The present invention relates generally to laser ranging devices and methods of calculating target ranging information and, more particularly, to tilt compensated laser ranging devices and methods of calculating target ranging information accounting for the vertical angle to a target.
Portable laser range finders are frequently used to determine the range from the user to a selected target. In this disclosure the term target is defined simply as the object for which ranging information is desired. A target may be simply a reference point on some structure, a topological feature, a building or a survey point. The target may also be the desired impact point of a ballistic projectile such as an arrow, a bullet, or even a golf ball. In determining ranging information regarding this second type of target, sportsmen, marksmen and others frequently use laser range finders. These laser range finders are typically handheld or are mounted on a tripod or other portable device. Current laser range finders measure the time-of-flight distance, otherwise know as the line-of-sight distance, between the range finder and the target. The time-of-flight distance is calculated by such laser range finders by first measuring the elapsed time between the firing of a laser pulse at the target and the detection of the reflected pulse, then multiplying that elapsed time by the appropriate value of the speed of light.
However, when used to provide ranging information for launching a ballistic projectile, all current laser range finders on the market have a common flaw. They do not give the user the correct aiming parameters for making uphill or downhill shots.
FIGS. 1a and 1b illustrate the ballistic projectile problem which this invention solves. For purposes of illustration, assume that the user is an archer and that an arrow is the projectile and a bow is the launcher. In FIG. 1a, the desired target 3 is on the same horizontal plane as the user 1. Or, as alternately expressed, the user 1 and target 3 are on the same vertical level. The laser range finder 2 provides the time-of-flight range 4 to the target 3, which is also the horizontal range 5 in this example. The user corrects the aim point of the launcher based on the horizontal range 5 and the ballistic equation. The arrow 6 follows the ballistic path 7.
It is a characteristic of the ballistic equation in a gravitational field that the distance a projectile is displaced in the downward vertical direction is dependent solely on the time of flight. Assuming that a projectile maintains a constant horizontal speed, it is the horizontal range to the target that determines how much the projectile drops when arriving at the target's horizontal range. This characteristic can present a problem to the user if the target is not in the same horizontal plane, but is displaced by some vertical distance as well as horizontal distance. Current laser range finders will provide only the time-of-flight distance to the target.
Referring now to the archery illustration in FIG. 1b, target 2a is at a higher vertical level than the user 1, while target 2b is at a lower vertical level. If only a current laser range finder 3 is available, the user 1 can only determine time-of-flight ranges 4a and 4b. If the user 1 corrects the aiming point based on these ranges, the result is an over shoot of the arrow 6 along ballistic paths 7a and 7b. However, the horizontal ranges 5a and 5b can be determined by a measurement of the angle at which of the line-of-sight direction to the target from the user intersects the user's horizontal plane. This is defined as the vertical angle. Since the time-of-flight ranges 4a and 4b are already known, the geometry of right triangles determines the horizontal ranges 5a and 5b. If the user 1 corrects the aiming point based on these horizontal ranges, the result is a target intersecting flight of the arrow 6 along corrected ballistic paths 9a and 9b. 
Users employing portable laser range finders must currently make manual, in-field determination of the vertical angle of the target and of the horizontal range. There is currently a need for a portable laser range finder that measures time-of-flight range and vertical angle, and determines the horizontal range. Such a system should be relatively simple, inexpensive, reliable, easy to manufacture, relatively quick, and very stable in a variety of applications.