Abstract: A method includes obtaining sensor data indicating a tension experienced by a tether while a payload coupling apparatus connected to the tether is lowered from an aerial vehicle using the tether. The method also includes determining, based on the sensor data, a ground contact time at which the payload coupling apparatus or a payload coupled thereto made initial contact with a ground surface. The method additionally includes determining a length of the tether released from the aerial vehicle at the ground contact time. The method further includes determining a tether-based altitude of the aerial vehicle based on the length of the tether released from the aerial vehicle at the ground contact time. The method yet further includes causing the aerial vehicle to perform an operation based on the tether-based altitude.

Abstract: A method includes, during a delivery process of an unmanned aerial vehicle (UAV), receiving, by an image processing system, a depth image captured by a downward-facing stereo camera on the UAV. One or more pixels are within a sample area of the depth image and are associated with corresponding depth values indicative of distances of one or more objects to the downward-facing stereo camera. The method also includes determining, by the image processing system an estimated depth value representative of depth values within the sample area. The method further includes determining that the estimated depth value is below a trigger depth.