## 1 Introduction  
The application of plant protection products by means of sprayers is the most widely used procedure for the protection of agricultural crops against pests and diseases. Spray drift is defined by the standard ISO 22866:2005 as the quantity of plant protection product that is carried out of the sprayed (treated) area by the action of the air currents during the application process. Most airborne spray drift measurements carried out today are made using collectors and tracers. The use of this type of methodology is costly and time-consuming. Furthermore, the information on the pesticide cloud is not time resolved, volume imaging of the cloud is not possible and collector efficiency is influenced by meteorological conditions [1]. In addition, because of the extensive variety of crop and meteorological conditions it is difficult to make an accurate assessment of the real spray drift hazard associated with each application technique. As a result, there has been a growing interest in the search for alternative methods which can be used either in the laboratory, with wind tunnels, or in the field. The use of optical systems like lidar (light detection and ranging) has emerged as one of the most feasible options. In the last 25 years lidar systems have been applied in several spray drift studies, some of the most relevant are cited below. The Atmospheric Environment Service of Canada constructed a Nd:YAG elastic-backscatter lidar (1064 nm, 50 mJ) to obtain near-real-time maps of the pesticide plumes in aerial applications [2; 3; 4]. This instrument allows to study the dynamics of the pesticides and the influence over them of aircraft wing-tip vortices. A more powerful Nd:YAG lidar system (1064 nm, 125 mJ), developed by researchers from the University of Connecticut, has been used to validate theoretical spray-drift models [5], to assess the influence of atmospheric stability over spray drift movement [6], to generate tri-dimensional images of the spray drift plume over an orange orchard [7] and to estimate the concentration of the pesticide cloud from the backscatter lidar signal [8; 9]. In [10], researchers from the University of Washington at Seattle monitored the pesticide plume over an apple orchard using an UV lidar (355 nm, 8 mJ) and compare these measurements with a spray simulation model. In a recent study [1], the authors of the present paper have obtained a strong relationship between spray drift measurements using simultaneously an UV lidar (355 nm, 16 mJ) and passive collectors. These studies show that lidar systems allow real-time monitoring of airborne spray drift, obtaining range-resolved images of the spray plume while requiring fewer personnel and consuming less time. However, despite the advantages of lidar systems for airborne spray drift monitoring, they have been used in only a limited way to date. This is because currently available lidar systems inherited their design architecture from atmospheric monitoring applications (high energy, low pulse-repetition-frequency systems), which make them expensive and require trained personnel for their operation [11]. In addition, many of these instruments are not eye-safe, preventing their practical application particularly in ground spray drift studies. In order to overcome previous limitations, this work presents the development of a new lidar system specifically suited for pesticide spray drift monitoring. Design specifications which this instrument needs to satisfy are:  
* Compactness. An easily transportable instrument suitable for field work is required. * Near-field and distance resolution measurement capabilities. Drift plumes generated by ground sprayers have relatively low dimensions, commonly just a few metres thick. For appropriate characterisation, the lidar system must have a high range resolution, ideally not greater than 3 m. A maximum reach of 500 m is sufficient. * Temporal resolution capability. Pesticide plumes are highly dynamic, with rapid variations in their shape and concentration.