# Small Multicopter-UAV-Based Radar Imaging: Performance Assessment for a Single Flight Track

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## Abstract

**:**

## 1. Introduction

## 2. Imaging System

- Small M-UAV platform: DJI F550 hexacopter able to fly at very low speeds (about 1 m/s), thus ensuring a small spatial sampling step and the ability to take-off and land from a very small area;
- Radar system: Pulson P440 radar is a light and compact time-domain device transmitting ultra-wideband pulses (about 1.7 GHz bandwidth centered at the carrier frequency of 3.95 GHz) with a low power consumption [25]. The radar system is mounted rigidly on the UAV body (strapdown installation) and no gimbal is adopted. The limited altitude dynamics experienced during flights (very low ground speed and wind speed conditions resulting in small and almost constant roll/pitch angles), the relatively large radar antenna lobes and the limited baseline between the radar antenna and the drone center of mass are such that altitude/pointing knowledge does not play a significant role;
- GPS receivers/antennas: two single-frequency Ublox LEA-6T devices are chosen, one mounted onboard the UAV and the other one used as a ground-based station. Both are connected to an active patch antenna. The antenna is directly placed on the ground (Figure 1b) in order to get from CDGPS a direct estimate of the height above ground for the antenna mounted on the drone;
- CPU controller: Linux-based Odroid XU4 is devoted to managing the data acquisition for both radar system and onboard GPS receiver, while assuring their time synchronization.

## 3. Radar Signal Processing

- Zero-timing;
- Background removal;
- Time-gating.

#### 3.1. Radar Imaging Approach

#### 3.2. Resolution Analysis

## 4. Experimental Results

## 5. Discussion

## 6. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**M-UAV radar imaging system: (

**a**) hexacopter with onboard equipment; (

**b**) ground-based GPS station.

**Figure 5.**Radar imaging with an ideal rectilinear flight path: (

**a**) 3D view; (

**b**) view in the y–z plane.

**Figure 6.**Contour plot of the across-track resolution ${\Delta}_{y}$ versus $d$ and $h$, expressed in meters, in the case of a rectilinear flight trajectory.

**Figure 7.**Point spread function (PSF) amplitude for a flight altitude $h=5\text{}m$: ideal rectilinear trajectory and point like target with offset: (

**a**) $d=0$ m, (

**b**) $d=2$ m. Curved trajectory as described by Equation (13) and point-like target located with offset: (

**c**,

**e**) $d=0$ m, (

**d**,

**f**) $d=2$ m. The white dashed line represents the trajectory and the white circle denotes the target.

**Figure 8.**PSF amplitude for $h=10$ m and a curved trajectory: (

**a**) point like target at $d=0$; (

**b**) point like target at $d=2$. The white dashed line shows the trajectory; the white circle denotes the target.

**Figure 10.**Raw radargrams: (

**a**) Track 1; (

**b**) Track 2. The white dotted line represents the variable UAV flight altitude h estimated by the Carrier-Phase Differential GPS (CDGPS) and transformed into the equivalent travel time by: ${t}_{h}=2h/{c}_{0}$.

**Figure 11.**Filtered radargrams: (

**a**) Track 1; (b) Track 2. The white dotted line represents the variable UAV flight altitude $h$ estimated by the CDGPS and transformed in the equivalent travel time by: ${t}_{h}=2h/{c}_{0}$.

**Figure 12.**Focused image of the scenario under test: (

**a**) Track 1; (

**b**) Track 2. The dotted white line represents the flight path as projected onto the investigated domain.

**Figure 13.**Reconstruction of a point target on image planes at different elevations. The true target (black triangle) is illuminated at the radar nadir and its reconstruction is represented by the red rectangles.

**Figure 14.**Reconstruction results in a three-target scenario (

**a**) image plane at z = 0 m; (

**b**) image plane at z = 0.2 m; (

**c**) image plane at z = 0.4 m.

Along-Track Resolution (m) | Across-Track Resolution (m) | |
---|---|---|

Rectilinear path, target offset $d=0$ m, flight altitude $h=5$ m | 0.04 | 0.95 |

Rectilinear path, target offset $d=2$ m, flight altitude $h=5$ m | 0.04 | 0.25 |

Path $y=0.15\mathrm{cos}\left(\frac{\pi x}{12}\right)$, target offset $d=0$ m, flight altitude $h=5$ m | 0.04 | 0.95 |

Path $y=0.15\mathrm{cos}\left(\frac{\pi x}{12}\right)$, target offset $d=2$ m, flight altitude $h=5$ m | 0.04 | 0.25 |

Path $y=-0.15\mathrm{cos}\left(\frac{\pi x}{12}\right)$, target offset $d=0$ m, flight altitude $h=5$ m | 0.04 | 0.95 |

Path $y=-0.15\mathrm{cos}\left(\frac{\pi x}{12}\right)$, target offset $d=2$ m, flight altitude $h=5$ m | 0.04 | 0.25 |

Path $y=0.15\mathrm{cos}\left(\frac{\pi x}{12}\right)$, target offset $d=0$ m, flight altitude $h=10$ m | 0.07 | 1.30 |

Path $y=0.15\mathrm{cos}\left(\frac{\pi x}{12}\right)$, target offset $d=2$ m, flight altitude $h=10$ m | 0.07 | 0.47 |

Parameters | Specification |
---|---|

Carrier Frequency | 3.95 GHz |

Frequency Band | 1.7 GHz |

Maximum Emitted Power | –13 dBm |

Maximum Dynamic Range | 75 dB |

Pulse Repetition Frequency | 14.28 Hz |

Received Signal Sampling Frequency | 16 GHz |

Errors (cm) | x | y | z |
---|---|---|---|

Track 1 | 4.3 | 4.6 | 9.4 |

Track 2 | 0.6 | 0.8 | 1.5 |

Experimental Along-Track Resolution (m) | Theoretical Along-Track Resolution (m) | Experimental Across-Track Resolution (m) | Theoretical Across-Track Resolution (m) | ||
---|---|---|---|---|---|

Track 1^{1} | Target 1 | 0.05 | 0.02 | 1.15 | 0.85 |

Target 2 | 0.04 | 0.02 | 0.99 | 0.85 | |

Track 2^{2} | Target 1 | 0.07 | 0.03 | 0.51 | 0.41 |

Target 2 | 0.09 | 0.03 | 0.50 | 0.41 |

^{1}$h=4m,\text{}d=0m$;

^{2}$h=10m,\text{}d=2m$

True Target Position | Retrieved Target Positions Versus Height of Image Plane | |||
---|---|---|---|---|

z = 0 m | z = 0.2 m | z = 0.4 m | ||

T1 | (−2, 0, 0) m | (−2, 0, 0) m | (−2, ±1.4, 0.2) | (−2, ±1.99, 0.4) m |

T2 | (0, 0, 0.2) m | - | (0, 0, 0.2) | (0, ±1.4, 0.4) m |

T3 | (2, 0, 0.4) m | - | - | (2, 0, 0.4) m |

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## Share and Cite

**MDPI and ACS Style**

Catapano, I.; Gennarelli, G.; Ludeno, G.; Noviello, C.; Esposito, G.; Renga, A.; Fasano, G.; Soldovieri, F.
Small Multicopter-UAV-Based Radar Imaging: Performance Assessment for a Single Flight Track. *Remote Sens.* **2020**, *12*, 774.
https://doi.org/10.3390/rs12050774

**AMA Style**

Catapano I, Gennarelli G, Ludeno G, Noviello C, Esposito G, Renga A, Fasano G, Soldovieri F.
Small Multicopter-UAV-Based Radar Imaging: Performance Assessment for a Single Flight Track. *Remote Sensing*. 2020; 12(5):774.
https://doi.org/10.3390/rs12050774

**Chicago/Turabian Style**

Catapano, Ilaria, Gianluca Gennarelli, Giovanni Ludeno, Carlo Noviello, Giuseppe Esposito, Alfredo Renga, Giancarmine Fasano, and Francesco Soldovieri.
2020. "Small Multicopter-UAV-Based Radar Imaging: Performance Assessment for a Single Flight Track" *Remote Sensing* 12, no. 5: 774.
https://doi.org/10.3390/rs12050774