A radar-based monitoring of the Collserola tower (Barcelona) G. Luzi n , M. Crosetto, M. Cuevas-González Remote Sensing Department – Division of Geomatics, Centre Tecnològic de Telecomunicacions de Catalunya (CTTC), Av. Gauss, 7, E-08860 Castelldefels (Barcelona), Spain article info Article history: Received 8 October 2013 Received in revised form 20 March 2014 Accepted 23 April 2014 Available online 15 May 2014 Keywords: Radar Interferometry Vibration Tall tower Cable force tension abstract This paper reports a set of experiments aiming at evaluating the capability of an innovative radar technique to measure the dynamic response of a 268 m high tower, the Collserola tower located in Barcelona, and its guys; the tension force of some guys is also estimated from the obtained vibration frequencies. The applied procedure was based on the use of a coherent radar system: temporal samples acquired using different observation geometries were processed to retrieve the vibration characteristics of both the tower and its guys. This was attained by observing the tower in a fully remote sensing mode, i.e. several hundred metres from the object, without installing any reflector on the tower, and under micro-tremor and wind-induced excitation. During the campaigns, which were spread over three years, the investigated technique demonstrated its capability to measure the dynamic response for a number of different points of the tower with high repeatability. The performed experiments also allowed distinguishing and characterizing the different contributions of the tower and the guys. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Articles focused on the use of microwave remote sensing to measure the dynamic response of a variety of structures, as bridges [1–3], buildings [4,5], wind farms [6], chimneys [7], ancient towers [8] and guyed masts [9] among others, have been widely disseminated in the last ten years. The capability to measure displacements with sub-millimetre accuracy and consequently, to identify the characteristics of key vibration modes, from distances up to hundreds of metres, and with negligible influence of the weather conditions, are major advantages of this technique. Although radar interferometry is only able to provide an estimate of the radar Line-Of-Sight (LOS) component of the actual displacement, it can contribute to understand the behaviour of complex structures, which is particularly useful when the use of contact sensors is costly or needs hazardous installations, as in the case of bridges, tall structures, guys, etc. In this paper, experimental results collected during three different monitoring campaigns are reported. The monitored structure is a tall tower located in the Barcelona neighbourhoods, namely the Collserola tower [10]. The data acquired using Ku-band real-aperture interferometric radar, with different observation geometries, are analysed to retrieve vibration characteristics of the structure and to estimate the tension force of the guys. The first experimental test was performed in 2008 but main data here analysed are from the following campaigns, carried out in 2012 and 2013. The paper is organized in five sections. This brief introduction is followed by a short outline of the technique and the used radar sensor, and a description of the monitored structure. A selection of the data acquired in the last years is analysed Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ymssp Mechanical Systems and Signal Processing http://dx.doi.org/10.1016/j.ymssp.2014.04.019 0888-3270/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel. þ34 936 452 900. E-mail addresses: gluzi@cttc.cat, guido.luzi@gmail.com (G. Luzi). Mechanical Systems and Signal Processing 49 (2014) 234–248