Renewable Energy 33 (2008) 13–21 In-service testing of wind turbine towers using a microwave sensor Massimiliano Pieraccini a,Ã , Filippo Parrini a , Matteo Fratini a , Carlo Atzeni a , Paolo Spinelli b a Department of Electronics and Telecommunications, University of Florence, via Santa Marta 3, 50139 Florence, Italy b Department of Civil Engineering, University of Florence, via Santa Marta 3, 50139 Florence, Italy Received 29 November 2006; accepted 28 February 2007 Available online 20 April 2007 Abstract A remote-sensing technique based on microwave interferometry is applied to dynamic testing of wind turbine towers for power generation. A high-speed interferometric radar is able to sample the structure at a rate high enough for modal and transient analysis. An experimental campaign is reported carried out on wind turbine towers of a power plant in north Sardinia, Italy. r 2007 Elsevier Ltd. All rights reserved. Keywords: Wind turbine; Remote sensing; Radar; Interferometry; Dynamic monitoring; Structural engineering 1. Introduction The dynamic behavior of large structures is of para- mount interest in structural engineering practice. Dynamic testing is currently implemented by networks of acceler- ometers installed on the structure. Such sensors are accurate and reliable, but need to be positioned in contact with the surveyed structure, and often their installation requires long interruptions of the service. If the structure is part of an industrial plant continuously operated, service interruption can cause heavy economic losses: this is the case of the wind turbine towers for power generation. The number of wind turbine towers installed worldwide is over several tens of thousands and is growing at rates exceeding 39% annually [1,2]. Although all these structures are tested soon after installation, they need a periodical monitoring. Engineering practice suggests the key requirements for this kind of in-field testing are: (1) in-service operation, (2) use of monitoring equipments easy and fast to operate, (3) direct measurement of the deflection (i.e., absolute displacement) of the structure under test. In particular, the latter is an open challenge [3] for the current state of art. Accelerometers can provide the displacement measure only after a double integration in time, and traditional transducers can only be used for relative displacement measurement. Laser sensors [4] have been proven often unpractical for in-field applications. A proposed solution is to use a global positioning system (GPS), but its accuracy is not good enough to meet the strict measurement requirements. Recently, the authors designed and realized an inter- ferometric radar able to image a scenario with a sampling rate high enough to track the movements of great architectural structures [5]. The technique has been tested on a variety of structures, such as bridges [5,6] and a historical masonry tower [7]. In this paper, the application of the novel interferometric technique to in-field dynamic control of in-service wind turbine towers of a large power farm in Sardinia (Italy) is described. Furthermore, a specific operation mode able to test several wind towers through a single acquisition is reported. 2. Working principle and radar equipment The equipment employed is basically a continuous-wave step-frequency radar [5–7]. It transmits, step by step, continuous waves at discrete frequency values, sampling a bandwidth B at a constant interval Df. Range resolution DR is determined by the bandwidth B of the transmitted signal through the basic relationship DR ¼ c 2B , (1) ARTICLE IN PRESS www.elsevier.com/locate/renene 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2007.02.001 Ã Corresponding author. E-mail address: massimiliano.pieraccini@unifi.it (M. Pieraccini).