Paul B. Hays

Molecular optical air data systems (MOADS)

1. ABSTRACT The Molecular Optical Air Data System (MOADS) is a compact optical instrument that can directly measure wind speed and direction, density, and temperature of the air surrounding an aircraft. From these measurements, a complete set of air data products can be determined. Single-axis wind tunnel testing of wind speed and density has just been completed for the current prototype. These wind tunnel measurements have shown that the current prototype meets wind speed accuracy predictions and initial results from density testing indicate a high level of correlation with absolute pressure transducer measurements. A preliminary design for the next generation instrument, the Joint Optical Air Data System (JOADS), has been completed and is intended to meet Joint Striker Fighter (JSF) requirements. Work is also underway to evaluate the application of MOADS to Unmanned Air Vehicles (UAVs), Reusable Launch Vehicles (RLVs), helicopters and weapon systems. Extensions of MOADS technology to wind shear, gust alleviation, and clear air turbulence detection for commercial aircraft are also being pursued. The basic instrument operation, preliminary ground testing (wind tunnel) results, comparison of these results to simulations, next generation instrument capabilities, and plans for a flight demonstration are discussed.

Molecular optical air data system (MOADS) prototype II

The Molecular Optical Air Data System (MOADS) is a compact instrument designed to measure aircraft airspeed as well as the density of the air surrounding the aircraft. Other air data products provided by the instrument include density altitude, angle of attack (AOA), angle of side-slip (AOS), and Mach number. MOADS is a direct-detection LIDAR that measures these air data products from fringe images derived from a Fabry-Perot etalon. Determination of airspeed and direction is achieved through three telescopes that view a fixed air volume ahead of the aircraft turbulent flow field. This method reduces the measurement error as compared to traditional measurements made from within this turbulent region. As a direct detection LIDAR instrument, MOADS is capable of collecting both molecular and aerosol LIDAR returns, which allows operation in clear air as well as in aerosol-filled atmospheric regions. A second prototype was designed, built and tested. This MOADS prototype has been validated in a laboratory wind tunnel. Presented here are the airflow velocity measurement results from ground testing and vibration test measurements.

Molecular optical air data system (MOADS) flight experiment

The Molecular Optical Air Data System (MOADS) is a compact optical instrument that can directly measure aircraft velocity, as well as the density of the air surrounding the aircraft. From these measurements, many air data products can be determined. Successful MOADS operation has been demonstrated in the laboratory using a wind tunnel. Recently, a MOADS prototype was designed and built in order to complete an upcoming flight experiment aboard a Beechcraft King Air 300. This flight program will be a significant milestone for direct detection lidar systems configured as an air data system aboard an aircraft. The background of the technology, ground experimentation summary of results, flight experiment approach, flight prototype design, and flight experiment planning are discussed.

Performance and comparison of 532-nm and 355-nm groundwinds lidars

GroundWinds 2nd Generation (2nd Gen.) New Hampshire (NH) and GroundWinds Hawaii (HI) are direct detection Doppler LIDAR instruments that operate at 532nm and 355nm, respectively. These ground based incoherent LIDARs utilize backscatter from Rayleigh and Mie scattering to measure Doppler shifts in the atmosphere. The NH and HI instruments routinely make wind measurements from 0.5 to 15 kilometers and achieve sub-meter per second accuracies in the lower troposphere. This paper will provide a brief review of each instrument, and detail the instruments performance and achievements in wind measurement.

GroundWinds New Hampshire and the LIDARFest 2000 campaign

The GroundWinds New Hampshire instrument is a direct detection Doppler LIDAR system that utilizes backscatter signal from both Rayleigh and Mie scattering to measure Doppler shifts in the atmosphere from the ground. This system is the first of two planned systems that will be used to validate the technology and improve the design for other potential implementations. As a means to that end, a validation campaign was conducted in September 2000 to compare the GroundWinds measurements to that from four other systems. These were the GLOW instrument, the NOAA Mini MOPA system, and a Microwave sounder from the National Weather Service. This paper will review the design of the GroundWinds instrument, as well as summarize some of the preliminary GroundWinds results from the field experiment.

Image processing, simulation and performance predictions for the MicroMak star tracker

The MicroMak device is a new, high-precision, very compact star sensor weighing less than 100 grams, with three independent 4-degree square fields of view. The collection telescope is a Maksutov design that incorporates three telescopes into a single sensor head. The sensor is designed for star identification and spacecraft attitude determination with a device that offers unprecedented low cost, volume and mass. While star trackers have achieved sub-arcsecond accuracy by utilizing sophisticated algorithms and complex hardware, the MicroMak sensor must rely on fairly efficient algorithms that utilize data from only the image sensor. This paper will discuss the attitude determination algorithm as well as a complete end-to-end simulation of the system that was used to optimize the design and predict performance. This simulation accepts various star and sensor parameters as inputs, and generates error estimates of attitude of the sensor. The inputs include color temperatures and magnitudes of stars, focal length, receiver aperture, reflectivity curves of mirrors, modulation transfer function of the telescope system, vignetting effects, jittter characteristics, spacecraft spin rate and spin axis, detector pixel size, read noise, dark noise, sensor update rate, quantum efficiency as a function of wavelength, and detector fill factor. A complete forward model of the optical train has been built, and used with a maximum likelihood estimator to generate estimates of sensor attitude. A Monte Carlo algorithm was used to generate error distributions on the attitude error given the noise and distortions injected into the measurement.

GroundWinds 2000 field campaign: demonstration of new Doppler lidar technology and wind lidar data intercomparison

A field campaign featuring three collocated Doppler wind lidars was conducted over ten days during September 2000 at the GroundWinds Observatory in New Hampshire. The lidars were dissimilar in wavelength and Doppler detection method. The GroundWinds lidar operated at 532 nm and used fringe-imaging direct detection, while the Goddard Lidar Observatory for Winds (GLOW) ran at 355 nm and employed double-edge filter direct detection, and the NOAA mini-MOPA operated at 10 microns and used heterodyne detection. The objectives of the campaign were (1) to demonstrate the capability of the GroundWinds lidar to measure winds while employing several novel components, and (2) to compare directly the radial wind velocities measured by the three lidars for as wide a variety of conditions as possible. Baseline wind profiles and ancillary meteorological data (temperature and humidity profiles) were obtained by launching GPS radiosondes from the observatory as frequently as every 90 minutes. During the final week of the campaign the lidars collected data along common lines-of-sight for several extended periods. The wind speed varied from light to jet stream values, and sky conditions ranged from clear to thick clouds. Intercomparisons of overlapping lidar and radiosonde observations show that all three lidars were able to measure wind given sufficient backscatter. At ranged volumes containing thicker clouds, and those beyond, the wind sensing capability of the direct detection lidars was adversely affected.

Lidar measurements of temperature turbulence in the atmosphere

Optical turbulence is an important characteristic for laser propagation in the atmosphere. The optical turbulence causes refractive index fluctuations in the air. The accumulative atmospheric refractive index fluctuations can incur deleterious effects to a laser beam propagating in the atmosphere by distorting its wavefront. Accurate characterization of turbulence in the atmosphere is important to many light propagation studies and related applications. Temperature fluctuations/turbulences in the atmosphere are highly correlated with the atmospheric optical turbulence. In many occasions, optical turbulences profiles in the atmosphere were obtained through the measurements of temperature turbulence and other other atmospheric quantities, for example, air humidity. Therefore determination of temperature fluctuations/turbulences parameter is helpful to the measurement of optical turbulence. Thermosonde was commonly seen in measurements of C2T in the past. Here we show that a direct lidar system is able to fulfill the task of C2T measurements. And compared to the thermosonde measurements the proposed lidar method has many advantages.

GroundWinds balloon fringe-imaging Doppler lidar mission concept and instrument performance

Over the past few years, the GroundWinds program has produced two operational, ground based, multi-order fringe imaging direct detection Doppler wind LIDARs. The two existing instruments are located in Bartlett, NH and Mauna Loa, HI and operate at a wavelength of 532 nm and 355 nm, respectively. Both systems employ Fabry-Perot etalons as the wavelength resolving element and are capable of detecting Doppler shifts in both Aerosol and Molecular backscatter from 0.25 km to 18 km. Patented technologies demonstrated and developed through this program, such as Photon-Recycling (U.S. patent #6,163,380) and the Circle To Line Optic (U.S. Patent #4,893,003), will be incorporated into the next generation interferometer design and flown on a high altitude (30km) balloon. The opportunity to view the entire troposphere from a downward looking high altitude platform will serve as an empirical reference point for scaling to space. This paper will discuss the BalloonWinds mission concept and top-level specifications of the instrument subsystems. Additionally, this paper will report on the testing and progress of the instrument build and present performance projections based on the as built system.

Instrument specifications and performance prediction for 2005 high altitude (30 km) balloon demonstration of GroundWinds fringe imaging Doppler lidar

The GroundWinds direct detection Doppler wind LIDARs located in NH and HI are operational, ground based, multi-order fringe imaging systems capable of detecting Doppler shifts in both Aerosol and Molecular backscatter from 0.25 km to 18 km. The technology developed through these GroundWinds programs will be incorporated and flown on a high altitude (30km) balloon in 2005. The demonstration of GroundWinds Fabry-Perot based incoherent LIDAR technology from a high altitude, downward looking platform to measure winds throughout the entire troposphere and boundary layer will be a significant milestone toward the validation of this technology. Key questions will be answered about the phenomenology of direct detection LIDAR, especially its effectiveness in the optically thick boundary layer. The extensive characterization of the 532nm GroundWinds NH and 355nm GroundWinds HI LIDARs serve as excellent reference points from which performance estimates and technology requirements can be determined to ensure a successful balloon mission. This paper will describe the baseline BalloonWinds instrument specifications; including etalon specifications, system component transmissions, transmit/receive specifications, required laser power, and detector characteristics. This paper will also present performance estimates based on model simulations that employ the baseline system specifications.