## 2 Literature Background
### Background Articles on Reference Frames and Structure Monitoring  
Before delving into this section, it's crucial to highlight the comprehensive review conducted by [PERSON] and [PERSON] on the physical applications of GPS CORS. This work begins with an overview of GPS, including its constellation and signals, infrastructure, surveys, and positioning methods. It then thoroughly discusses topics related to space geodesy reference systems, coordinate transformations, and GPS position time series. The review also emphasises other aspects of GPS CORS, such as tectonic and crustal deformation, natural hazard mitigation systems, atmospheric monitoring, and climate [41]. Since their development, the main applications of CORS have concerned reference frame definition and land monitoring (in terms of subsidence, seismology, and volcanology). [PERSON] and [PERSON] investigated the permanent network (EPN) of the Regional Reference Frame Sub-Commission for Europe (EUREF) with reference frame maintenance, analysing the quality of the coordinate time series and the site velocity estimation, involving continent-wide spread CORS [19]. The review by [PERSON] et al. is also noteworthy, where they assessed the techniques and software packages for optimal noise modelling in GPS CORS time series, along with processing strategies and error mitigation [42]. [PERSON] and [PERSON] also carried out a case study on a new set of parameters (for transforming positional coordinates and velocities) between the International Terrestrial Reference Frame of 2000 (ITRF00) and the North American Datum of 1983 (NAD 83) using CORS data by Natural Resources Canada and the U.S. National Geodetic Survey (NGS) [43]. Regarding the South American subcontinent, the _Sistema de Referencia Geocentricro para las Americas_ (SIRGAS) was presented in _[PERSON]_ et al. [44; 45]. As is well known, SIRGAS represents the geocentric reference system for the South Americas; its definition is equivalent to the International Earth Rotation and Reference Systems Service (IERS) International Terrestrial Reference System (ITRS), and it is realised by regional densification of the IERS International Terrestrial Reference Frame (ITRF). In 2018, [PERSON] et al. presented the results of North China Reference Frame 2016 (_NChina16_), which deals with landslides and subsidence using five years of continuous GPS observations from 12 CORS fixed on the North China Craton [46]. Other studies at national or regional scales can be further explored, e.g., those of [PERSON] and [PERSON] about GNSS CORS in Thailand [23], [PERSON] et al. for the TUSAGA-Aktif Network (Turkish national CORS) [24], [PERSON] and [PERSON] for the geodetic infrastructure in the Gulf of Mexico Region [47], [PERSON] et al. for a subsidence test area in Houston metropolitan area, Texas, U.S.A. (Houston16) [48] or, also, [PERSON] 2011 et al. for the land uplift model NKG2016 LU, with a new land model for Finnish-Scandinavia (Fennoscandia) peninsula and the neighbouring Baltic Region [49]. The use of CORS was also aimed at seismic displacement assessment operations, especially in the most geo-seismically active areas around the world, such as the South American plates [50], Chile and Mexico [51], Turkey [52; 53; 54], and the USA [55]. Another field of application of CORS concerns the investigation of ground deformations by analysing the estimated time series of velocity fields. Based on the important scientific contribution given to these applications by the review of [PERSON] et al. [56], many other studies have been developed in recent years in different countries, such as the Sierra Nevada (USA) [57], Andes (Chile) [58], Caribbean region [59], Himalaya (India) [60; 61], Turkey [62], Greece [63], and the Alpi region (Italy) [64]. It is worth mentioning that CORS were also used in other recent applications concerning tunnelling via BDS [65], bridges [66] and dams [67], monitoring, mining [68], and combining different satellite techniques, such as GNSS and Interferometric Synthetic Aperture Radar (InSAR) [69; 70; 71].