# C Band 360\({}^{\circ}\) Triangular Phase Shift Detector for Precise Vertical Landing RF System
Recently, a solution based on triangular phase shift detection [25] has been proposed, which significantly reduces the computations needed to obtain the correction instructions in the landing maneuver and increases the ODR to values provided by the drone stabilization systems. The radio frequency (RF) circuit allows a control based on the detection of the phase shift of three input signals coming from an oscillator located at the landing point. The system is capable of tracking in an inverted conical volume, where the vertex would be the LP, and the tracking area increases with height and decreases as frequency rises. The phase detection is performed with the AD8302 circuit, which has an integrated signal compression system, so that the analog detector or multiplier does not vary the output voltage as a function of the input voltage. However, the tracking area determined by the unambiguous phase detection range is less than the theoretical value of \(\pm\)90\({}^{\circ}\). The reduction in the detection range to \(\pm\)80\({}^{\circ}\) is mainly due to mismatching and unbalancing of the RF branches of the circuit. In addition, the proposed circuit is designed at 2.5 GHz, which implies a relatively large antenna array size [27] and lacks an interference protection system [28]. Regarding phase detectors, the digitals and those that combine double multiplication are appropriate to increase the detection range and frequency. The digital ones can cover detection ranges greater than \(\pm\)180\({}^{\circ}\) and are immune to amplitude variations in the input signal. However, the limited operating frequency of about 10 GHz is their main drawback [29; 30; 31], as are FPGA-based phase detectors below 1 GHz [32; 33]. Another more complex measurement method, limited in frequency and occasionally used, is based on the phase variation of a reference oscillator and subtraction with each of the input signals to locate a null [34]. However, the solution requires time to sweep all phase values, so it is used when the phase does not vary rapidly. There are other solutions with small detection range that sum the input signals (0 V output voltage when phase shift is 180\({}^{\circ}\)) and obtain good resolutions when the input phase shift varies \(\pm 60^{\circ}\) over 180\({}^{\circ}\) of phase shift reference [35]. To change the reference value, phase shifters are introduced in one of the branches [36]. Thus, with a 180\({}^{\circ}\) phase shifter, it would operate around 0\({}^{\circ}\). The phase detection range based on the analog phase detector can be extended to 360\({}^{\circ}\) by a double product in phase (I) and quadrature (Q) of the two input signals [37; 38]. These circuits are integrated up to 100 GHz [39; 40], but in all cases, the quadrature condition must be met, which also requires good balancing between the branches that carry the signals from the antennas (RF input) to the integrated circuit. A variant of these circuits is the phase detector with double multiplication, in phase and phase-shifted, which avoids the need to fulfill the quadrature condition [41]. This facilitates circuit design in any frequency band where multipliers (mixers) are available. However, the calibration and modeling of these detectors can become very complex, especially when the input amplitude can vary and be different at each of the input RF ports [42]. The phase shift is obtained from an interpolation process using two multidimensional matrices of calibrated data. Accuracy increases with the number of points contained in the matrices, but a powerful data processing unit is required to reduce the computing time. If the frequency increases, the synchronization requirements of the generators used to characterize the phase detector further complicate the calibration and measurement process [43]. Nevertheless, these processes are simplified if the input powers do not vary and the bandwidth is very narrow [41]. A more detailed comparative analysis of RF phase detectors can be found in [43].