In this paper we develop spectral models for windowed
speckle-modulated continuous coherent ladar signals with
arbitrary coherence time as a function of the window duration and shape. This processing chain (compute the spectrum
of the window random signal) is typical for many coherent ladar applications where the target is distributed in range,
such as wind lidar. We link the peak of the spectrum to the signal's carrier-to-noise ratio (CNR) and the number of
integrated signal photoelectrons. We also develop theoretical models for the SNR of the integrator output and the
resultant coherent integration efficiency. In addition, we generalize Goodman's rectangular integrator diversity model to
arbitrary window functions and show that the product of diversity and efficiency is close to one for arbitrary integration
times and window functions.
These general models are then applied to various signal autocorrelation functions and window functions. The analysis
demonstrates that the shape of the window function has little impact on the output SNR, efficiency and speckle diversity.
We show that if the coherence time is long compared to the integration time then the conventional CNR expression is
valid, and the signal has unit diversity and the coherent integration efficiency is 100%. However, when the coherence
time is short compared to the integration time, the CNR is reduced by roughly the ratio of these times and the diversity is
increased by roughly the same factor. Finally, we also show how signal detectivity is related to SNR and diversity.
An important issue in synthetic aperture ladar is phase noise mitigation, since phase noise corrupts image quality. There
are many phase noise contributors including, residual platform motion, local oscillator phase/frequency instability,
atmospheric turbulence, and additive receiver noise. The Phase Gradient Autofocus (PGA) algorithm is a common phase
noise correction algorithm utilized in synthetic aperture radar. The Cramer-Rao Lower Bound for the phase-difference
estimate variance of PGA can be found in the radar literature. This lower bound describes the precision of the phasedifference
estimate between any two pulses as a function of the carrier-to-noise ratio (CNR). However, this lower bound
does not account for speckle saturation limitations, present in both synthetic aperture ladar and radar.
This paper extends the PGA performance theory to include a high CNR saturation term which accounts for speckle
decorrelation. This term is shown to be proportional to the ratio of the image spot size to the laser pulse repetition
frequency (PRF). This paper also describes impact of PGA estimate variance on image cross-range resolution. We
show, given a fixed PRF and fixed PGA phase-difference estimate variance, that resolution initially improves with
increasing dwell times but eventually saturates to a level proportional to the product of the PGA estimate variance and
the laser PRF.
The Advanced Ladar Signal Simulator (ALASS) is a comprehensive laser radar simulator that synthesizes ladar signals
for complex three dimensional dynamic diffuse targets in the presence of a dynamic turbulent atmosphere. ALASS
provides single realization random signals (speckle) or the associated mean signals (ensemble rough target average).
ALASS is radiometrically correct, accurately models receiver diffraction and defocus for both coherent and direct
detection transceivers with single or multi-element detectors, and generates signals with correct three dimensional
speckle statistics. Signals are computed using the target plane formulism; for coherent detection this involves the
calculation of the back propagated local oscillator (BPLO) while for direct detection the back propagated impulse
response (BPIR) is used. ALASS's primary functions are to serve as a laser radar sensor design tool, data product
generator for exploitation, and a decision aid for implementing system designs. This paper provides an overview of
ALASS, describes its functionality, presents validation results, and displays example imagery.
Pulsed coherent Doppler lidar systems have matured rapidly, especially at solid-state wavelengths. Turnkey systems are commercially available and are being deployed for various aviation applications. Doppler lidar data is used in the airport terminal area to map hazardous wind shear and turbulence levels and to detect and track wake vortices. Future applications could include slant path visibility monitoring. Several permanent installations and rapidly deployable instrument configurations have been achieved. The benefit of the infrared Doppler lidar relative to its microwave counterparts is the ability to sense clear air hazards, especially those in and around local terrain features. The fact that the lidar beam is quite narrow eliminates artifacts associated with sidelobe-induced ground clutter. This paper summarizes our autonomous pulsed lidar developments and reviews sample results.
A theoretical performance analysis of a heterodyne ladar system incorporating a single-rnode fiber receiver has been perforrned. For our purposes, the perforrnance parameters of interest are the coupling and mixing efficiency of the ladar receiver, as they relate to the overall system carrier-to-noise ratio. For a receiver incorporating a single-mode fiber mixer, the received and local-oscillator fields are matched both spatially and temporally at the detector, yielding 100% mixing efficiency. We have therefore focused our efforts on determining an expression for the efficiency with which a diffuse return from a purely speckle target can be coupled into the receiving leg of a monostatic, untruncated cw ladar system. Through numerical analysis, the expected coupling efficiency for a ladar system with negligible truncation of the transmit beam has been determined to be 30.6%.