###### Abstract  
The use of unmanned aerial vehicles (UAV) to provide services such as the Internet, goods delivery, and air taxis has become a reality in recent years. The use of these aircraft requires a secure communication between the control station and the UAV, which demands the characterization of the communication channel. This paper aims to present a measurement setup using an unmanned aircraft to acquire data for the characterization of the radio frequency channel in a propagation environment with particular vegetation (Caatinga) and a lake. This paper presents the following contributions: identification of the communication channel model that best describes the characteristics of communication; characterization of the effects of large-scale fading, such as path loss and log-normal shadowing; characterization of small-scale fading (multipath and Doppler); and estimation of the aircraft speed from the identified Doppler frequency. U 2021 22 2021 22  
solutions to mitigate such effects, especially in UAV communications, that directly affect the flight safety and distance range. In 2013, the United States of America created a special committee (SC-228), formed by the Technical Commission for Radio for Aeronautics (RTCA), whose objective was to define the minimum performance standards for UAV operation in regard to the command and control data link [3]. The first version of the standards document was published in [4]. One way to characterize the channel is to use first-order statistics such as the cumulative distribution function--CDF--and the probability density function--PDF--to describe the channel. However, an important feature in mobile communication channels is the presence of the Doppler effect, which can cause communication problems as this effect becomes stronger. The communication channels used by these aircraft are subject to this effect, due to the relative mobility between the aircraft and the ground station [5]. Therefore, it is important to characterize the Doppler effect and its impact as the aircraft moves. The characterization of the Doppler effect can be performed by second-order statistical methods, such as the level crossing rate--LCR--which corresponds to the frequency at which the signal strength crosses a certain threshold in the positive or negative direction [6; 7; 8]. More specifically, the communication between the aircraft and the ground station requires high reliability due to the challenges imposed by the environment, such as relief variation, vegetation, and urbanization. In Brazil, an important biome present in the northeast region is the Caatinga, which means \"white forest\" in the Tupi language. This region comprises an area of 850,000 km\({}^{2}\) covering 10 Brazilian states. It is a typical tropical vegetation with trees up to 8 m high and covers a population of more than 53,000,000 inhabitants, which makes it a significant economic area [9]. The use of UAVs in this region has grown over the years, and there is not much work related to the characterization of radio frequency channels in this environment. Another significant factor that influences the communication signal is the presence of lakes, as the surface of these places may reflect, scatter, or attenuate the electromagnetic signal. Many studies have been carried out to characterize the communication channel used by UAVs, as can be seen in Table 1. We notice that most of the works focus on large-scale fading statistics, such as the path loss exponent and shadowing, but only few of them characterize the communication channel in relation to the Doppler effect. This paper aims to identify the channel model that best describes the large and small-scale effects in the environments of the Caatinga and lakes. For this, we use the Phantom 3 Standard UAV equipped with an XBee module to transmit data on the frequency channel of 915 MHz.