A proposed instrument would project a narrow laser beam that would be frequency-modulated with a pseudorandom noise (PN) code for simultaneous measurement of range and velocity along the beam. The instrument performs these functions in a low mass, power, and volume package using a novel combination of established techniques. Originally intended as a low resource-footprint guidance sensor for descent and landing of small spacecraft onto Mars or small bodies (e.g., asteroids), the basic instrument concept also lends itself well to a similar application guiding aircraft (especially, small unmanned aircraft), and to such other applications as ranging of topographical features and measuring velocities of airborne light-scattering particles as wind indicators.
Several key features of the instrument’s design contribute to its favorable performance and resource-consumption characteristics. A laser beam is intrinsically much narrower (for the same exit aperture telescope or antenna) than a radar beam, eliminating the need to correct for the effect of sloping terrain over the beam width, as is the case with radar. Furthermore, the use of continuous-wave (CW), erbium doped fiber lasers with excellent spectral purity (narrow line width) permits greater velocity resolution, while reducing the laser’s power requirement compared to a more typical pulsed solid-state laser. The use of CW also takes proper advantage of the increased sensitivity of coherent detection, necessary in the first place for direct measurement of velocity using the Doppler effect. However, measuring range with a CW beam requires modulation to “tag” portions of it for time-off-light determination; typically, the modulation consists of a PN code. A novel element of the instrument’s design is the use of frequency modulation (FM) to accomplish both the PN-modulation and the Doppler-bias frequency shift necessary for signed velocity measurements. This permits the use of a single low-power waveguide electro-optic phase modulator, while simultaneously mitigating the effects of speckle as a noise source in the coherent detection.
The instrument (see figure) would include a narrow-line-width CW laser, the output of which would be split into a local oscillator and signal arm. Within the instrument, optical beams would be routed, split, and combined by use of fiber and planar integrated optics. The signal arm beam would be frequency modulated by the electro-optic phase modulator, fed by a serrodyne waveform generated either in software or hardware, then sent through a multiplexer to a fixed-focus collimating telescope. The combination of a fiber-coupled Faraday circulator and a quarter-wave polarization retarder forms an effective transmit/receive multiplexer, as well as outputting the desired circular polarization.
The return signal, coupled back into the instrument by the same telescope, would be mixed with the local oscillator beam in a semiconductor optical receiver. The resulting heterodyne signal would be filtered, then directly digitized for processing in the digital signal processor, where frequency demodulation and PN-code correlation would be performed, with phase-edge detection and tracking for increased range accuracy.
This work was done by Daniel Chang, Greg Cardell, Alejandro San Martin, and Gary Spiers of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category.
This Brief includes a Technical Support Package (TSP).
Coherent Laser Instrument Would Measure Range and Velocity
(reference NPO-40403) is currently available for download from the TSP library.
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