# Small-UAV Radar Imaging System Performance with GPS and CDGPS Based Motion Compensation
## 1 Introduction  
Radar imaging by Unmanned Aerial Vehicle (UAV) platforms [1; 2] is worth attention in the remote sensing community as a cost-effective solution to cover wide and/or not easily accessible regions, with high operative flexibility [3]. Indeed, Multicopter-UAVs (M-UAV) have vertical lift capability, allow take-off and landing from very small areas, without the need of long runways or dedicated launch and recovery systems, and, they are able to hover and move in any directions. These peculiar features allow their use at any places and under different flight modes, thus introducing new attractive possibilities in radar remote sensing [4]. UAV based radar imaging can be used in several civil [4] and military applications [5], such as surveillance, security, diagnostics, monitoring in civil engineering, cultural heritage and earth observation, with particular emphasis on natural disasters [6]. At the state of the art, M-UAV radar imaging has been proposed in biomass mapping [7], glaciology [8] and for precision farming [9]. In addition, M-UAVs have been also exploited to perform Synthetic Aperture Radar (SAR), for monitoring small areas and avoiding large platforms. In this context, a first experimentation concerning interferometric P and X band SAR sensors on board a UAV platform has been reportedin [10]; moreover, a novel UAV polarimetric SAR system [11] and a multiband drone-borne SAR system [12] have been recently proposed. UAVs have been also exploited in the frame of subsoil exploration by means of Ground Penetrating Radar (GPR) for mine detection [13; 14; 15; 16], soil moisture mapping [17], and detecting subsurface Improvised Explosive Devices (IEDs) [18]. In all of these promising examples, different solutions have been devised to estimate the UAV flight trajectory. For instance, in [7] the ultra-wideband radar PulsON 410 (3.1-5.3 GHz) is used and altitude variations during the flight are corrected by exploiting the reflections from the ground. In [15], a wideband Frequency-Modulated Continuous-Wave (FMCW) GPR, working in the frequency range from 1 GHz to 4 GHz and in a bistatic configuration is proposed; the flight is controlled by manual piloting and a Light Detection and Ranging (LIDAR) is used as the only positioning device for measuring the flight altitude above ground. A standalone Global Positioning System (GPS) is used in [17] for positioning a stepped-frequency continuous-wave (SFCW) radar working in the range from 1 MHz to 6 GHz. Conversely, a sophisticated positioning system is exploited in [14; 18] to perform SAR imaging. In [14], a compact pulse radar working in the 3.1 to 5.1 GHz frequency band is considered and a Real Time Kinematic (RTK) system as well as a LIDAR altimeter are used to achieve the cm-level accuracy positioning. In [18], high-resolution 3D SAR imaging is carried out by means of an M-sequence Ultra-Wide-Band (UWB) radar covering a frequency range from 100 MHz to 6 GHz. This radar system is mounted on a UAV, which autonomously flies over a region of interest. The flight is controlled by means of the UAV flight controller, an Inertial Measurement Unit (IMU), a barometer, and a Global Navigation Satellite System (GNSS) receiver. In addition, a laser rangefinder and a dual-band RTK system are exploited to enhance the accuracy of positioning data. According to the survey just presented, general considerations are:  
(i) Accurate UAV positioning data improve the radar imaging performances by avoiding defocusing and localization errors [19];  
(ii) Accurate knowledge of the UAV flight trajectory depends strongly on the quality of both the embarked navigation sensors and the deployed ground-based aids [20]. Furthermore, small M-UAVs are constrained by the maximum payload mass and by cost considerations, thus, limiting number and typology of on-board devices. This work deals with the impact of standalone GPS and Carrier based Differential Global Positioning System (CDGPS) positioning accuracy on small M-UAV radar imaging performed in sounder modality.