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MACAWS
Point of Contact:
MACAWS Home Page: http://weather.msfc.nasa.gov/macaws/
- Dr. Jeffry Rothermel
- Global Hydrology and Climate Center
- NASA/ Marshall Space Flight Center / Code: SD60
- Huntsville, Alabama 35805
- Office Telephone: (256)961-7965
- Fax: (256)961-7723
- email: jeffry.rothermel@msfc.nasa.gov
Brief Instrument Description of the
Multicenter Airborne Coherent Atmospheric Wind Sensor (MACAWS)
Platform
NASA DC-8 research aircraft (service ceiling 12.5 km, duration 10 hr or more, optimum speed for wind measurements <450 kt)
Overview
MACAWS is an airborne side-scanning Doppler laser radar (lidar) which measures two dimensional wind fields, vertical wind profiles, and aerosol backscatter from clear air and clouds. Range varies from 10-30 km depending on aerosol abundance and cloud attenuation. Upon exiting the aircraft, the lidar beam is completely eye-safe. MACAWS is developed and operated cooperatively by the atmospheric lidar remote sensing groups of NASA Marshall Space Flight Center, NOAA Environmental Technology Laboratory, and Jet Propulsion Laboratory.
Instrument Description
MACAWS consists of: a frequency-stable pulsed transverse-excited atmospheric pressure carbon dioxide laser emitting 0.5-1.0 J per pulse at 10.6 micron wavelength at a nominal pulse repetition frequency (PRF) of ~20 Hz; a coherent receiver employing a cryogenically-cooled HgCdTe detector; a 0.3 m off-axis paraboloidal telescope shared by the transmitter and receiver in a monostatic configuration; a ruggedized optical table and three-point support structure; a scanner using two counter-rotating germanium wedges to refract the transmitted beam in the desired direction; an inertial navigation system (INS) for frequent measurements of aircraft attitude and speed; data processing, display, and storage devices; and an Operations Control System (OCS) to coordinate all system functions.
Principal of Operation
During flight laser pulses are transmitted through the scanner, which is mounted within the left side of the aircraft ahead of the wing. A portion of the signal is backscattered to the aircraft by aerosols or cloud, with a Doppler frequency shift proportional to the aircraft and wind components along the line-of-sight (LOS). INS measurements of aircraft pitch, roll, and velocity are input to the OCS, which, in order to maintain precise beam pointing, rapidly adjusts the scanner to compensate for aircraft attitude and speed changes. Using the INS measurements and scanner settings the OCS estimates and subtracts the frequency contribution to the Doppler-shifted signal due to the component of aircraft motion along the line of sight, yielding LOS wind velocities with respect to earth coordinates. The method to obtain two-dimensional wind fields is described below. LOS velocity, backscatter intensity, and wind fields are displayed in real-time. Data stored for later analysis include digitized in-phase and quadrature components of the amplified detector output (in limited quantities), LOS velocity and intensity from multi-lag covariance calculations, aircraft housekeeping, scanner settings, and INS outputs. Nominal LOS velocity accuracy is ~1 meter per second. LOS resolution, which may be varied over 150-450 m during signal processing, is nominally set to 300 meters. Along-track resolution (1-3 km) varies with airspeed, PRF, and scan pattern.
Multi-dimensional Measurement Capabilities
MACAWS has the capability to direct the lidar beam with varying degrees of sophistication. A field of two-dimensional winds, or scan plane, is measured by alternately directing the beam forward and aft by 20 deg relative to the flight heading. Two-dimensional wind velocity is then calculated at the intersections using trigonometry. The scanner and OCS compensate for drift angle and turbulence using INS measurements of aircraft attitude and speed. When profiles are required at several vertical levels, the scanner can direct the beam at up to five vertical angles to produce multiple scan planes, to a limit of plus or minus ~25 deg elevation. In the simplest case, one-dimensional vertical profiling of LOS velocity and backscatter above or below flight level is achieved by refracting the beam to the limits of the scanner, plus or minus ~32 deg. Profiles at steeper angles may be achieved by banking the aircraft.
History
The concept of wind measurements with airborne Doppler lidar was first demonstrated in 1981. Science demonstration flights with MACAWS, which uses a much more powerful laser, were made in September 1995 including the first Doppler lidar LOS velocity measurements within a hurricane (Juliette). MACAWS was subsequently flown in June-July 1996 to study a variety of boundary layer and free tropospheric flows.
References
Rothermel, J., D.R. Cutten, R.M. Hardesty, R.T. Menzies, J.N. Howell, S.C. Johnson, D.M. Tratt, L.D. Olivier, and R.M. Banta: The Multi-center Airborne Coherent Atmospheric Wind Sensor, MACAWS, Bull. Amer. Meteor Soc., 79, 580-598 (1998). Rothermel, J., D.R. Cutten, R.M. Hardesty, J.N. Howell, R.T. Menzies, D.M. Tratt, and S.C. Johnson: Application of airborne Doppler laser radar to hurricane research, Preprints, 22nd Conf. Hurricanes and Tropical Meteorology, May 19-23, Ft. Collins, CO, Amer. Meteorol. Soc., 57-58 (1997). Rothermel, J., L.D. Olivier, R.M. Banta, R.M. Hardesty, J.N. Howell, D.R. Cutten, S.C. Johnson, R.T. Menzies, and D.M. Tratt: Remote sensing of multi-level wind fields with high-energy airborne scanning coherent Doppler lidar, Optics Express, 2, 40-50 (1998).
