Target spectral estimation using direct detection and coherent detection ladar

Abstract

The ability to estimate the mean frequency, peak frequency, and frequency spread of angularly unresolved hard targets is examined using both coherent and direct detection ladar simultaneously. It has been proposed that the direct detection of the return speckle intensity can be used to enhance the coherent detection ladar spectral estimates and signal processing algorithms. The direct detection ladar uses the orthogonally polarized speckle E-field return, with respect the local-oscillator laser, and does not affect the coherent detection sequence. We are concerned in obtaining precise frequency information with only coarse range requirements and, therefore, consider Q-switched laser pulses whose spectral width is much narrower than the target’s spectral bandwidth. We use spinning diffuse cones for examples. The direct detection speckle intensity spectrum is computed and is shown to be corrupted by a strong DC-component and interference between the positive and negative frequencies, which causes additional frequency spread of nearly twice the target spectral width. However, useful target spectral width information can be obtained by direct detection to help in the coherent detection signal processing. Three algorithms are described which are each shown to be within a factor of about 2 times the Cramer-Rao lower bound estimate on mean and peak frequency precision. Surprisingly robust performance of the autocorrelation function first-lag algorithm (“pulse-pair”) is demonstrated for these targets.