The DARPA Dynamic Optical Tags (DOTs) program has as its goal the development of a low cost, small, robust, persistent, low probability of intercept, 2-way tagging, tracking, and locating device that also supports error free data rates in excess of 100 kbps and can be interrogated at ranges up to and beyond 1Km. The program has selected several promising candidates for this device and is in the process of evaluating individually their performance against predetermined milestones to ascertain whether the technology is feasible and the program should continue for further development. In all cases the candidate devices operate as retro-reflecting optical modulators. Upon interrogation by a laser at the correct wavelength and with the correct code, the tags will proceed to modulate the return retro-reflection. While data for the candidate devices are not yet in hand, nevertheless this paper will provide an overview of the nature of the devices under investigation and speculate on how these devices could be employed for unattended ground sensor applications.
The DARPA Dynamic Optical Tags (DOTs) program has as its goal the development of a low cost, small, robust, persistent, low probability of intercept, 2-way tagging, tracking, and locating device that also supports error free data rates in excess of 100 kbps and can be interrogated at ranges up to and beyond 1Km. The program has selected several promising candidates for this device and is in the process of evaluating individually their performance against predetermined milestones to ascertain whether the technology is feasible and the program should continue for further development. In all cases the candidate devices operate as retro-reflecting optical modulators. Upon interrogation by a laser at the correct wavelength and with the correct code, the tags will proceed to modulate the return retro-reflection. While data for the candidate devices are not yet in hand, nevertheless this paper will provide an overview of the nature of the devices under investigation and speculate on how these devices could be employed for both national security applications.
The goal of the DARPA Dynamic Optical Tags (DOTs) program is to develop a small, robust, persistent, 2-way tagging, tracking and locating device that also supports communications at data rates greater than 100 kbps and can be interrogated at significant range. These tags will allow for two-way data exchange and tagging operations in friendly and denied areas. The DOTs will be passive and non-RF. To accomplish this, the DOTs program will develop small, thin, retro-reflecting modulators. The tags will operate for long periods of time (greater than two months) in real-world environmental conditions (-40° to +70° C) and allow for a wide interrogation angle (±60°). The tags will be passive (in the sleep mode) for most of the time and only become active when interrogated by a laser with the correct code. Once correctly interrogated, the tags will begin to modulate and retro-reflect the incoming beam. The program will also develop two tag specific transceiver systems that are eye-safe, employ automated scanning algorithms, and are capable of short search and interrogate times.
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%.
The ability of a ladar system to resolve two or more separate returns from a combined echo is related to the effective correlation bandwidth of the pulse emitted by the ladar system. Phase modulation of an outgoing pulse introduces additional frequency components, which increases the effective correlation bandwidth of the pulse and thus improves the range resolution of the system. In this paper, we discuss the general theoretical basis for achieving improved range resolution using a modulated waveform and a matched filter receiver. We then demonstrate these concepts by considering the particular case of improved range resolution for a sinusoidally phase modulated carrier with a rectangular amplitude function. We also perform computer simulations with a realistic pulsed ladar envelope possessing the same modulation function. Our calculations indicate that the resolution of a pulsed ladar system may be improved by a factor of 70 with a phase-modulated pulse and a matched-filter receiver.
A 2.09-μm ladar system is built to compare coherent to incoherent detection. The 2.09-μm wavelength is of interest because of its high atmospheric transmission and because it is eyesafe. The 2.09-μm system presented is capable of either a coherent or incoherent operational mode, is tunable in a small region around 2.09 μm, and is being used to look at the statistical nature of the ladar return pulses for typical glint and speckle targets. To compare coherent to incoherent detection the probability of detection is investigated as the primary performance criterion of interest. The probability of detection is dependent on both the probability of false alarm and the probability density function, representing the signal current output from the detector. These probability distributions are different for each detection technique and for each type of target. Furthermore, the probability of detection and the probability of false alarm are both functions of the dominating noise source(s) in the system. A description of the theoretical expectations of this system along with the setup of the ladar system and how it is being used to collect data for both coherent and incoherent detection is presented.
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