Method and apparatus for terrain avoidance

A method for detecting the presence of an orographic protrusion extending from the surface of the earth, comprising the steps of:

• locating at least two spaced points in the earth's atmosphere which lie along a line parallel to the nominal surface of the earth;
• sensing the horizontal potential component of the electrostatic field existing in the earth's atmosphere at each of the two spaced points;
• and,
• comparing the two sensed horizontal potential components, a difference in potential therebetween indicating the presence of an orographic protrusion extending from the surface of the earth, wherein the step of locating the two spaced points includes the step of defining an artificial horizontal plane in the earth's atmosphere, the line joining the two spaced points being parallel to said plane.

A method for detecting the attitude relative to an orographic protrusion extending from the surface of the earth of at least one axis of an airborne vehicle operating in the electrostatic field existing in the earth's atmosphere, the protrusion distorting the shape and position of equipotential surfaces in the field which normally are parallel to the nominal surface of the earth, the method comprising the steps of:

• defining an artificial horizontal plane in the earth's atmosphere which is parallel to the nominal surface of the earth;
• aligning the axis of the airborne vehicle with said defined plane;
• sensing the horizontal potential component of the electrostatic field at each of two spaced points, the points lying along the axis of the airborne vehicle;
• and,
• generating a potential difference signal proportional to the magnitude of the difference in the two sensed horizontal potential components, the potential difference signal being indicative of misalignment between the axis of the airborne vehicle and the equipotential surfaces surrounding the orographic protrusion.

A method for operating an airborne vehicle to cause said vehicle to avoid orographic protrusions extending from the surface of the earth, comprising the steps of:

• defining an artificial horizontal plane;
• sensing the horizontal potential component of the electrostatic field existing in the earth's atmosphere at each of at least two spaced points effectively on the airborne vehicle, the spaced points effectively lying in the artificial horizontal plane;
• generating a potential difference signal proportional to the magnitude of the difference in the two sensed potential components;
• and,
• displacing the position of at least one of the points to cause the potential difference signal to become zero.

Apparatus for operating an airborne vehicle in the earth's atmosphere to cause said vehicle to avoid orographic protrusions extending from the surface of the earth comprising:

• at least two electrostatic potential sensing probes disposed on the airborne vehicle;
• means electrically connected to the probes for receiving the horizontal potential components of the electrostatic field sensed by said probes and for producing a first output signal proportional to the difference in the sensed components;
• means for effectively referencing the attitude of the airborne vehicle to an artificial horizon and for producing a second output signal proportional thereto;
• means for referencing the first output signal of the first mentioned means to the second output signal and for producing a third output signal indicative of the attitude of the vehicle with respect to the artificial horizon and of the difference in the horizontal potential components between the two probes;
• and,
• means for adjusting the attitude of the airborne vehicle relative to the surface of the earth in response to the third output signal.