## 2 Literature Background
### Background Articles on GNSS-CORS Network Monitoring  
Many works deal with the theoretical network monitoring of CORS; indeed, numerous studies have been developed following the guidance on implementation, materialisation, and executive design by _[PERSON]_ et al. [10]. The _MultiRef_ approach ([PERSON] et al. and [PERSON] [15; 16]) was investigated to analyse the influence of the network geometry design on the obtainable accuracy. These analyses showed that the best results are gained when reference stations are distributed at a regular distance; furthermore, CORS stations located outside the study area do not significantly increase accuracy [16]. Two papers by [PERSON] et al. discuss the stations' separation, the effects of CORS geometry, and the data reduction technique on the reliability of the rover positioning solution considering that of the Ohio CORS [17; 18]. _[PERSON] and [PERSON]_ identified the main factors affecting the quality of coordinates' time series in the EPN (EUREF Permanent Network) in Europe, installed for reference frame maintenance [19]. [PERSON] et al., [PERSON], [PERSON], and [PERSON] analysed the _Online Positioning User Service_ (OPUS), a full Web-based software that processes GPS observation from the United States Department of Commerce's _National Geodetic Survey_ (NGS) CORS network, bringing almost 200 organisations together at various levels: academics, private sector, and also governmental [20; 21; 22]. Among the most cited articles, there are the experiences of [PERSON] and [PERSON] and [PERSON] et al. ; the first author describes the context in which the Global Geodetic Observing System (GGOS) was established, discussing the role of GPS/GNSS network infrastructure in the implementation of the GGOS mission and providing an update status in Thailand of GNSS CORS [23]; the second paper discusses the infrastructure of the Turkish RTK CORS Network denominated Turkriye Ulusal Sabit GNSS Agt--Aktif (TUSAGA-Aktif) [24]. Of particular significance in the literature is the work of [PERSON] et al. that studied real-time kinematic precise point positioning (PPP-RTK) with centimetric precision employing the Perth GPS Network regional CORS [25], while [PERSON] and [PERSON] studied the benefits of array-assisted CORS ambiguity resolution [26]. In more recent years, the efforts of scientists in the design of GNSS CORS have explored the applicability of these services to wide-area satellite navigation systems. [PERSON] et al., for instance, reported their experience in the People Republic of China on the use of ground-based augmentation systems (GAS) using the BeiDou global navigation satellite system (BDS-3). Many tests have been conducted on the performance of the GAS service, showing its suitability for positioning at different levels of accuracy [27]. Another recent study, carried out in Asia by [PERSON] et al., discussed the contribution of CORS to a long-standing issue in India, in particular regarding the transport infrastructures and their mapping. In this regard, the Indian mapping agency _Survey of India_ (SOI) is deploying a CORS network to improve the precision of three-dimensional positions obtained via GNSS [28]. Analogously, in Egypt, the _Egyptian Surveying Authority_ (ESA) has established a CORS network consisting of more than 40 stations covering the Nile valley and its delta to assess the quality of ESA-CORS data to monitor tectonic movements. The study analyses the reliability of free Canadian Spatial Reference System Precise Point Positioning (CSRS-PPP) software over more than 30 ESA-CORS claiming millimetric accuracy [29]. The augmentation of the GNSS CORS network has been crucial in providing continuous positioning in various scientific areas. However, gaps in the system, interference from buildings and vegetation, and problems in deep mining pits and underground mines limit the possibility of using augmented GNSS CORS in all situations.