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Coherent Laser Instrument Would Measure Range and VelocityA 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-of-flight 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 electrooptic phase modulator, while simultaneously mitigating the effects of speckle as a noise source in the coherent detection.
Document ID
20110014748
Acquisition Source
Jet Propulsion Laboratory
Document Type
Other - NASA Tech Brief
External Source(s)
http://www.techbriefs.com/component/content/182?task=view
Authors
Chang, Daniel (California Inst. of Tech. Pasadena, CA, United States)
Cardell, Greg (California Inst. of Tech. Pasadena, CA, United States)
San Martin, Alejandro (California Inst. of Tech. Pasadena, CA, United States)
Spiers, Gary (California Inst. of Tech. Pasadena, CA, United States)
Date Acquired
August 25, 2013
Publication Date
February 1, 2005
Publication Information
Publication: NASA Tech Briefs, February 2005
Subject Category
Man/System Technology And Life Support
Report/Patent Number
NPO-40403
Distribution Limits
Public
Copyright
Public Use Permitted.

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IDRelationTitle20110014712Collected WorksNASA Tech Briefs, February 2005
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