## 1 Introduction  
Repeated surveys (i.e., GPS campaigns) with the repetition rate of several consecutive days per year and with session durations of 8-24 h were frequently referred to in the history of GPS positioning [1; 2; 3]. GPS short occupations (i.e., \(<\)24 h) are biased because of the severe atmospheric conditions on the GPS signal, multipath environment, bad satellite-receiver geometry, etc. [4]. One needs to use complete 24 h data to overcome the above disturbances. Furthermore site occupations should last several consecutive days to eliminate bad solutions from the available results. All possible irregularities occurring with the above mentioned reasons can only be sampled in one complete day. The next day, the same satellite constellation and hence the same observation pattern repeats four minutes earlier. Positioningfrom shorter occupations generally results in coarser accuracy [5; 6]. There were several reasons GPS campaign measurements must be carried out with short site occupations. First, field operators wanted to take advantage of the day light. Second, it was secure to employ receivers before sunset, and field operators wanted to avoid unfavorable overnight conditions. Then continuously operating reference system (CORS) networks started to be used however they were not installed as dense as desired in all types of deformation monitoring works. Obviously, CORS were costly and thus were not adopted by many researchers in all kind of deformation monitoring tasks. GPS campaigns were used in sea level experiments [7], determination of geopotential values [8], monitoring of regional tectonics [9; 10; 11], natural hazard monitoring such as landslides [12; 13], and others [14; 15; 16]. In each of those experiments the goal was the same; estimating the deformation rate. CORS then became widespread, especially in the tectonically active areas. The GPS time series were accumulated and deformation rates obtained from those time series were assessed. The character of the seasonal signal on the vertical component was determined [17]. The optimum length of time series in obtaining true velocities (i.e., deformation rates) was experimentally determined. The noise characteristics of the time series and the effect on the estimated deformation were studied [18; 19; 20]. At times, GPS experiments were carried out in which the site occupation was performed in a rate of once per year with 8-10 h of the observation session. This was the worst case scenario for repeated GPS surveys because the researcher was not able to eliminate bad observations from a set of site occupations. In many studies, the velocity estimated from GPS campaigns was assumed to be equivalent to the velocity derived from continuous GPS. This assumption could be true only if a proper processing strategy (e.g., short baseline lengths to reference stations processed by commercial software) was adopted [21]. According to the theory of [PERSON] [17], there was an annually repeating significant seasonal motion on the vertical GPS component, and the GPS data sampled at the rate of once per year would not reveal that effect on the positioning results. [PERSON], et al. [22] noticed this and estimated the velocity of tectonic plates (i.e., the horizontal motion) using campaign GPS measurements from the data sampled once per year with the observation session duration of 8-12 h. The sampling rate for the vertical component was increased to once per month with the same session duration to reveal the effect of sinusoidal motion. The study mainly targeted to criticize the work performed with site occupations less than 24 h and repeated less frequently. They compared the velocity estimated from the GPS campaigns based on short occupations with that of GPS campaigns using the full 24 h observation period. Based on the analyses performed with 95% reliability, only 30-40% of horizontal velocities from 8-12 h GPS campaigns agreed with the velocities derived from 24 h sessions.