Russell Targ

Coherent lidar airborne windshear sensor: performance evaluation

National attention has focused on the critical problem of detecting and avoiding windshear since the crash on 2 Aug. 1985 of a Lockheed L-1011 at Dallas/Fort Worth International Airport. As part of the NASA/FAA National Integrated Windshear Program, we have defined a measurable windshear hazard index that can be remotely sensed from an aircraft, to give the pilot information about the wind conditions he will experience at some later time if he continues along the present flight path. A technology analysis and end-to-end performance simulation measuring signal-to-noise ratios and resulting wind velocity errors for competing coherent laser radar (lidar) systems have been carried out. The results show that a Ho:YAG lidar at a wavelength of 2.1 microm and a CO(2) lidar at 10.6 microm can give the pilot information about the line-of-sight component of a windshear threat from his present position to a region extending 2-4 km in front of the aircraft. This constitutes a warning time of 20-40 s, even in conditions of moderately heavy precipitation. Using these results, a Coherent Lidar Airborne Shear Sensor (CLASS) that uses a Q-switched CO(2) laser at 10.6 microm is being designed and developed for flight evaluation in the fall of 1991.

Coherent launch-site atmospheric wind sounder: theory and experiment

The coherent launch-site atmospheric wind sounder (CLAWS) is a lidar atmospheric wind sensor designed to measure the winds above space launch facilities to an altitude of 20 km. In our development studies, lidar sensor requirements are defined, a system to meet those requirements is defined and built, and the concept is evaluated, with recommendations for the most feasible and cost-effective lidar system for use as an input to a guidance and control system for missile or spacecraft launches. The ability of CLAWS to meet NASA goals for increased safety and launch/mission flexibility is evaluated in a field test program at Kennedy Space Center (KSC) in which we investigate maximum detection range, refractive turbulence, and aerosol backscattering efficiency. The Nd:YAG coherent lidar operating at 1.06 µm with 1-J energy per pulse is able to make real-time measurements of the three-dimensional wind field at KSC to an altitude of 26 km, in good agreement with our performance simulations. It also shows the height and thickness of the volcanic layer caused by the volcanic eruption of Mount Pinatubo in the Philippines.

Investigation of airborne lidar for avoidance of windshear hazards

A generalized windshear hazard index is defined, which is derived from considerations of wind conditions at the present position of an aircraft and from remotely sensed information along the extended flight path. Candidate airborne sensor technologies based on microwave Doppler radar, Doppler lidar, and infrared radiometric techniques are discussed in the context of overall system functional requirements. Initial results of a performance and technology assessment study for competing lidars are presented. Based on a systems approach to the windshear threat, lidar appears to be a viable technology for windshear detection and avoidance, even in conditions of moderately heavy precipitation. The proposed airborne CO2 and Ho:YAG lidar windshear-detection systems analyzed here can give the pilot information about the line-of-sight component of windshear threat from his present position to a region extending 1 to 3 km in front of the aircraft. This constitutes a warning time of 15 to 45 seconds. The technology necessary to design, build, and test such a brassboard 10.6 micron CO2 lidar is now available. However, for 2-micron systems, additional analytical and laboratory investigations are needed to arrive at optimum 2-micron rare-earth-based laser crystals.

Coherent launch-site atmospheric wind sounder

The coherent launch-site atmospheric wind sounder (CLAWS) is a lidar atmospheric wind sensor designed to measure the winds aloft at space launch facilities to an altitude of 20 km. Candidate lidar systems analyzed for use in CLAWS include Nd:YAG, Ho:YAG, and CO2. Detailed simulations were carried out by Coherent Technologies, Inc. The results of our development studies include: (1) definition of lidar sensor requirements, (2) definition of a system to meet those requirements, and (3) a concept evaluation with recommendations for the most feasible and cost-effective lidar system for use as an input to a guidance and control system for a missile or spacecraft launch. A field test program will begin in August 1991, in which the ability of CLAWS to meet NASA goals for increased safety and launch/mission flexibility at Kennedy Space Center (KSC) will be evaluated with regard to maximum detection range, refractive turbulence, and aerosol backscattering efficiency at the three lidar wavelengths. It is found that the shorter wavelength solid-state lasers will afford better performance (longer detection range), are more energy efficient, and are more compact for operation in the humid, postvolcanic aerosol environment found at KSC. Finally, the Ho:YAG (2.1 micrometers ) lidar gives the best performance at an eyesafe wavelength and would be applicable for detecting winds aloft during descent as well as during ascent.

Airborne lidar wind detection at 2 μm

NASA and the FAA have expressed interest in laser radars capabilities to detect wind profiles at altitude. A number of programs have been addressing the technical feasibility and utility of laser radar atmospheric backscatter data to determine wind profiles and wind hazards for aircraft guidance and navigation. In addition, the U.S. Air Force is investigating the use of airborne lidar to achieve precision air drop capability, and to increase the accuracy of the AC- 130 gunship and the B-52 bomber by measuring the wind field from the aircraft to the ground. There are emerging capabilities of airborne laser radar to measure wind velocities and detect turbulence and other atmospheric disturbances out in front of an aircraft in real time. The measurement of these parameters can significantly increase fuel efficiency, flight safety, airframe lifetime, and terminal area capacity for new and existing aircraft. This is achieved through wind velocity detection, turbulence avoidance, active control utilization to alleviate gust loading, and detection of wingtip wake vortices produced by landing aircraft. This paper presents the first flight test results of an all solid-state 2-micrometers laser radar system measuring the wind field profile 1 to 2 km in front of an aircraft in real time. We find 0.7-m/s wind measurement accuracy for the system which is configured in a rugged, light weight, high- performance ARINC package.

Performance analysis and technical assessment of coherent lidar systems for airborne wind shear detection

The feasibility of using a pulsed coherent lidar for an on-board airline wind shear monitoring system was studied using detailed computer simulations of the lidar wind measuring process. Lidar performance was studied using NASA-LaRC-provided data fields of an actual microburst event that occurred on August 2, 1985 at the Dallas-Ft. Worth International Airport. Both CO2 and Ho:YAG lidar systems performed well in the dry micro-burst test case. Both lidar systems are able to measure wind shear in the severe weather of the wet microburst to ranges greater than 1.4 km giving minimum warning times of approximately 15 sec.

Windshear Avoidance: Requirements And Proposed System For Airborne Lidar Detection

A generalized windshear hazard index is defined, which is derived from considerations of wind conditions and an aircrafts present and potential altitude. Based on a systems approach to the windshear threat, lidar appears to be a viable methodology for windshear detection and avoidance, even in conditions of moderately heavy precipitation. The proposed airborne CO2 and Ho:YAG lidar windshear detection systems analyzed in this paper can each give the pilot information about the line-of-sight component of windshear threat from his present position to a region extending 1 to 3 km in front of the aircraft. This constitutes a warning time of 15 to 45 s. The technology necessary to design, build and test such a brassboard 10.6-μm CO2 lidar is at hand.

Wind hazard detection with airborne laser radar sensors

The National Aeronautics and Space Administration (NASA), the Federal Aviation Administration (FAA), and the United States Air Force have all expressed interest in the ability of laser radar to detect wind profiles at altitude. A number of programs are addressing the technical feasibility and utility of using laser radar atmospheric backscatter to determine wind profiles and wind hazards for aircraft guidance and navigation. This paper presents the results of ground and flight tests of a coherent 10.6 micrometers airborne laser radar for wind hazard detection. It also discusses the emerging capabilities of an airborne, all-solid-state, 2 micrometers laser radar for performing these functions in a rugged, smaller, lighter weight, high- performance package.

Infrared lidar windshear detection for commercial aircraft and the edge technique, a new method for atmospheric wind measurement

National attention has focused on the critical problem of detecting and avoiding windshear since the crash on August 2, 1985, of a Lockheed L-1011 at Dallas/Fort Worth International Airport. As part of The NASA/FAA National Integrated Windshear Program, we have defined a measurable windshear hazard index that can be remotely sensed from an aircraft, to give the pilot information about the wind conditions he will experience at some later time if he continues along the present flight path. Our technology analysis and end-to-end performance simulation, which measured signal-to-noise ratios and resulting wind velocity errors for competing coherent lidar systems, showed that a Ho:YAG lidar at a wavelength of 2.1 μm and a CO2 lidar at 10.6 m can give the pilot information about the line-of-sight component of a windshear threat in a region extending from his present position to 2 to 4 km in front of the aircraft. This constitutes a warning time of 20 to 40 s, even under conditions of moderately heavy precipitation. Using these results, a Coherent Lidar Airborne Shear Sensor (CLASS), using a Q-switched CO2 laser at 10.6 μm, is being designed and developed for flight evaluation in early 1992. The edge technique is a powerful new method for the measurement of small frequency shifts which allows high accuracy measurement of atmospheric winds (0.2 to 1 m/sec) with high vertical resolution (10 meters) using currently available technology.

Automatic Frequency Control of a Laser Local Oscillator for Heterodyne Detection of Microwave-Modulated Light

Microwave-modulated light is demodulated in an optical heterodyne receiver. An automatic frequency control (AFC) system acquires the incoming microwave-modulated laser signal. The AFC system makes use of an optical single-sideband suppressed-carrier modulator to offset the frequency of the laser local oscillator by 2.5 Gc/sec, and also to track the rapid variations in difference frequency between the signal and local oscillator.