## 2 Literature Background
### Background Articles on CORS Accuracy  
In recent years, the development of information technology (IT) infrastructures related to CORS networks has made possible the investigation of their accuracy for classical topographical applications, of which many works have been mentioned. Two papers by [PERSON], [PERSON], and [PERSON] et al. represent some CORS research milestones: in the first, [PERSON] et al. present the PPP-RTK method based on the non-differentiated GNSS observations studied in Delft in the 1990s on two CORS differing in inter-station distances (Hong Kong CORS Network, 30 km, and GPS Network Perth, 70 km) and the method's performances. In the second paper, analogously, [PERSON] et al. applied the method to the other two GPS CORS networks with inter-station distances ranging from 60 to 100 km. The tested CORS networks were the northern China network and the Australian Perth network. In the two papers, the fast GPS surveying is based on the _Last-squares AMBiguity Decorrelation Adjustment_ (LAMBDA) [72; 73; 74]. Furthermore, [PERSON] et al. reported the processing of ten days of GPS data to investigate the accuracy of a three-dimensional relative position vector between GPS antennas, depending on the chord distance between these antennas [75]; also, [PERSON] et al. describe the accuracy of Online Positioning User Service (OPUS) solutions for short observation sessions (1 to 4-h). [PERSON] and [PERSON] also describe network-based geometry-free models for three carriers' ambiguity resolution (TCAR) and phase bias estimation with a double-differenced (DD) solution [76]. Most cited articles analysing instead the long inter-distances between the sites of the CORS are published by [PERSON] et al. [77], [PERSON] et al. [78], and [PERSON] et al. [79]. Articles by [PERSON] et al. [80] (USA), [PERSON] et al. [81] and [PERSON] [82] (United Kingdom, UK), and also [PERSON] et al. [83] (Australia) on the evaluation of the accuracy of nationwide networks are the most widely rated in the literature. Also, the evaluation of CORS accuracy represents one of the main topics analysed in low-cost receivers [84; 85; 86] and UAV trajectory [87; 88; 89]. As is well known, the accuracy of GNSS CORS is dependent on various aspects, such as RFI or multipath problems. However, there are another series of limitations to the use of data from CORS, which differ in nature and consistency. Certainly an important role plays in the noise reduction of time series: [PERSON] et al. have since 2004 investigated the influence analysis of the error of continuous GNSS CORS position time series, with lengths of the series ranging from a few months to over 10 years. In particular, using maximum likelihood estimation (MLE), the noise was taken to be only white noise, a combination of white noise plus flicker noise, or a combination of white noise plus random walk noise. All noise components show a latitude addictiveness in their amplitudes (greater in equatorial sites) coupled with a bias towards larger values in the southern hemisphere [90]. Also, [PERSON] et al. have addressed the issue of GNSS CORS time series reduction with a performance of Narrow Correlator spacing in the receiver [91]. More recently, [PERSON] et al. carried out methods to determine noises in the GNSS vertical coordinate time series (as the white and flicker noise) using time series from the Crustal Movement Observation Network of China (CMONC). The results show that the average flicker noise in the time series was reduced from 19.90 mm/year \({}^{0.25}\) to 2.8 mm/year \({}^{0.25}\)[92]. Also, another limitation was the GNSS CORS signal extraction: [PERSON] et al. propose new strategies to detect spoofing attacks on GNSS signals through a signal quality monitoring (SQM) technique [93], while [PERSON] et al. investigate an analysis of signal propagation and signature extraction for GNSS indoor positioning [94].