Rough impacts like this one are prevented by Creare’s Height Sensor.

On Christmas Eve 2006, a stranded helicopter crew in the mountains close to the Pakistani border, caught in a snowstorm and in danger of freezing to death, received survival equipment, rations, and ammunition from a high altitude precision airdrop. Incidents like this one are proving the value of the Army’s efforts to develop and deploy precision airdrop systems. The Army is currently developing a suite of such systems that will be able to deliver loads ranging between 200 lbs and 30,000 lbs from heights up to 25,000 ft to ground targets with an accuracy that is better than 50 m. The systems use guided parafoils and rely on the global positioning system (GPS) for navigation.

As part of this effort, Creare has developed a height sensor that provides accurate height-above-ground measurements and enables the guided systems to flare just before landing. This maneuver reduces their velocity at impact and increases the delivery accuracy by minimizing target overshoots and undershoots. Our height sensor is unique in its use of audible sound as its sensing modality. The sensor, known as a sodar (sound distance and ranging) sensor, emits bird-like chirps and determines its height by detecting the return echo from the ground. Sound is well suited to this application because it remains robust in the presence of clouds, fog, and dust. Furthermore, the frequency we selected is particularly good at penetrating vegetation.

We assembled a multi-disciplinary team for the sensor development effort. Acoustic test engineers optimized the sound emitting and detection transducers; electrical engineers and technicians developed the driver and control electronics; mechanical engineers, technicians, and machinists designed and fabricated the packaging and mounting components; signal processing engineers devised the detection algorithms; and software engineers programmed the sensor’s digital signal processing (DSP) chip. The resulting Sodar Height Sensor (SHS) has a range of 200 to 500 ft and a height accuracy of 2 ft. This performance is a major improvement over the 60 ft accuracy available with GPS. The sensor has survived numerous hard landings during drop tests in the Arizona desert, and has successfully commanded flare landings for 200 lb and 30,000 lb systems. Creare has partnered with Kaman Aerospace to commercialize this technology.

We are now developing a lidar (light distance and ranging) sensor for systems that require a larger range (1500 ft) and are not sensitive to errors due to vegetation. Both sensors may be used together for a very robust sensor solution.

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