Passive remote sensing at microwave wavelengths

Passive remote sensing at microwave frequencies has applications which range from meteorology to oceanography and geology. The meteorological applications are the most fully developed and include measurements of the temperature profile of the atmosphere and of the atmospheric distribution of H2O and O3. Such measurements can he made from space or from the ground by utilizing the microwave resonances of O2, H2O, and O3which occur near 1-cm wavelength. Although infrared observations permit similar meteorological measurements, such optical devices are much more sensitive to aerosols and clouds. The small but finite nonresonant attenuation of most moderate clouds at microwave frequencies also permits their liquid water content to be estimated. At wavelengths longer than 2 cm the microwave properties of the terrestrial surface dominate observations from space, and measurements as a function of polarization and viewing angle yield information about surface temperature and emissivity. Such measurements of the ocean should also permit the sea state to he inferred. The review has two major parts. The first part reviews the physics of the interactions, the mathematics of data interpretation, and the instrumentation currently available. The second part is applications-oriented and emphasizes the types, accuracy, and relevance of possible meteorological measurements.     

Remote sensing

With the advent of powerful workstations and affordable software, data from remote-sensing satellites are at last being winnowed and sifted for special-purpose information, sometimes in combination with other inputs. Still more data will become available with the launch of new commercial satellites with high-resolution remote sensing. The author discusses the image processing aspects of remote sensing, describing how the digital images are transformed into maps. In particular the author discusses data fusion and how the spatial resolution limits are overcome by using spectral signatures of objects     

Remote sensing of atmospheric water content from satellites using microwave radiometry

Analysis is presented which substantiates the high correlation achieved in relating integrated water vapor and liquid water to brightness temperatures at frequencies near the 22.235 GHz water vapor line. The influence of atmospheric and surface variability is shown to be minimal over low emissivity sea surfaces. Determination of atmospheric water content using regression techniques is shown to follow directly from radiation transfer theory. Satellite data from the Nimbus-E Microwave Spectrometer (NEMS) aboard Nimbus-5 are compared with radiosonde water vapor measurements and cloud images recorded by the Temperature Humidity Infrared Radiometer aboard Nimbus 5.     

Thermography at millimeter wavelengths

Radiometer measurements show that the human body emits thermal signals at millimeter wavelengths which can produce a thermographic image potentially useful for detection of "hot spots." The spatial resolution which can be obtained at a frequency of 45 GHz is in the order of 1.3 cm, and the temperature resolution is 0.1 K.     

Remote sensing of water vapor in the near IR from EOS/MODIS

The LOWTRAN-7 code was used to simulate remote sensing of water vapor over 20 different surface covers. The simulation was used to optimize the water vapor channel selection and to test the accuracy of the remote sensing method. The channel selection minimizes the uncertainty in the derived water vapor due to variations in the spectral dependence of the surface reflectance. The selection also minimizes the sensitivity of the selected channels to possible drift in the channel position. The use of additional MODIS channels reduces the errors due to the effect of haze, subpixel clouds and uncertainties in the temperature profile. Remote sensing of the variation of water vapor from day to day will be more accurate, because the surface reflectances vary slowly with time. The method was applied to Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data     

Atmospheric attenuation at submillimetre wavelengths

A high-resolution interference spectrometer has been used to observe the absorption of water vapour in the 500?1500 Gc/s region. The centre frequencies and relative strengths of the resonances are compared with earlier theoretical calculations of several workers and point to an extrapolation error in one previous experimental measurement.     

Helical feeds at millimetre wavelengths

The properties of helical aerials have been investigated between 26.5 GHz and 40 GHz. Beamwidths and polarisations are given as functions of helix parameters and frequency. With a suitable choice of dimensions, helixes can be used successfully as broadband feeds for parabolic reflectors and lenses at millimetre wavelengths.     

Remote sensing of dipole rings

Historical satellite-derived sea surface temperature (SST) data are reanalyzed with a zebra color palette and a thermal separatrix method. The new results from this reanalysis are as follows: (a) thirteen observational sequences of six rings from the Gulf Stream and the Brazil Current, which have historically been interpreted as solitary vortices or monopoles are shown to have a dipolar character; (b) some of these dipole rings have been observed in the open ocean, thereby eliminating the possibility that they are sustained by topographic interactions with the continental slope; (c) whether interacting with other features or evolving as isolated circulations, dipoles are seen to rotate within a relatively narrow range of approximately 4-8° day ^-1 (interacting) and 10-15° day-1 (isolated); and (d) feature tracking delineates energetic fluid in both vortices and eliminates the possibility of interpreting dipole rings as transient features produced by active monopoles and patches of entrained fluid     

Astronomical refraction at millimeter wavelengths

Measurements of the difference between radio and optical astronomical refraction atlambda=8.6, 4.3, 3.1, and 2.2 mm are described. The measurement technique utilized solar limb crossing times observed with a 16-foot radio telescope. Results show the expected dependence upon atmospheric water vapor belowlambda= 3mm, but radio refraction tends to approach optical refraction at the shortest wavelength.     

Sea echo at laser wavelengths

Experimental data on sea echo at a 1.06-micron laser wavelength at normal incidence are discussed. The experimental data over a variety of sea states indicate that the return signal is definitely range-square dependent and that the effective target cross sections (σ⁰) are 0.15±0.08 and 0.24±0.12 for smooth and rough surface water conditions, respectively. Additional considerations concerning laser spot size at the ocean and its effect on signal returns are discussed.     

Solar observations at millimeter wavelengths

Solar radiations at millimeter wavelengths consist of a basic component of thermal radiation from an undisturbed sun, a slowly varying component originating from active regions on the sun, and a solar burst component. Eclipse measurements of the brightness distribution have led to a quiet sun model of the chromosphere containing small jet-like spicules. High-resolution millimeter-wave measurements of the solar disk show that the slowly varying component is enhanced radiation from bright spots correlating with plage areas. An increase in brightness of a plage area is observed as early as one day before a plage region develops a flare. Both gradual-rise-and-fall bursts and outbursts have been observed at 4.3 mm. The large outbursts occur in the flash phase of a solar flare. Some of these outbursts had higher flux at 4.3 mm than at longer wavelengths of 3.15 cm and 9.4 cm.     

Circular polarization at light wavelengths

Results of experiments are described which show that circularly polarized laser light provides improved visibility through mist.     

Cloud attenuation at millimeter wavelengths

Total atmospheric attenuation under conditions of complete and partial cloud cover was measured at frequencies of 15 and 35 GHz in the Boston area. The attenuations were actually inferred from extinction measurements using the Sun as a source. Measurements were made at 29 elevation angles from 1° to 20°, and the angular dependence of the attenuation was examined. For most cloud conditions the attenuation was found to be proportional to the slant path distance through the absorbing atmosphere. For elevation angles above about 8°, a flat Earth approximation is valid and the slant path distance is proportional to the cosecant of the elevation angle. For low elevation angles the slant path distance is a function of the effective earth radius and the effective height of the attenuating atmosphere, in addition to the elevation angle. A statistical technique for determining the radius and height is described. A zenith attenuation was extrapolated from each set of data. The humidity and frequency dependence of the attenuation was examined. An algorithm for estimating total atmospheric attenuation as a function of elevation angle, frequency, and surface absolute humidity was derived     

Rain backscatter measurements at millimeter wavelengths

The US Army Ballistic Research Laboratory (BRL) conducted an experiment in 1973 to measure the properties of radar backscatter from rain at millimeter wavelengths. Rain backscatter and attenuation were measured with pulse radars operating simultaneously at 9.375, 35, 70, and 95 GHz over a wide range of rain intensities while continuous measurements of raindrop size and rainfall rate were made. This report describes the measurement technique, details of the instrumentation, the data analysis procedure, and the rain backscatter data obtained from A-scope photographs and video tapes     

High power sources at millimeter wavelengths

Design considerations for high power millimeter-wave oscillators and amplifiers are discussed. Typical structures, electron gun and beam systems, electrode configurations, and interaction processes are described, particularly with regard to their applications in the millimeter-wave portion of the spectrum. The advantages of the linear beam approach in the design of these sources become evident. Typical operating characteristics for general classes of amplifiers and oscillators are reviewed. A survey is presented of the various individual sources which are commercially available or have reached a practical prototype status.     

A Barkhausen-Kurz oscillator at centimeter wavelengths

This paper describes a new device for the generation of high-frequency oscillations in the centimeter wavelength region with high efficiency and low starting currents. The tube is fundamentally a Barkhausen-Kurz oscillator [1] utilizing a resonant cavity and a magnetically focused electron beam, but with no accelerating grids. The important features of this tube are the use of a parabolic potential distribution to utilize multiple transit electron motion, and the use of the magnetic field to control coupling between the electron stream and the resonator field and to reduce to negligible value the current intercepted at the resonator gap. A preliminary theory of the electronic energy interchange of this oscillator is provided, and the background theory of the oscillator design is discussed. Both X-band and K-band tubes have been tested. In the X-band tube, a maximum output power of 1.8 watts and a maximum efficiency of 13.5 per cent were obtained. At K band, the maximum power output was 430 milliwatts, and the maximum efficiency was 12.2 per cent. Starting currents belaw 0.1 ma have been observed at X band.     

Plasma harmonic generation at millimetre wavelengths

A millimetre-wave harmonic generator using a gas discharge formed between two electrodes inside a waveguide has been investigated. Significant amounts of second-harmonic power were obtained from a 34 Gc/s c.w. source.     

Fundamental mode waveguide at submillimetre wavelengths

Experiments with a TE10 waveguide at 460 GHz are described using a simple cross-guide multiplier and a Golay cell. Electroforming production techniques give components with high attenuation, but unconventional methods give acceptable losses.