This invention generally relates to a laser doppler velocimeter, and more particularly to such a velocimeter having laser backscatter discrimination.
It is known in the art that a laser may be used in a velocimeter to determine the velocity of particles in a moving stream of liquid or air, as, for example, in a wind tunnel, to infer the motion of the transporting medium in the vicinity of a solid obstacle. Coherent detection may be used for line-of-sight velocity measurements, while correlation methods may be used to determine the velocity perpendicular to the line-of-sight. A velocimeter of this type, however, uses the return from aerosols or suspensions as its primary signal and would not, therefore, discriminate against such signals in favor of those from a solid target. Generally, a velocimeter of this type also uses a focussed beam to define a sample volume, the velocity of which is to be measured. Focussed beam techniques are not applicable, however, to the small apertures and long ranges typically required for the field operation of a velocimeter designed to measure the velocity of distant, solid targets.
It is also known in the art that a laser may be used in a laser doppler velocimeter (LDV) to determine the relative velocity between an optical platform and another target by measuring the doppler frequency shift using coherent detection techniques. A velocimeter of this type, to be reliable, must provide highly accurate velocity measurements. However, a well known problem with such a device is that created by backscatter from atmospheric conditions such as rain, snow, dust, fog, and the like.
As an attempt to solve the backscatter problem, most prior art designs use some form of frequency modulation to discriminate between legitimate signals from the target and false signals from atmospheric backscatter. However, the use of frequency modulation has disadvantages. The FM transmitter can greatly increase the cost of the device over an unmodulated (CW) approach. The modulated waveform must be carefully controlled and monitored. The discrimination technique based on range can be obscured at short range in the presence of modest winds. Range-doppler coupling can lead to velocity measurement errors especially if the platform is rotating.
As another attempt to solve the backscatter problem, some prior art designs use separate transmit and receive apertures to reduce the signals received from nearby atmospheric scatterers. Separating the apertures, however, results in only a modest decrease in returns from atmospheric backscatter, introduces higher costs due to the extra optics, and greatly increases the sensitivity to optical misalignment.
The laser backscatter discriminator (LBD) of the present invention optimizes the detection of signals from solid targets while rejecting signals due to atmospheric backscatter to detect the valid doppler signal for highly accurate velocity measurements. The laser backscatter discriminator of the present invention allows the use of a simple CW transmitter, and yet provides the system at substantially reduced costs and increased reliability and accuracy as compared to the FM approach.
By way of summary, the system of the present invention combines a simple, low cost, unmodulated (CW) transmitter with a compressive surface acoustic wave (SAW) spectrum analyzer receiver to provide unambiguous, high accuracy velocity measurements with a high utility. Signals from atmospheric backscatter and noise signals are rejected based on bandwidth and amplitude thresholds, while legitimate doppler signals are enhanced using spectral averaging and derivative processing techniques.
The system of this invention can be used in any application that requires an accurate measurement of the line-of-sight relative velocity between the transmitting platform and another target. The platform may be in a moving vehicle, or can be stationary with the beam directed at or scanned in the direction of a moving target.