## 4 The Como Test
### The Surveying  
To impose displacements, a device (slide) that allowed controlled horizontal and vertical displacements was used (Figure 4). It was designed and built at Department of Environment, Land and Infrastructure Engineering, Politecnico di Torino. It is composed of a calibrated hardened steel bar (\(X\) axis), with a special support for a GNSS antenna on a vertical steel bar (\(Y\) axis). When the slide is perfectly leveled, the \(X\) axis materializes as a horizontal direction while the \(Y\) axis is the vertical direction. Figure 4: The slide of the Como Test. (**a**) The device: \(X\) displacement 1.3 m, \(Y\) displacement 0.3 m; and (**b**) the total station used to orient the horizontal \(X\) axis exactly to the northeast (azimuth of 45\({}^{\circ}\) from north) direction. Figure 3: Milano test. Vertical residuals of hourly sessions. BSW5.2 in blue, LGO in red. The movements of the antenna support are controlled by a wheel and pivot activated manually. Two millimetric tapes allow the user to control the imposed displacements for \(X\) and \(Y\) axes. The system guarantees the check of the displacements with a precision better than 1 mm, which is certainly sufficient for our test. In total, horizontal displacements up to 1.30 m and vertical displacements up to 30 cm can be applied. The slide was fixed on the roof of the building in Castelnuovo street. The \(X\) axis of the slide was oriented clockwise 45\({}^{\circ}\) from the north direction with the help of a total station. In this way, horizontal displacements are equally distributed in the north and east directions. In total, three receivers were used:
* The Como European Permanent Network (EPN) station [31]. The station consists in a Topcon Odyssey RS receiver and its antenna mounted on a pillar on the roof of the building in Valleggio street: the building, with its height of 30 m, is the dominant structure inside the campus and provides optimal open sky conditions in all directions. This was used as the primary reference station in the data processing scheme. * One u-blox NEO-7P receiver. This antenna was installed on a point with well-known coordinates, at about 3 m from the Como EPN station. This was used as the secondary reference station in the estimation of the rover. * One u-blox NEO-7P receiver which simulated a monitored rover. It was installed on the slide, at about 130 m in the northwest direction with respect to the reference stations on the roof of a building (Castelnuovo street), about 10 m in height. NEO-7P is a consumer-grade GNSS receiver equipped with u-blox next generation GPS platform NEO-7P and an active GPS antenna [32]. The main technical characteristics of u-blox NEO-7P are:  
* high-precision GPS < 1 m, in the configuration satellite-based augmentation system (SBAS) + precise point positioning (PPP),
* differential GPS by SBAS or Radio Technical Commission for Maritime Service (RTCM),
* raw measurement data (GPS),
* 56 channels: GPS L1 C/A; GLONASS L1 FDMA; QZSS L1 C/A; SBAS,
* GNSS receiver: GPS, GLONASS, quasi-zenith satellite system (QZSS), and Galileo,
* antenna gain: 27 dB. The rover u-blox used its default antenna. The secondary reference u-blox was paired with a Tallysman TW3150 GPS L1 single frequency antenna: it is still a low-cost device as its retail price is approximately $80, but has higher gain (50 dB), lower noise (<1.5 dB), better multipath suppression and a more stable mean phase center. The antenna was courteously provided by Stefano Caldera of G-Red. NEO-7P is a multiconstellation GNSS receiver: however, just one constellation at a time can be selected for a session of registration. Clearly, we chose GPS, that is the most complete and larger constellation in orbit. Each NEO-7P was connected to a laptop which ensured the power supply and allowed us to access the receiver and to configure it. The roof that hosted the slide is surrounded by hills that partly obstruct signals for very low elevations (below 5\({}^{\circ}\)).