Photon-counting combined with M-ary PPM is practically the most efficient means for implementing freespace
optical communications. Data rates are limited by the speed of the counting devices. We calculate here
the performance with soft decision coding that one can expect for speeds so fast that device-limiting timing
jitter is the primary source of measurement error.
Photon-counting is known to be the practically most efficient means
for detection of free-space optical communications. Data rates will
always be limited, however, by the speed at which such devices can
operate. We calculate here the performance one can expect as one
demands speeds so fast that device-limiting timing jitter substantially
corrupts the measurements.
KEYWORDS: Frequency modulation, Fermium, LIDAR, Signal to noise ratio, Signal processing, Interference (communication), Speckle, Laser applications, Modulation, Laser processing
Laser vibration sensing has traditionally relied on the use of limiters and frequency modulation (FM) discriminators to process frequency modulated laser radar returns. The performance of the traditional FM discriminator approach can be limited by laser radar target characteristics and motion (speckle noise) and laser temporal coherence. In this paper we examine a novel laser vibration signal processor, a spectrogram processor, and compare its performance with the traditional limiter/FM discriminator signal processor used to process laser radar vibration measurements. The two processes are also compared using some laser radar measurement data.
ISAR is a form of range-Doppler imaging in which object motion is used to form an image. An account is presently given of the use of laser radar to conduct ambiguity function-like (AFL) and subaperture AFL (SAFL) imaging with all-digital processing. Attention is given to waveform resolution and ambiguities, SAFL image samples, the application of correlation-mixing 'stretch processing', and a plausible design for a heterodyne receiver and generic waveform processor for all-digital stretch processing.
This paper deals with the theory of centroid tracking of range-Doppler images created with a heterodyne-detection laser radar. A brief description of the system model is presented and the noise statistics of the image pixels are characterized. A center-of-mass centroid algorithm is used to determine the object's image location within the range-Doppler window. The effects of thresholding and frame averaging upon the performance of the centroid estimator are investigated. An analytical approach for setting the threshold is developed which is based on the criterion of making the estimator unbiased. A constant probability of detection algorithm in conjunction with an acceptable bias condition is described. Performance curves and simulation results are presented which support the theory. Center-of-mass centroid tracking performance, using simulated image data and data collected at the Firepond site in Westford, MA, is shown to be in agreement with the theoretical performance.
The effects of optical and microwave heatings and thermally-induced birefringence in a CdTe modulator crystal on the performance of a linear FM CO2 laser radar are examined. This is conducted in terms of reductions in beam Strehl ratio and dynamic ranges of the Doppler shift and range for given optical and microwave powers. An analysis of the thermal lenses generated by these heatings is presented.
This paper deals with the theory of monopulse angle tracking with a laser radar employing heterodyne detection. The detector is assumed to be of the quadrant type. The target's angular location is inferred from the location of its diffraction pattern in the detector plane. A generalized gain characteristic for the detector can be used to relate the strength of the matched filter outputs, which correspond to the spatial regions (halves) of the detector surface, to the corresponding angular location of the target. The maximum likelihood angle estimator is derived for a speckle target. The performance of an approximation to the maximum likelihood angle estimator is evaluated for both the speckle and glint targets. The angle estimator performance is also evaluated in the presence of atmospheric turbulence and beam jitter.
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