Several experimental and theoretical studies have shown that path-integrated rain rates can be determined by means of a direct measurement of attenuation. For ground-based radars, this is done by measuring the backscattering cross section of a fixed target in the presence and absence of rain along the radar beam. A ratio of the two measurements yields a factor proportional to the attenuation from ...

The performances and characteristics of a satelliteborne radar operating in the millimeter wavelength region of the spectrum with emphasis placed on the 35 and 94 GH3 frequency bands are discussed. It is concluded that millimetric wavelengths provide an acceptable solution for the design of satelliteborne active microwave equipment.

The Goddard Space Flight Center 13.9 GHz 12 ns pulse compression radar was flown on extended NASA CV-990 aircraft missions in 1977 and 1978 for the primary purpose of advancing a simple microwave radar technique for measuring ocean wave directional spectra from aircraft and satellite platforms, and secondarily for advancing in general the state-of-knowledge of microwave interaction with sea and ice surfaces. The radar is a versatile, rather complex, instrument having basically three functional modes that are associated with three different antenna systems. These modes are: a. Altimeter (ALT. A nadir-pointing horn antenna is used for significant wave height measurements to within the 1.5 m resolution capability of the radar. This well-known technique makes use of the time-stretch of the leading edge of the average pulse return in the presence of waves. b. Real Aperture Imaging Radar (SLAR). A slotted waveguide antenna mounted on the aircraft instrument sled is used for wave imagery at low altitude (<10 k ft.). c. Directional Wave Spectrometer (DWS). In the Fall 1978 mission, a 6 rpm rotating printed circuit (PC) antenna, boresighted at a nadir angle and having a elevation and azimuth beamwidth, was used in the DWS mode. (In the 1977 mission an adjustable elevation angle PC antenna fixed at broadside azimuth was used to study incidence angle dependence.) The DWS concept, briefly, makes use of the azimuthal averaging of ocean wave contrast features to isolate Fourier contrast wave components traveling in the radar look direction. Spectral analysis of the return for the contrast wave modulation over azimuth angle yields a directional contrast spectrum that is in a first approximation proportional to the directional spectrum of total wave slope variance. Modulation signal-to-Rayleigh clutter noise ratios are improved by Doppler time domain filt- - ering prior to the spectral analysis. A preliminary analysis of the digital data from flights in the Northeastern Pacific over several environmental buoys and ocean weather ship PAPA, and from a flight in the Norwegian Sea over an instrumented weather ship during storm conditions is presented. The directional modulation spectrum is related to the directional wave height spectrum through a physical scattering model: essential similarity with the directional slope spectrum is shown. The short pulse DWS and its sister dual-frequency DWS concepts are discussed with reference to future satellite DWS instrument design.

A three-term approximation and a discrete drop-size distribution have been used to obtain an analytical expression for the microwave attenuation coefficient for precipitation. Results compared to an exact calculation using the same drop size distribution showed a relative difference of 50% or less over a broad range of rainrates (5-150 mm/hr) and wavelengths (2-5 cm). Since the use of a discrete drop-size distribution is computationally complex, the exponential size distribution formulated by Marshall and Palmer (1948) was studied. This distribution resulted in an overprediction of the attenuation coefficient of as much as 100% at a 2 cm wavelength and a 150 mm/hr rainrate, although the prediction was much closer at lower rainrates and different wavelengths.

In September 1975, the Goddard Space Flight Center operated a short pulse radar during ocean wave measuring experiments off the coast of West Germany in the North Sea. The experiment was part of JONSWAP-75. The radar system and operations during the experiment are described along with examples of data.

A real-aperture radar technique is under development at the Goddard Space Flight Center to remotely sense ocean gravity waves from spacecraft. Experimental data obtained from aircraft demonstrations indicate correlation between wave profilometer and radar-derived spectra, and results from an analytical model for scattering from rough surfaces shows promise of explaining the sensor response. This paper will present the basic concept of the sensing system and a summary of the research program and results.

Sensors for remote sensing of ocean wave directional spectra are needed for global wave forecasting application. Backscatter from off-nadir, short pulse, radar-illuminated ocean waves gives the wave impulse response. Wave slope and wave length can be determined from the applitude time radar signature. Analysis of some preliminary wave spectral data taken with a 10 nanosecond aircraft mounted radar will be compared with wave spectra derived from laser prolifometer measurements.

The NASA interference and propagation program was planned by a multiagency committee at the request of the Director of Telecommunications Policy. Initially, emphasis was placed on preparation of the United States position on interference between satellite Earth stations and microwave relay links at frequencies below 10 GHz for the 1971 World Administrative Radio Conference (WARC-71). The program objective is to acquire, analyze, and disseminate space propagation data for a user community of space systems designers, operators and regulatory agencies. Measurements and analyses are being performed which lead to a centralized data base for characterization of radio frequency phenomena required to design space communications systems.