# Obstacle Avoidance Drone Control Dr. Norman Thurston, one of the first to use the word “UAV” to refer to an unmanned aerial vehicle (or UAV), laid out a simple but powerful way of measuring the obstacle avoidance drone faced by a drone: four-way intersections. He made this claim in 1971 and later clarified it in an article published in the Journal of Unmanned Aerial Vehicles and Spaceflight.

Quadrant Intersections are four-point intersections. You can think of them as little circles, but with a diameter of 1 mile. In a quadrant intersection, the quadrant intersection would be the point where the circle intersects the quadrant line or the circle that divides the quadrant into two intersecting halves.

An obstacle avoidance drone occurs when the quadrant intersection is not entirely within the circle’s perimeter. The quadrant intersection can be very close to the perimeter line or too far away to prevent an obstacle from passing between the intersection point and the circle’s inner perimeter.

As seen from the above image, quadrant intersections are very low in size; most people cannot see them on the ground. Drone pilots can usually see them from above or hover over them in the air.

## Computer Graphics

In order to determine the proximity of a quadrant intersection to the circle’s perimeter. We can use computer graphics or a laser sensor. On the ground, an obstacle may appear as a little dot in the center of the quadrant line. If we track that dot along the quadrant line, we will know the distance that the quadrant intersection is from the circle’s perimeter.

The problem is that obstacle avoidance is not the same on the ground as it is in the air. A quadrant intersection near the circle’s perimeter may not pose a problem for a drone flying above it. Since the drone can maneuver away from the intersection and fly up. In contrast, a quadrant intersection on the ground will create an obstacle avoidance problem if the drone flies near the intersection.

However, the quadrant intersection near the circle’s perimeter will pose an obstacle avoidance problem for a drone flying in the air. It can’t move up to avoid the intersection, because it can’t see the boundary. At a distance of one mile, the speed of sound is about 3,800 miles per hour.

## Know The Basics

The distance from the circle’s outer perimeter to the point where the quadrant intersection meets the circle’s inner perimeter is also called the obstacle avoidance distance. This distance is computed as follows. Take the speed of the quadrant intersection at that point as given by Dr. Thurston: