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Abstract

This optical velocimeter may be installed in an aircraft to measure the true airspeed and the angles of attack and of sideslip of the aircraft. Alternatively, the velocimeter may be mounted in a wind tunnel to measure the velocity of the airstream passing through the wind tunnel. In either case, the points of reference for measurement are the large number of aerosol particles that are entrained in the air and are assumed to have the same velocity as the oxygen and nitrogen molecules of the air itself. The velocimeter comprises a plurality of optical transmitters and optical receivers. Each transmitter includes a laser operated in the continuous-wave mode and a laser operated in the pulsed mode. The laser operated in the continuous-wave mode generates a 'precursor light sheet' at some distance from the aircraft whose velocity is to be measured or in the wind-tunnel airstream whose velocity is to be measured. When an aerosol particle intercepts the precursor light sheet, the laser operating in the pulsed mode, after a suitable time delay, generates a 'first successor light sheet,' which is in turn intercepted by the aerosol particle. The time of such interception is registered and initiates a delay period for passage of the aerosol particle to intercept 'a second successor light sheet', also generated by the laser operated in the pulsed mode. The interception by the aerosol particle of the precursor light sheet and then in turn the two successor light sheets causes scintillations which propagate light back to the aforementioned receivers in the velocimeter. The receivers and their associated electronic circuitry record the time of passage of the aerosol particle between the two successor light sheets, as a first measure of the velocity of the aircraft with respect to the particles, or of the particles with respect to some fixed benchmark. By having a plurality of transmitters forming pairs of light sheets, such pairs being neither parallel nor orthogonal to one another, it is possible to resolve the components of motion of the respective aerosol particles and to recombine the resolved components to compute the vector components of the true velocity of the aircraft with respect to its own coordinate system. Certain other computations enable determination of the angle of attack and the angle of sideslip, if any, of the aircraft. Still further, this velocimeter calculates the Mach rate of the aircraft. These calculated outputs are then used as inputs to the control system for the aircraft, which may be inherently unstable and completely dependent upon such control inputs.

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