## 4 Experimental Setup and Data Acquisition  
Two continuously colocated GNSS stations (KBG1 and KBG2) were constructed in August 2020 in the city of Luxembourg, Kirchberg campus, on the roof of the JFK building (6.2\({}^{\circ}\) N, 49.6\({}^{\circ}\) E, 404.9 m). The two GNSSs are equipped with Trimble Alloy GNSS reference receivers. These receivers are attached to a Trimble geodetic Choke Ring GNSS antenna with IGS naming, TRM159800.00, integrated with a radome-type SCIS. The antennas are mounted on a 3.5-m aluminium mast (LECLERC type). The two sites use a common receiver/antenna and only differ in their serial number at the site, with KBG1 having SCIS-5850337009 and KBG2 having SCIS-5628351662. The two Trimble antennas have also been individually calibrated in a clean environment within an anechoic chamber of the University of Bonn for phase centre offset (PCO) and phase centre variation (PCV) models [53]. The model values are provided in ANTEX version 1.4 format. The anechoic chamber antenna calibration provides models for 25 frequencies for BeiDou (C01, C02, C05, C06, C07, and C08), Galileo (E01, E05, E06, E07, and E08), GLONASS (R01, R02, R03, R04, and R06), GPS (G01, G02, and G05), QZSS (J01, J02, J05,and J06) and SBAS (S01 and S05). The two stations are located in challenging locations but very close to each other with a distance of 5 m, as shown in Figure 3, which elucidates the experimental setup; to the left is  
Figure 2: The Screenshot of the first Fresnel Zone at KBG1/KBG2 on the roof of the JFK building, Kirchberg campus, Luxembourg city, using elevation angles of 5, 10, 15, and 30 degrees. The coloured ellipses represent all GPS satellites visible at KBG1/KBG2 location. An antenna height of 3.5 metre is used. Credit of background image: Google Earth. station KBG1, and to the right is station KBG2. The station KBG1 antenna is mounted on a single three-metre LECLERC aluminium mast with four-legged three-metre LECLERC aluminium mass supporting the wooden frame carrying the Eccosorb material. As indicated in the introduction, one strategy to potentially mitigate the consequences of signal scattering is to cover the scatterer with microwave-absorbing material. The KBG1 station is equipped with a lightweight, flexible, polyurethane foam broadband microwave-absorbing material, specifically an Eccosorb AN-W-79 sheet sealed with neoprene-coated nylon fabric. A single Eccosorb has 11.4 cm nominal thickness polyurethane sheets cut into 61 cm \(\times\) 61 cm sections. It consists of nine sections with a total area of 3.35 m\({}^{2}\) surrounding the perimeter of the KBG1 antenna, with the individual unit weighing 2.95 kg. For fairly wet and high humidity environments, a sealed version of Eccosorb AN-W-79 is preferable, providing improved outdoor properties. This particular Eccosorb material reflects less than \(-\)17 dB of normal incident energy above 600 MHz, which is below the signal frequencies that operate the GNSS signals. Figure 4's left and middle depict an image of an Eccosorb AN-W-79 covered by an olive green colour attached to the perimeter of the KBG1 station antenna, whereas the antenna for station KBG2 without the Eccosorb material is shown on the right of Figure 4. The Eccosorb material should block scattering or diffraction at least within 90 cm from the antenna from any direction, including the wooden frame and the Choke Ring of the antenna. This means that the scatterer located in the near-field of the antenna has negligible electromagnetic coupling with the antenna. Another GNSS antenna next to the KBG1 GNSS station is also installed with a Trimble TRM159800.00 Choke Ring GNSS antenna, which is a radome (type SCIS) mounted on a single three-meter LECLERC Aluminium mast, as shown in Figure 3, to the right. The two stations have collected multi-GNSS data continuously since 20 September 2020 (day of year (DoY) 264). The data help us assess the characteristics of multipath effects with two configurations, i.e., with and without the Eccosorb material.