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.
Project Objectives and Challenges
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:
- Optics Selection
- Real-Time Performance Analysis of Laser Pulsing
- Real-Time of Laser On-Target Performance
- Single System Design
- Protection from Extreme Environmental Conditions
- Integrated Design for Testing Multiple Lasers Simultaneously
Addressing the Challenges
“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.
Real-Time Performance Analysis of Laser Pulsing
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.
Real-Time Analysis of Laser On-Target Performance
Controlling Image Capture: Camera control via hardware triggering was a key element for lining up a very small integration window to match the timing of the laser beam pulse, when both timings were very narrow – only nanoseconds. “This requires that you can predict very accurately when the next pulse that the camera can catch occurs. And this has to happen over 100 times per second,” says Director of Engineering, Jamie Grier. By utilizing knowledge of when the camera will finish exposing, timing of the very last pulse, and the average time between the last set of pulses, the system was able to trigger the camera so that the laser was seen on the target in every image, 110 times per second, for hours at a time.
Maximizing Image Quality: Software was developed to remotely control motors for adjusting the focus and selecting the spectral band filter. For camera focus, a physical motor was selected for its high precision, capable of stepping at 40,000 steps per mm. A separate motor enabled selection of one from the eight filters pre-loaded into the filter wheel.
2D Spatial Analysis: On-the-fly image processing enabled detection of the laser beam spot and associated statistics including its centroid, diameter, average power intensity, and intensity distribution. A 3D image plot, 2D image, and associated statistics were displayed in real-time.
A single system was essential for tight coupling of the photodiode statistics and the camera triggering. The design exceeded the high throughput demands (100 MB/second) as well as the long-duration recording (multi-hour) requirements. In addition, all data was time stamped enabling synchronized playback (see Figure 2).
Extreme Environmental Conditions
System requirements demanded operation under variable temperature, precipitation, humidity, dust and particulate matter, and vibration exposure. Several measures were taken to overcome these challenges. All parts were housed within a hermetically sealed aluminum structure for protection. All optical components where suspended in the housing on an “optical bench” for maximum stability. To prevent internal moisture condensation, valves were added to the enclosure that facilitated the purging of air and injection of nitrogen. Finally, the enclosure was coated for optimum passive thermal control.
Testing Multiple Lasers Simultaneously
Networking Systems: To support detecting, recording, and profiling two lasers pulsing at different frequencies simultaneously, the system was duplicated and then networked into a master-slave configuration. IRIG time stamping, a time source standard, ensured the data from different systems was correlated.
Image Fusion: Registration was performed followed by fusion of the two visual streams into a single image. The result provided a real-time view of the two laser spots striking the single target.
Testing and product performance verification are essential elements for continual engineering innovation and design improvement. Equipment such as the laser scoring system ensures that accurate measurements and evaluation are obtained. One key to success was overcoming a variety of technical challenges utilizing analytic imaging expertise. Various engineering elements were leveraged such as specialized optics, highly sensitive sensors (photodiode and camera), high-performance recording (100 MB/second throughput and 1 GHz signal sampling), and analysis (1D and 2D).
This article was written by Carlos Jorquera, Founder and CEO of Boulder Imaging, Louisville, CO. For more information, visit http://info.hotims.com/22926-159.