## 4 Global Navigation Satellite System (GNSS) Measurements
### Installation of Continuous Global Navigation Satellite System Instruments  
When planning to install a continuous GNSS instrument for volcano monitoring, several points have to be taken into consideration:  
* The location with respect to the volcano and other monitoring equipment;
* The availability of bedrock and stability of the site to create a monument to mount the antenna;
* Access to power and data transmission (communication) facilities;
* An unobstructed clear view of the sky in all directions (\(360^{\circ}\) when possible) considering human activity, buildings, and vegetation coverage;
* A convenient path and path length for running the cables;
* Protection against lightning strikes;
* The safety of the equipment (secured from vandalism).  
Following these considerations, both islands were scouted in early 2017 to find suitable locations for two continuous GNSS instruments (one on each island). After thoughtful consideration, two locations were chosen co-located with already operating seismometers: \"SABA\" on Saba and \"SEUT\" on St. Eustatius (Figure 2). At both of these sites, a monument was constructed for the stable placement of the antenna. This monument consists of a concrete base and a stainless steel support anchored into bedrock (Figure 11). The coupling of the antenna to the bedrock is guaranteed by the use of a one piece, 1.7 m long stainless steel rod, drilled 70 cm into solid bedrock and epoxied in place, covering the full length of the monument. Four additional rebar rods improve the coupling to the bedrock of the concrete base of the monument, and rebar rings reinforce the base. The stainless steel support, covering the top end of the long rod, is screwed onto the concrete base and held in place by a bolt at the top. The long rod is topped with a SECO mount to allow precise leveling of the PolaNt choke ring B3/E6 antenna with spike and its precise orientation towards the north. The skyview at both sites is mostly unobstructed (Figure 12), although vegetation coverage will need to be controlled especially on St. Eustatius. The receiver, a Sepentrrio PolaRx5S, is placed inside the same building as the broadband seismometer (SABA), where access to power, using a wall outlet, and data transmission, using ethernet, is secured. To minimise disturbances from human activity and to prevent obstruction by buildings, the antennas were placed away from the building, necessitating a relatively long (35-40 m) cable to connect the antenna to the receiver. To limit data loss, a high quality flexible LMR-600 cable was used, covered with a protective conduit. To prevent damage from lightning strikes, the cable was protected by a Times Protect LP-GPX-05 lightning and surge protector upon entering the building. The receivers are configured to acquire three data streams with different frequencies: every 15 s in 24 h blocks (15s24h in RINEX format), at a frequency of 1 Hz in 1 h blocks (1 hz1h in RINEX format), and at a frequency of 20 Hz in 1 h blocks (20 hz1h in SBF format). Data are saved internally as well as sent to KNMI four times a day.  
Figure 11: (**a**) Sketch of the monument design ([PERSON], 2018); (**b**) A one piece stainless steel rod is drilled 70 cm into solid rock and epoxied in place. Four additional steel rods improve coupling to the bedrock of the concrete base; (**c**) Construction of the concrete base at SEUT; (**d**) A stainless steel support, screwed to the concrete base, is topped with the SECO mount to allow precise leveling of the antenna and orientation towards the north. A LMR-600 cable is cut to length and crimped on site.