Blas Pablo Dorta-Naranjo

This paper presents a novel design for precise vertical landing of drones based on the detection of three phase shifts in the range of ±180°. The design has three inputs to which the signal transmitted from an oscillator located at the landing point arrives with different delays. The circuit increases the aerial tracking volume relative to that achieved by detectors with theoretical unambiguous detection ranges of ±90°. The phase shift measurement circuit uses an analog phase detector (mixer), detecting a maximum range of ±90°and a double multiplication of the input signals, in phase and phase-shifted, without the need to fulfill the quadrature condition. The calibration procedure, phase detector curve modeling, and calculation of the input signal phase shift are significantly simplified by the use of an automatic gain control on each branch, dwhich keeps input amplitudes to the analog phase detectors constant. A simple program to determine phase shifts and guidance instructions is proposed, which could be integrated into the same flight control platform, thus avoiding the need to add additional processing components. A prototype has been manufactured in C band to explain the details of the procedure design. The circuit uses commercial circuits and microstrip technology, avoiding the crossing of lines by means of switches, which allows the design topology to be extrapolated to much higher frequencies. Calibration and measurements at 5.3 GHz show a dynamic range greater than 50 dB and a non-ambiguous detection range of ±180°. These specifications would allow one to track the drone during the landing maneuver in an inverted cone formed by a surface with an 11 m radius at 10 m high and the landing point, when 4 cm between RF inputs is considered. The errors of the phase shifts used in the landing maneuver are less than ±3°, which translates into 1.7% losses over the detector theoretical range in the worst case. The circuit has a frequency bandwidth of 4.8 GHz to 5.6 GHz, considering a 3 dB variation in the input power when the AGC is limiting the output signal to 0 dBm at the circuit reference point of each branch. In addition, the evolution of phases in the landing maneuver is shown by means of a small simulation program in which the drone trajectory is inside and outside the tracking range of ±180°.

This paper presents a 360º phase detector in the 2-18 GHz frequency band (160% relative bandwidth) that uses a fixed delay line as a phase shifter and a double multiplication, in-phase and out-of-phase, between the input signals. Initially, several classic 90º power splitter designs are analyzed under the new perspective that they can be used when integrated in this type of topology, where it is not necessary to comply with the quadrature condition. Simulations show increases of up to 600% of the relative bandwidth, going from a typical value of 20% (9-11 GHz) in branchline coupler structures, to 140% (3-17 GHz). It is slightly lower in those that use a Schiffman phase shifter (325%), where it goes from 40% (8-12 GHz) to 170% (1.5-18.5 GHz). This is followed by the design of the phase detector, with the main innovation being the use of a fixed delay line that can easily be implemented in other frequencies and technologies, and which presents a theoretical phase shift of 90±72º between 2 GHz and 18 GHz. The prototype is intended to show the possibilities of the design and the conceptual idea. In order to be part of a measurement array, the circuit has a cell-type topology. It features four switches that prevent the use of power dividers and line crossover, which facilitates its implementation in other frequencies and technologies, but increases the measurement time. Phase errors derived from the measurement system and amplitude noise at the multiplier output are analyzed. The measurement system uses a procedure based on quasi-synchronized generators, which guarantees a measurement error of less than ±3º. The manufactured prototype shows a deviation of quadrature of β=±72º in the 3 GHz to 18 GHz range, but at the 2 GHz frequency it displays a value of β=73º which considerably increases the phase errors compared to noise in the output voltages of the multiplier. The errors stay below ±5º in the 4 to 18 GHz band when considering an amplitude variation of ±7.5 mV (±1.5 ADC voltage resolution) in the output voltages of the multiplier, yet they increase to ±7º y [+6.5º -10.5º] at 3 GHz and at 2 GHz, respectively. The error decreases below ±1.5º between 4 and 17 GHz for a variation of ±2.5 mV (±0.5 ADC voltage resolution), increasing to ±2.3º at 18 GHz, ±3.3º at 3 GHz and ±4.6º at 2 GHz.

This article presents an amplitude and 360° phase shift array measurement system. The basic cell of the measurement system uses a novel amplitude and phase detector based on switched dual multipliers. The phase shift measurement is characterized using an analog phase detector (mixer), detecting a maximum range of ±90°, and a double multiplication of the input signals, in phase and phase shifted. This method broadens the frequency and amplitude range beyond other solutions that require fulfilling the quadrature condition. This method broadens the frequency and amplitude beyond other solutions requiring fulfilling the quadrature condition or phase and amplitude balance. Thus, it enables to compensate significant phase imbalance in the 90° hybrid or use amplitudes out of the range that ensures the switching operation of mixer diodes. The circuit calibration that allows compensation for errors (amplitude, phase shift, mismatching, etc.) is detailed, and its relation to the required measurement accuracy is discussed. The design can be easily extrapolated to other frequency ranges because it uses commercial RF devices available in a wide frequency range and avoids the need of crossing lines or complex 90° hybrid. A prototype with $3 \times 3$ cells has been built to evaluate various test conditions on $1 \times 3$ cell configurations that show the advantages of the procedure. It should be highlighted that the cell prototype uses devices that will be operating outside the frequency and amplitude ranges recommended by their manufacturers. A calibration from 2.6 to 6 GHz and −15 to −3 dBm was performed to evaluate the measurement errors. An analysis of the isolation between cells and different calibration configurations is performed to analyze the measurement errors. Measurements show compensation of +30°/−25° phase imbalance and 13-dB power lower than mixer manufacturer recommendation.

The use of drones or UAVs (unmanned aerial vehicles) for commercial or leisure purposes is increasingly common in our society. However, UAVs can also be used for other purposes that may compromise the privacy and safety of the population. For this reason, it has become necessary to enable the monitoring and identification of UAVs through a radio system with a license to avoid unwanted interferences. This article proposes the AIS (Automatic Identification System) as an alternative to monitor drone activities in near-real time. To do this, a prototype has been designed and integrated in an amateur UAV unit, enabling the transmission of UAV parameters through this system. AIS density testing has been carried out in selected zones of Gran Canaria (Spain), demonstrating the compatibility between the canonical usage of the AIS for vessels and its new proposed purpose for amateur UAVs.

Thermographic imaging has been the preferred technology for the detection and tracking of wildfires for many years. Thermographic cameras provide some very important advantages, such as the ability to remotely detect hotspots which could potentially turn into wildfires if the appropriate conditions are met. Also, they can serve as a key preventive method, especially when the 30-30-30 rule is met, which describes a situation where the ambient temperature is higher than 30 ∘ C, the relative humidity is lower than 30%, and the wind speed is higher than 30 km/h. Under these circumstances, the likelihood of a wildfire outburst is quite high, and its effects can be catastrophic due to the high-speed winds and dry conditions. If this sort of scenario actually occurs, every possible technological advantage shall be used by firefighting teams to enable the rapid and efficient coordination of their response teams and to control the wildfire following a safe and well-planned strategy. However, most of the early detection methods for wildfires, such as the aforementioned thermographic cameras, lack a sufficient level of automation and usually rely on human interaction, imposing high degrees of subjectivity and latency. This is especially critical when a high volume of data is required in real time to correctly support decision-making scenarios during the wildfire suppression tasks. The present paper addresses this situation by analyzing the challenges faced by a fully autonomous wildfire detection and a tracking system containing a fully automated wildfire georeferencing system based on synthetic vision technology. Such a tool would provide firefighting teams with a solution capable of continuously surveilling a particular area and completely autonomously identifying and providing georeferenced information on current or potential wildfires in real time.

Underwater Wireless Sensor Networks (UWSNs) using electromagnetic (EM) technology in marine shallow waters are examined, not just for environmental monitoring but for further interesting applications. Particularly, the use of EM waves is reconsidered in shallow waters due to the benefits offered in this context, where acoustic and optical technologies have serious disadvantages. Sea water scenario is a harsh environment for radiocommunications, and there is no standard model for the underwater EM channel. The high conductivity of sea water, the effect of seabed and the surface make the behaviour of the channel hard to predict. This justifies the need of link characterization as the first step to approach the development of EM underwater sensor networks. To obtain a reliable link model, measurements and simulations are required. The measuring setup for this purpose is explained and described, as well as the procedures used. Several antennas have been designed and tested in low frequency bands. Agreement between attenuation measurements and simulations at different distances was analysed and made possible the validation of simulation setups and the design of different communications layers of the system. This leads to the second step of this work, where data and routing protocols for the sensor network are examined.

The authors present full wave simulations and experimental results of propagation of electromagnetic waves in shallow seawaters. Transmitter and receiver antennas are ten-turns loops placed on the seabed. Some propagation frameworks are presented and simulated. Finally, simulation results are compared with experimental ones.

Shallow seawaters are problematic for acoustic and optical communications. Sensor networks based on electromagnetic (EM) communications are evaluated in this environment. In order to characterize the underwater channel, several measurement systems have been designed, built and tested in the sea obtaining very reliable results. Experiments carried out with dipoles and loop antennas showed serious disagreement with the state of the art, especially when dipole antennas are used. Dipoles performance was poor while magnetic loops showed much lower attenuation. A measurement system is described in detail and real attenuation of the underwater channel is obtained for several distances and antennas. Finally, measured and simulated results are compared and reveal good agreement.

Submillimeter-wave imaging radars for standoff detection require broadband and fast-switch signal-generation architectures in order to achieve a high-range-resolution and near-video-rate imaging system. The signal generation has a strong influence on the radar image quality, primarily due to the transmitted phase noise. Two direct digital synthesis-based architectures for continuous-wave linear-frequency-modulated signal generation have been fabricated and tested for a high-range-resolution imaging radar at 300 GHz for standoff sensing of person-borne concealed threats. In order to select the signal-generation architecture that ensures the proper operation of the imaging radar with a cost-saving objective, three figures of merit have been used: 1) radar image quality; 2) power consumption; and 3) cost. The impact of the signal-generation architectures on the imaging radar indicates that both architectures present a similar radar performance in terms of radar image quality, although the narrowband direct-digital-synthesis/phase-locked loop scheme is a cost-effective solution compared with the broadband direct-digital-synthesis scheme in order to develop an affordable and energy-efficient high-performance preindustrial radar prototype.