Antenna Selection for Bridge Deformation Monitoring: Comparison of Multipath Mitigation Characteristics for Three Types of Antennas

The area of bridge deformation monitoring using GNSS has been growing and developing over the last 10 – 15 years. The University of Nottingham is continuing research in this field with recent monitoring campaigns on the Avonmouth viaduct which is a steel-box girder structure, the Severn Crossing (suspension bridge) as well as the Soehae and Machang bridges in Korea which are cable-stayed bridges. In many of these tests large choke ring antennas have been used both at the reference and rover sites in order to mitigate the impact of multipath. The GNSS rover points are sometimes located using clamps on the handrail (Humber, Avonmouth) or poles affixed to the actual suspension cables, as was done in the Severn bridge tests. In these instances having a lightweight antenna would be desirable. For the Severn and Avonmouth tests lightweight choke-ring antennas were used. However the use of an even lighter antenna would be desirable. As a result a comparison test was conducted using three different types of Leica antennas in two different environments. The Leica AT504 choke ring antenna, the AT503 lightweight choke ring and the AX1202 (which is the lightest of the three antennas) were tested. The environments were a medium to high multipath environment close to a building and then a high multipath location which was simulated by placing a 2m wide reflective metal sheet close to the antenna.
The results showed that the AX1202 compared favourably with the other two choke ring antennas under medium to high multipath conditions. However under severe multipath the AT504 appeared more robust compared to the others. This suggests that where it is critical to have a lightweight antenna for bridge deformation monitoring, the AX1202 shows good multipath mitigation characteristic and can be used in conjunction with a standard choke ring antenna at the base station.