The Charles Stark Draper Laboratory, Inc.

Glenn Mamon

J Arnold Soltz

Jeffrey A Volp

Douglas G Youmans

William A Drohan

A method and apparatus for surveying ground terrain from an airborne vehicle is disclosed. According to the method and apparatus, an inertial measurement unit, a position tracking device, and a terrain altimeter are carried by the airborne vehicle in a known vector relationship to one another. The inertial measurement unit provides continuous accelerometer data from which a first determination of vehicle position can be made. Simultaneously, data from the tracking device representative of the position of the tracking device relative to cooperating reflection means on the ground, if within sight of the airborne vehicle, provides, in combination with gimbal angle sensor outputs from the inertial measurement unit, an independent determination of the position of the vehicle. The two position measurements, when available, are continuously compared in real time on board the vehicle and error corrections are generated therefrom by estimation theory. The generated error corrections are stored and updated in real time and error correction signals are preferably provided to at least the inertial measurement unit to correct errors due, for example, to drift in the unit. The airborne vehicle can thereby be out of sight of the cooperating reflection means for substantial time durations without substantially degrading the accuracy of the position information. Corrected positional information, which includes the generated error corrections, and the correlated output of the terrain altimeter are stored to provide a profile of the region over which the vehicle flies. In preferred embodiments of the invention, both the position tracking device and the terrain altimeter are laser devices.

Airborne surveying apparatus and method

A system for determining the range between a reference point and a target by transmitting a pulse toward the target and measuring the time until receipt of a pulse reflected by the target. In an airborne laser altimeter configuration, the reference point may be located on an aircraft and a pulsed laser is directed along a vertical path to the ground. The system measures the duration of a pulse sequence including a first (or start) pulse, representing a pulse transmitted from the reference point, and the last of one or more associated subsequent return pulses, where the return pulses represent reflections of the transmitted pulse from both the ground and interfering objects located between the reference point and the ground. The system generates a replica of the start pulse and subsequent pulses, with the replica being delayed by a predetermined period from the start pulse and associated subsequent return pulses. The undelayed pulse sequence is processed to determine the number of return pulses in that sequence, and this detected number of pulses is utilized to enable a gate which passes only the last return pulse in the delayed pulse sequence. A time interval measurement is performed by a single precision time interval counter which is initiated by the start pulse of the delayed pulse sequence and is disabled by the gated last, or stop, pulse of the delayed pulse sequence.

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Chestnut Hill, MA, United States of America

Glenn Mamon