Point of Contact:

Gerry Heymsfield

NASA/Goddard Space Flight Center

Mesoscale Atmospheric Processes Branch/Code 912

Greenbelt, MD 20771

Office Telephone: (301) 286-6369

Fax: (301) 286-1762

E-mail: heymsfie@carmen.gsfc.nasa.gov

EDOP Home Page: http://rsd.gsfc.nasa.gov/912/edop/edop.htm

Brief Instrument Description of the
ER-2 Doppler Radar

EDOP is an X-band (9.6 GHz) Doppler radar nose-mounted in the ER-2. The instrument has two antennas: one nadir-pointing with pitch stabilization, and the other forward pointing. The general objectives of EDOP are the measurement of the vertical structure of precipitation and air motions in mesoscale precipitation systems and the development of space borne radar algorithms for precipitation estimation such as will be used for the Tropical Rain Measuring Mission (TRMM). The EDOP instrument collected the first reflectivity measurements during CAMEX-1 (September 1993) and first Doppler measurements during January 1995 flights and CAMEX-2 (August 1995).

EDOP measures high-resolution time-height sections of reflectivity and vertical hydrometeor velocity (and vertical air motion when the hydrometeor fall speed and aircraft motions are removed). The forward beam in addition measures the linear depolarization ratio (LDR) which provides useful information on orientation of the hydrometeors (i.e., the canting angle), hydrometeor phase, size, etc. The dual beam geometry has advantages over a single beam. Along-track horizontal air motions can be calculated by using the displacement of the ER-2 to provide dual Doppler velocities (i.e., forward and nadir beams) at a particular altitude.

EDOP has been designed as a turn-key system with real-time processing on-board the aircraft. The R-F system consists of a coherent frequency synthesizer which generates the transmitted and local-oscillator frequencies used in the system, a pulse modulator which converts the transmitted frequency into a 0.25-2.0 microsecond pulsed signal, a high gain 20 kW Traveling Wave Tube Amplifier (TWTA) which is coupled through the duplexer to the antenna, and the receiver which is comprised of a low-noise (~2dB) GaAs preamplifier and mixer for each of the receive channels. The composite system generates a nadir oriented beam with a co-polarized receiver and a 35 degree forward directed beam with co- and cross-polarized receivers. The antenna design consists of two separate offset-fed parabolic antennas with tri-mode feed horns mounted in the nose radome of the ER-2. The antennas are 0.76 m diameter resulting in a 3 degree beamwidth and a spot size of about 1.2 km at the surface (assuming a 20 km aircraft altitude). The two beams will operate simultaneously from a single transmitter.

EDOP uses a real-time processor to accommodate the very high data and processing rates required by the system's 4400 Hz pulse repetition frequency. The system obtains high vertical resolution profiles (37.5 meter spacing) of measured quantities. The EDOP system was designed to have both high sensitivity for detection of weaker precipitation returns and a large dynamic range since variations in one vertical profile can exceed 90 dB (including return from the surface). A new data system is currently being implemented that serves both as a digital IF receiver and a data processing system. This new linear receiver has a wider dynamic range for Doppler measurements as compared with the current system. Minimum detectable reflectivity for the current system is about -5 dBZ at an altitude of 15 km and for a 0.75 m range gate spacing.