon ( which is explained by difficulties in collocation of doris beacon and vlbi antenna due to radio frequency interference ) .
.results of analysis of variation of baseline lengths : baseline length ( l ) , km , number of epochs ( n ) processed and found in the ivs data base , linear trend ( rate ) , mm , amplitude of annual term ( as ) , mm , amplitude of semiannual term ( asa ) , mm .
[ cols="^,^,^,>,>,>,^ , > , > , > , > , > , > " , ]     it is also remarkable that influence of atmospheric loading on baseline length rate is significant for many baselines .
evidently , this effect must be investigated more carefully and properly accounted during geodynamical analysis .
figure [ fig : lbacc ] shows dependence of error in baseline length rate on length of baseline .
it is interesting that for gps data error is practically the same for all baselines unlike vlbi data .
1em    further steps of our work will include new re - computation of epn coordinate time series based on new combination of individual epn analysis center solutions , reprocessing of vlbi data with new version of software , and more complete analysis of various factors effected variations of baseline lengths .
analysis of variations in vertical component of station displacement is also planned .
this research was partially supported by a grant of the st.petersburg scientific center of the russian academy of sciences .
campbell j. the european geodetic vlbi project  an overview . in : p.
tomasi , f.  mantovani , m.  prez - torres ( eds . ) proc .
14th working meeting on european vlbi for geodesy and astrometry , castel san pietro terme , 89 sep 2000 .
lanotte r. , c. ferraro , a. nardi , c.  sciarretta , f.  vespe .
the cgs vlbi eur98 geodetic solution and comparison with the cgs gps results . in : w.  schlueter , h.  hase ( eds . )
13th working meeting on european vlbi for geodesy and astrometry , viechtach / wettzell , feb 1213 , 1999 , 101107 .
malkin z. m. , n. a. panafidina , e. a. skurikhina .
comparison of baseline lengths derived from gps and vlbi observations on european network .
presented at the all - russian astronomy conference , st.petersburg , russia , 612 aug 2001 .
panafidina n. , malkin z. on computation of an homogeneous euref coordinate time series . in : abstracts `` vistas for geodesy in the new millennium '' . presented at the iag 2001 scientific assembly ,
budapest , hungary , 27 september 2001 .
tornatore v. , f. sacerdote , m. crespi .
comparison of deformation resulting from gps and vlbi data : problems and preliminary results . in : w.  schlueter , h.  hase ( eds . ) proc .
13th working meeting on european vlbi for geodesy and astrometry , viechtach / wettzell , feb 1213 , 1999 , 206216 .
results of vlbi and gps observations were analyzed with goal to investigate differences in observed baseline length derived from both techniques . 
 vlbi coordinates for european stations were obtained from processing of all available observations collected on european and global vlbi network . 
 advanced model for antenna thermal deformation was applied to account for change of horizontal component of baseline length . 
 gps data were obtained from re - processing of the weekly epn ( european permanent gps network ) solutions . 
 systematic differences between results obtained with two techniques including linear drift and seasonal effects are determined .