## 1 Introduction  
The Global Positioning System (GPS) was developed for single-point positioning (i.e., Standard/Precise Positioning Service). The use of carrier-phase measurements in relative positioning was explored by [1, 2, 3]. Since then, GPS has been widely used in high-precision positioning applications, such as attitude determination [4, 5], geodetic surveying [6, 7], geodynamic and geophysical studies [8, 9], and engineering surveying [10]. A reference receiver is needed for relative positioning, and the determined position of the rover station is aligned in the same reference frame as the reference station. Organizations/companies needed a GPS receiver for the reference station and a GPS receiver for the rover station in the 1980s and 1990s. The US National Geodetic Survey (NGS) and the International GNSS Service (IGS; was called the International GPS Service for Geodynamics) developed the Continuously Operating Reference Stations (CORS; also known as tracking stations) for precise orbit determination in the 1990s. Moreover, the CORS stations serve as the reference stations for relative positioning. In addition, some GPS receiver manufacturers have built CORS in some countries/regions, such as Leica and Trimble. Surveyors only need a rover receiver for engineering surveying when there is a CORS network in the region. Usually, the nearest CORS station is selected for baseline processing. In baseline processing, any errors at the reference station are propagated to the estimated position of the rover. In network real-time kinematic (RTK) approaches, such as the Master-Auxiliary Concept (MAC) [11] and Virtual Reference Station (VRS) [12], errors at reference stations affect the RTK corrections, modeled biases/errors, and raw measurements. Site selection is a step in network RTK approaches. We prefer selecting reference stations with high-quality raw measurements over selecting the nearest reference stations. The quality of raw measurements at reference stations is affected by cycle slips, measurement noise, and multipath errors. The IGS website does provide hourly time-series plots of the number of cycle slips in GPS L1 and L2 measurements and pseudorange multipath errors in GPS L1 and L2 measurements (i.e., MP1 and MP2) per station. It is worth mentioning that \"multipath mitigation should be disabled in the GNSS receiver\" [13] (guideline no. 2.2.19), excluding special antenna designs such as choke-ring antennas. GPS and GLONASS modernization is being undertaken. Galileo will have its full operational capacity (FOC) soon (scheduled for 2020; it has 23 \"usable\" satellites as of 20 June 2022). The BeiDou Navigation Satellite System (BDS) started FOC in June 2020. The Global Navigation Satellite System (GNSS) is the collective term for GPS, GLONASS, Galileo, and BDS. Currently, IGS only provides the daily average pseudorange multipath errors in GPS L1 and L2 measurements (i.e., average MP1 and MP2) and the number of cycle slips (but not specific to L1 or L2) per CORS/tracking station. High-precision GNSS users may want to use either GPS, GLONASS, Galileo, or BDS only. The quality of GNSS measurements (i.e., all constellations and multi-frequency) at each CORS station is essential for station selection in baseline post-processing and (conventional/network) RTK. This paper proposes a multi-GNSS and multi-frequency data quality assessment approach for CORS stations with a focus on carrier-phase data quality. There is no absolute approach for GNSS data quality assessment. The proposed approach is designed to minimize the effect of other GNSS biases/errors on the assessed data quality. The proposed approach is demonstrated with case studies on the Hong Kong and Canada/USA CORS networks. An analysis of the data quality at some selected CORS stations will be given. GNSS CORS data quality is affected by the antenna and receiver hardware and firmware, the monument type and height, the site environment and location, etc.