## 2 Materials and Methods
### Determination of Site Suitability Criteria and Constraints
#### 2.1.1 GNSS CORS Installation Requirements  
According to the CORS guidelines released by the Intergovernmental Committee on Survey and Mapping (ICSM) [9], GNSS CORS are divided into three categories in terms of the station's purpose and stability (i.e., Tier-1, -2, and -3). CyCLOPS has deployed  
Figure 2: The suitability analysis procedure for the determination of the CRs’ final location.  
Tier1/2 GNSS CORS. According to UNAVCO, IGS, and EPN [10; 11; 12], such a site should fulfil specific requirements to achieve maximum accuracy during measurements:  
* Highly Stable monument (on bedrock);
* Open Sky visibility (increased satellite availability);
* Avoidance of potential Radio Frequency Interference (RFI) sources;
* Security from unauthorized access;
* Clear of reflecting surfaces (e.g., fences, metal poles);
* Ease of access;
* Data Accessibility;
* No local crustal instabilities-controlled vegetation;
* Power Supply and energy redundancy;
* Internet connectivity (through wired or wireless broadband internet);
* Augmentation of the existing national networks (filling the gaps for the availability of network solutions).  
The requirements mentioned above are vital to the operation and reliability of the GNSS CORS. Since the objective of CyCLOPS was the deployment of Tier1/2 GNSS reference stations, having very stable monuments installed on solid bedrock was the most critical requirement. Following GSD suggestions, an in-situ visit has been carried out in each one of the candidate locations. The objective was to (a) inspect the geological background, (b) verify whether the locations fulfil the above-mentioned site selection requirements, (c) check on the existence of physical or man-made obstacles that can compromise sensor performance and (d) detect potential security threats. After the initial inspection visit, 24-h GNSS data acquisition campaigns were carried out in each candidate location to assess actual GNSS satellite availability, signal quality, multipath and potentially detect signal jamming sources. The datasets were collected using Trimble R9s receivers equipped with Zephyr Geodetic 2 and 3 antennas. The analysis was then processed with G-Nut/Anubis, which has been developed by the Geodetic Observatory Pecny and is the standard software used in the European Plate Observing System (EPOS) EU ESFRI [13].  
The results from the analysis allow for the assessment of the data quality for each satellite system (GPS, GLONASS, GALILEO, BeiDou and QZSS) which is tracked from a particular location. The related quality metrics that have been computed from the daily GPS/GNSS datasets at the candidate locations include the following: multipath maps, number of tracked satellites per epoch, histograms of phase measurements per elevation angle of tracked satellites, number of observations per epoch or satellite, number of cycle slips and ambiguities, measurement precision for each satellite arc, estimated receiver clock corrections and their precision, standard deviation of the estimated kinematic coordinates and RMS of the final PPP-estimated coordinates. An example of these metrics for the candidate site ALEV is illustrated in Figure 3.