  This paper presents a circuit for precise vertical landing of drones based on
a three phase-shifts detection of a single frequency transmitted from the
landing point. The circuit can be considered as a new navigation sensor that
assists in guidance corrections for landing at a specific point. The circuit
has three inputs to which the signal transmitted from an oscillator located at
the landing point arrives with different delays. The input signals are combined
in pairs in each of the three analog phase detectors, after having passed
through 3 dB@90 o hybrid couplers that guarantee a theoretical non-ambiguous
phase-shift range of +-90 degree. Each output has a voltage that is
proportional to the phase-shift between each of the input signals, which in
turn depend on the position relative to the landing point. A simple landing
algorithm based on phase-shift values is proposed, which could be integrated
into the same flight control platform, thus avoiding the need to add additional
processing components. To demonstrate the feasibility of the proposed design, a
triangular phase-shift detector prototype has been implemented using commercial
devices. Calibration and measurements at 2.46 GHz show a dynamic range of 30 dB
and a non-ambiguous detection range of +-80 degree in the worst cases. Those
specs let us to track the drone during the landing maneuver in an inverted cone
formed by a surface with a +-4.19 m radius at 10m high and the landing point.
