Description:
OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Directed Energy (DE);Space Technology
The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.
OBJECTIVE: The objective of this project is to develop and demonstrate an aircraft detection and avoidance system that would allow astronomical and space situational awareness observatories and other atmospheric laser operators to avoid accidental illumination of aircraft from eye-hazardous lasers. Specifically, the system would need to be configured in a way that's appropriate for small (e.g., less than 1 meter) telescopes. The objective is to develop the necessary aircraft detection and avoidance components and demonstrate them on-sky, in conditions that are representative of typical sites for ground-based observations of earth-orbiting satellites. These components could be demonstrated on government, university, or civilian telescopes.
DESCRIPTION: AFRL supports the US Space Force in researching and developing effective, affordable techniques to identify, track, and characterize satellites in Earth orbit. Radar, although it is expensive to build and operate, works for satellites in low-Earth orbit. However, because of the distances involved, only a few specialized ground-based radars are capable of tracking satellites in geosynchronous orbit. Compared to ground-to-space radars, ground-based optical telescopes are less expensive to build and operate; in addition, they work well for satellites in all orbits. However, atmospheric turbulence limits the resolution and effectiveness of ground-based optical telescopes. Laser-beacon adaptive optics is an established technique to overcome the effects of atmospheric turbulence. However, laser beacons are not usually eye-safe and present a significant hazard to pilots and the safe operation of aircraft. One type of laser beacon that is not hazardous to pilots is ultraviolet (UV) Rayleigh laser beacons. However, UV laser beacons have a number drawbacks. Before discussing these drawbacks, it is helpful to discuss the different types of laser beacons used for adaptive optics. There are two main types of laser beacons used in adaptive optics, Rayleigh beacons and sodium beacons. Rayleigh beacons are formed by scattering light from molecules of nitrogen and oxygen lower in the atmosphere; typical altitudes range from 10 km to 20 km. Pulsed lasers are typically used for Rayleigh beacons so that the light may be sampled from a particular altitude by using a technique called range gating. Because Rayleigh scattering is much stronger for shorter wavelengths of light, common wavelengths for Rayleigh beacons are 355 nm and 532 nm. Typically, the 355 nm (UV) beacons are eye-safe, but the 532 nm (visible) beacons are not eye-safe. Because Rayleigh beacons rely on scattering from air molecules, they are limited to relatively low altitudes where the density of air molecules is higher. Light from the beacon traverses a cone of air above the telescope, with the beacon at the apex of the cone and the telescope pupil at the base of the cone. If a Rayleigh beacon is used for a larger telescope, the cylindrical column of air above the telescope will not be well sampled. Because of this cone effect, Rayleigh beacons are suitable only for smaller telescopes of up to about 2 m in diameter. Sodium beacons are formed from scattering light from a layer of ionic sodium that is centered at an altitude of 90 km above the ground. Because of their high altitude, sodium beacons sample a much larger cone of air when compared to Rayleigh beacons. So, they are better suited for use with large telescopes. So, UV Rayleigh beacons are suited only for smaller telescopes. Now that we have discussed the different types of laser beacons, we can put the drawbacks of UV laser beacons in context. Astronomical telescopes usually use a series of mirrors to reflect and focus light onto sensors. The best coating for these mirrors, especially in smaller telescopes, is protected silver. However, silver does not reflect UV light efficiently. The reflectivity of typical silver coatings at 355 nm wavelength is about 0.5. A typical AO system at Nasmyth focus would have at least 5 silver-coated mirrors before the wavefront sensor. This mean about 3 percent of the UV light would make it to the wavefront sensor. Now, UV-enhanced silver coatings have much higher reflectivity at 355 nm, but that would required recoating several large mirrors, which would be costly. In addition to the issue with silver-coated mirrors, pulsed UV lasers with good beam quality required for laser beacons do not have sufficient power to form beacons bright enough for observatories with strong turbulence. For most astronomical observatories, this is not a problem, because they are located in places with weak atmospheric turbulence. However, observatories for space situational awareness (SSA) and ground stations for laser communications (lasercom) are typically located in places with stronger atmospheric turbulence. To make matters worse for SSA observatories, when a ground-based telescope tracks a satellite in low-Earth orbit, it must slew quickly across the sky. This, in effect, creates a situation that is equivalent to a strong wind blowing across the aperture of the telescope. This means the adaptive optics system must operate at a higher frame rate and higher gain to compensate for atmospheric turbulence. In addition, there's a growing need for SSA and lasercom systems to operate during the day, which means the atmospheric turbulence is much wors than it is at night. The combination of these factors means a laser beacon for SSA and lasercom purposes must be much brighter than a laser beacon for astronomy. Thus, UV Rayleigh beacons are not ideal for some applications. In the past, observatories have used human aircraft spotters and radar systems to avoid illuminating aircraft. However, human spotters are expensive to employ and they can not observe for long periods of time in potentially very cold weather. In addition, human aircraft spotters have a very difficult time spotting aircraft during the day. As for radar systems, they produce radio-frequency interference, which can adversely affect sensitive electro-optical equipment. Radar systems are expensive to operate, maintain, and calibrate, plus they produce ionizing radiation that is hazardous to personnel. Radar system also have a difficult time detecting aircraft that have a small radar cross-section. One system that meets many of the requirements is the Transponder-Based Aircraft Detector (TBAD). (http://www.aircraft-avoid.com/). However, the TBAD antenna system is too large to install on small-aperture telescopes, which may have domes with relatively small openings. Furthermore, the current antenna format can act as a sail and catch wind, which may cause jitter of the optical telescope. Passive infrared detectors have been developed and used in the past, but these systems were less effective than TBAD. That said, passive infrared detectors do not rely on the aircraft having a transponder, thus they may be able to detect experimental aircraft, such as hang gliders. Thus, AFRL is seeking development of reliable, passive systems that would allow astronomical and space situational awareness observatories and other atmospheric laser operators to avoid accidental illumination of aircraft from eye-hazardous lasers, but is suitable for small telescopes and avoids the issues of telescope jitter due to wind buffeting.
PHASE I: As this is a Direct-to-Phase-II (D2P2) topic, no Phase I awards will be made as a result of this topic. To qualify for this D2P2 topic, the Government expects the applicant to demonstrate feasibility by means of a prior “Phase I-type” effort that does not constitute work undertaken as part of a prior SBIR/STTR funding agreement. "Phase I-type" deliverables include a report that describes thoroughly concepts, analyses, and simulations for aircraft avoidance systems that are suitable for SSA ground-to-space imaging applications that use small telescopes. These analyses and simulations must show that the proposed components are effective and affordable. The report should describe the components at a level suitable for a conceptual design review. (See https://en.wikipedia.org/wiki/Engineering_design_process#Concept_Generation) The report shall include a plan for demonstrating the aircraft avoidance systems on-sky, in conditions that are representative of typical sites for ground-based observations of earth-orbiting satellites. (Since this is a D2P2 topic, this section describes the content expected to substantiate that the proposer's technology is currently at an acceptable stage to award a D2P2.)
PHASE II: Phase II deliverables include a detailed design of aircraft avoidance systems that are suitable for SSA ground-to-space imaging applications that use small telescopes. This design must illustrate that the proposed components are effective and affordable. The design documents should describe the components at a level suitable for preliminary and critical design reviews. (See https://en.wikipedia.org/wiki/Design_review_(U.S._government)#Preliminary_Design_Review_(PDR), and https://en.wikipedia.org/wiki/Design_review_(U.S._government)#Critical_Design_Review_(CDR)) The report shall include a detailed plan for demonstrating the aircraft avoidance systems on-sky, in conditions that are representative of typical sites that use small telescopes for ground-based observations of earth-orbiting satellites. As cost and schedule constraints allow, a prototype aircraft avoidance system shall be built, tested, and demonstrated on-sky at government, university, or civilian observatory.
PHASE III DUAL USE APPLICATIONS: A Phase III effort would require identifying a suitable transition partner, which could be a government program office, a government contractor or other commercial entity, or a civilian astronomical observatory. Potential phase III applications include other defense SSA observatories in the US, Europe, and Australia; civilian astronomical observatories that wish to observe at visible wavelengths, which requires improved adaptive optics performance; and ground-to-space laser communications research facilities or ground sites.
REFERENCES:
1. A Radio System for Avoiding Illuminating Aircraft with a Laser Beam, Author(s): W. A. Coles, T. W. Murphy Jr., J. F. Melser, J. K. Tu, G. A. White, K. H.Kassabian, K. Bales, B. B. Baumgartner, Source:Publications of the Astronomical Society of the Pacific,Vol. 124, No. 911 (January2012), pp. 42-50;
2. Seán Meenehan, Emily Dunkel, Michael Cheng, "Automatic aircraft avoidance for laser uplink safety," Proc. SPIE 11993, Free-Space Laser Communications XXXIV, 119930S (4 March 2022)
KEYWORDS: laser beacons; adaptive optics; aircraft laser safety; aircraft avoidance
