Munitions accuracy provides an advantage in leveraging and applying force and has been sought after by political and military strategists for decades. Reflecting recent advances in technology, during Gulf Wars I and II, Americans watched video on the news of U.S. precision-guided bombs destroying tanks, flying through windows, and exacting coordinates on bridges utilizing laser-guided targeting. Yet, we forget the past challenges associated with target acquisition and destruction.During both World War II and the Korean War, the U.S. dropped excessive amounts of ordnance on targets without ever destroying them. During WWII, it took 108 B-17 bombers, crewed by 1,080 airmen, dropping 648 bombs, to guarantee a 96% chance of getting just two hits inside a 400 x 500-foot target. In contrast, in the Gulf War, a single-strike aircraft with one or two crewmen, dropping two laser-guided bombs, could achieve the same results with essentially a 100% expectation of hitting the target.
With the ongoing effort to refine and hone laser-targeted systems, the demands on qualifying them have grown. For this reason, the U.S. Air Force looked to Boulder Imaging to provide next-generation instrumentation for testing and evaluation equipment. Meaningful analysis was required for a test setup where two lasers in a moving plane pulsed 5,000 times per second onto a 10×10-meter ground-based stationary target. Each laser was pulsed at a different frequency and had its own pulsing characteristics. Measurements were to be taken across both spatial and temporal distribution with the system positioned 100 meters away (Figure 1).
System requirements included being able to detect weak signals from lasers, sample high-frequency laser pulsing at nanosecond accuracy, and synchronize image capture to align with the 20-ns window of laser pulsing. A multipronged approach was used to address this:
“It’s not rocket science to measure laser characteristics, after you select all the components, to capture samples and provide a good signal,” said Boulder Imaging’s Vice President of R&D, Jie Kulbida. “The real challenge is bringing together the most appropriate hardware, software, optical design, optical engineering, and more in order to get an extremely accurate, complete, and strong signal. Because we were able to engineer the optics, the first major hurdle was passed,” she added. The optics made detection of the signal by the system sensors – a photodiode and camera – possible.
The optics design provided two uniform, high-signal optical beams, each sized to match the sizes of the camera sensor and photodiode detector. It included appropriately sized field stops that excluded light from outside the target field of view, a filter wheel to confine the spectral band of the beam, objective and relay lenses that focused light arriving from the observed target into a collimated optical beam, as well as a beam splitter to direct the signal in two paths. The result was data ready for consumption and analysis by the analytic imaging system.
Sampling and Recording Laser Pulses from a Photodiode: To analyze the 1D analog signal, digitization had to cover a complete pulse, including its rising and falling edge. Because this system was designed to handle lasers pulsing at up to 5 kHz with pulse widths as small as 20 ns while guaranteeing zero data drops and 100% data accuracy, specialized hardware with a sampling rate of 1 GHz was selected. To guarantee no data loss, a multi-layered buffering system consisting of hardware and software components was implemented.
Maximizing Signal Quality: For optimal signal analysis, the analytic imaging system dynamically adjusted the gain so that the signal consistently fit in the dynamic range.
1D Time Domain Analysis: With uncorrupted, high-quality, digitized data acquired, several real-time statistical parameters including the average, minimum, and maximum pulse duration as well as the standard deviation for the pulse duration could be calculated. This information was also continuously updated to a display.
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