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A radiometer for the remote measurement of sea surface temperature is described. Two requirements are necessary for the attainment of an absolute accuracy of 1 or 2 K in molecular temperature. Although the first is inappropriate for discussion here, it is clear that corrections must be developed to account for perturbations caused by surface effects (roughness, foaming, and salinity changes) and for atmospheric effects (absorption and scattering). The second requirement, namely, the development of an instrument capable not only of high relative accuracy (i.e., resolution) but also of high absolute precision, is the subject of this paper. The concepts underlying the design of an instrument capable of an absolute accuracy of a few tenths degrees Kelvin in the measurement of brightness temperature at S band are described. The role of the antenna is discussed and the importance of high ohmic and beam efficiencies is stressed. The hardware itself is fully described, along with an outline on the design of a unique cryogenically cooled termination used to calibrate the whole radiometer, including antenna. Finally, some test results are presented that show that the design goals for the instrument have been closely approached.

Microwave radiometer measurements were conducted from a railroad bridge which spans the Cape Cod Canal in Massachusetts. Data were collected as a function of viewing angle and polarization at frequencies of 1.4, 4.0, and 7.5 GHz. The results compare differences in the microwave emmissivity of a smooth vs rough water surface. Results are also given which show the effects of roughness on the bistatic scattering of sunlight.

The blackbody temperature and the albedo of a planet, and the variation of both parameters with latitude, longitude, and time, are of great value in understanding the climatic and meteorological conditions of the planet. An unchopped radiometer with a wide but restricted field of view is capable of such temperature and albedo measurements. Coated thermistors mounted in highly reflective cones serve as detectors. Their performance as sensor elements is analyzed in detail herein to prove the feasibility of the measurement. The simplicity of the instrumentation and the low information bandwidth required make the experiment equally attractive for earth satellites and space probes.

A radiometer for the remote measurement of absolute sea surface temperature is described. Two requirements are necessary for the attainment of the desired goal of 1 or 2 degrees (K) in absolute accuracy. Although the first is inappropriate for discussion here, it is clear that corrections must be developed to account for perturbations caused by surface effects (roughness, foaming, and salinity changes) and for atmospheric effects (absorption and scattering). The second requirement, namely the development of an instrument capable not only of high relative accuracy (i e., resolution) but also of high absolute precision, is the subject of this paper. The concept underlying the design of an instrument capable of an absolute precision of a few tenths degrees Kelvin in the measurement of brightness temperature at S-band is described. The role of the antenna is discussed and the importance of high ohmic and beam efficiencies is stressed. Finally, a description of the hardware itself is presented, along with the development of a unique cryogenicalIy cooled termination used to calibrate the entire system, including antenna.

The reduced data of the Mariner 6 and 7 Infrared Radiometer Experiments are presented, along with a discussion of the reduction and calibration procedures. Evidence is presented showing that the surface of Mars is strongly nonhomogeneous in its thermal properties, on scales ranging from those of the classical light and dark areas to the limit of resolution of the radiometers. On the sunlit side, the mean thermal inertia, for admissible bolometric albedos, is 0.006 (cal cm^(-2) sec^(-1/2) °K^(-1)). The dark areas Syrtis Major and Mare Tyrrhenum, observed at night, require thermal inertias as high as 0.010. The temperatures measured over the circular basin Hellas require a bolometric albedo of 0.40 and also a high thermal inertia. The temperature measured over the south polar cap, 148° K, provides evidence that the major constituent of the frost deposit is CO_2.

To provide a general guide in the interpretation of air-borne microwave radiometer data over the ocean, a limited set of measurements of a simulated ocean surface was conducted. The experiment was designed to measure the apparent or brightness temperature, in the microwave region, of an air-water interface under a variety of conditions. We had available a system of radiometers on the frequencies of 9.3, 15.8, 22.2, and 34 GHz. These radiometers were installed in a NASA aircraft, and our experiment was conducted, with the aircraft parked, by erecting a temporary test tank 8 feet in diameter below the radome housing the radiometers (Figure 1).

The brightness temperatures at 10 and 20 micrometers measured by the Mariner 9 infrared radiometer differ substantially from those predicted by the radiometer results of Mariners 6 and 7. The results indicate a significant latitude-dependent contribution of the atmospheric dust to the observed thermal emission.

Solar radiation and atmospheric attenuation were measured at 4.3-mm wavelength The sun was scanned with a radio telescope consisting of a 10-foot precision paraboloid antenna (6.7-minute beamwidth) and a Dicke-type radiometer. Atmospheric attenuations were determined from the change in received solar radiation with changing elevation of the sun and from direct measurements of the thermal radiation from the atmosphere. The measured attenuations at the zenith for clear skies were between 1.6 and 2.2 db. At 4.3-mm wavelength, the sun (when it is free of sunspots) appears to be a uniform disk nearly one per cent larger than its optical size. From a large number of measurements over a period of 6 months, the solar brightness temperature was found to be 7000 °K with an uncertainty of about 10 per cent. Sunspot regions are slightly brighter than the quiet areas; the largest observed enhancement is 2 per cent.

Microwave radiometry data (1.55 cm) taken by aircraft over the Salton Sea have been corrected for viewing angle and atmospheric effects, rectified, and mapped. No fetch-limited conditions are observed along the upwind shore despite a 15 m/sec wind, which indicates that the radiometer is sensitive to the short wavelength surface roughness but not to the longer wavelengths. The brightness temperature field can be represented as a nearly linear function of the surface wind speed.

The variability of sea-surface conditions has been observed from a low-flying aircraft by a laser-wave profiling system and a scanning horizontally polarized 19.35-GHz passive microwave radiometer for fetch-limited wind speeds of 17 to 25 m/s in the North Sea. Wave profiles obtained with the laser system have been analyzed and show that wave growth occurs simultaneously at all frequencies and that an equilibrium value for the higher frequency components is eventually reached, but not before substantially higher (overshoot) values are obtained. Simultaneous observations of the microwave brightness temperature at vertical incidence show an increase with wind speed (or roughness of the sea surface) of 1°K/m/s. This increase, not in accord with theory, is shown to be a function of the percentage of foam coverage of the ocean surface.

The combination of seasonal and orbital changes in Martian insolation result in complex latitude dependent surface temperature variations that effect the total radiance of the planet as seen from the earth. These surface temperature variations have been calculated, based upon a computer simulation of the thermal environment of the planet. The temperature variations are then integrated to yield the total radiance of the planet as seen from the earth as a function of time. The absolute radiance of Mars was measured on April 4, 1971, with a balloon-borne radiometer system operating in the wavelength range between 10.5 and 12.5 μm. The average brightness temperature of the Mars disk determined from these measurements was 254°K with a 1 σ error of 4°K.

A combination of remote sensing from an aircraft and simultaneous surface measurements have confirmed the feasibility of identifying old and new sea ice according to its emission of thermal radiation at wavelengths between 0.3 and 3 cm. Emissivity of first-year thick ice with a surface temperature of about 260 K is 0.95 or greater for wavelengths between 0.81 and 11 cm; the emissivity of multiyear ice is 0.8 at 0.81 cm and 0.95 at 11 cm, increasing monotonically in this wavelength interval. The ease with which multiyear ice can be distinguished from first-year ice using a passive microwave radiometer is demonstrated by comparing mosaics prepared both from photographs and images of 1.55-cm radiation.

Preliminary 10- and 20-μm brightness temperatures of Mars are presented. More than 35% of the Martian surface was observed with a resolution better than 100 km. On the whole, the results confirm the thermal properties derived from the Mariner 6 and 7 radiometers, although the temperatures, on the average, were cooler and shifted with respect to the Martian day during the dust storm. Thermal inertias and radiometric albedos were derived for many areas; no clear correlation exists between these properties. Thermal structure again was found at the spatial limit of the radiometer; no cases were found in which it was necessary to invoke internal heat sources.

A three-instrument infrared spectral emittance experiment, comprising a rapid-scan spectrometer (6.7-13.3 ?m), radiometer (10.375-12.1 ?m), and boresight camera, has been flight tested over selected geological terrain in central and southern California and Nevada. Preand post-flight calibrations of the infrared spectrometer were performed both by using polished samples of "standard" rocks (quartz diorite and gabbro) as well as the more familiar blackbody radiance standards. From these latter spectra the instrument transfer function (A??) was derived. In-flight calibrations of wavelength were achieved by the rapid insertion and removal of a polystyrene film in the optical train of the spectrometer, as polystyrene is a material whose transmission spectrum is constant and well known. By flying over a body of water (?6.7-13.3?m = 0.98) and recording the radiance spectrum of that target one can determine the transmission spectrum of the atmospheric path between the aircraft and the water (at least to a first approximation) as both the spectral emittance of lake water and the optical transfer functions of the instrument are known or can be calculated. So far, flights have been made only at low altitudes (2000 ft above the lake), with the lake surface at 2000 ft (near Pisgah Crater, S. Calif.) or 6000 ft (Mono Lake, E. Calif.) above sea level. The lake should be in the area to be studied geologically.

A 3.4-cm wavelength radiometer aboard the Mars 2 and 3 orbiters observed the brightness temperature due to planetary thermal emission in two orthogonal polarizations as a function of position on Mars. Preliminary results for two orbits of Mars 3 show a correlation between subsurface temperature and dielectric constant, interpreted as an effect of porosity.

Measurements were made from aircraft of the 1.55-cm microwave emission from the North Sea and North Atlantic at surface wind speeds ranging from less than 5 to 25 m sec−1. Brightness temperatures in the nadir direction increased almost linearly with wind speed from 7 to 25 m sec−1 at a rate of about 1.2C (m sec−1)−1. At 70° from nadir the rate was 1.8C (m sec−1)−1. This increase was directly proportional to the occurrence of white water on the sea surface. At wind speeds

Circulation features of stratosphere derived from radiometric temperature measurements with Tiros VII satellite

Radiometric technique for absolute temperature measurement of microwave noise sources under field conditions

Cloud melting zone, noting increased antenna temperature for radiation trapping between cloud layer and ground

The infrared radiometer is designed to provide brightness temperatures of the surface of Mars by measuring the energy radiated in the 8 to 12 and 18 to 25 μ wavelength bands. The instrument is essentially the same as that flown on the Mariner Mars 1969 missions, modified only to define more sharply the field of view. Because Mariner Mars 1971 will orbit Mars, a given area of the planet will be observed at a variety of local times, and the characterization of the various areas by their thermophysical properties will be more complete than that obtained by Mariner Mars 1969.

Tungsten strip lamps spectral radiance calibration, discussing accuracy requirements for radiation constants, wavelength and black body temperature

During July 1964, Venus was observed with a tunable radiometer at eight frequencies in the interval 20.6-24.0 GHz and again at six frequencies in the interval 18.5-24.0 GHz during an eight-month period beginning approximately one month after the inferior conjunction of 1967. Microwave spectra for these frequency intervals were computed using the results from Mariner 5 and Veneras 4-7 and compared to these data. One block of data suggests a lower percentage of H2O than that reported by the Venera probes, while other blocks may be consistent with the probe data. These data therefore suggest that the distribution of water vapor down to atmospheric levels probed at these frequencies may vary with time or else the Venera probes may have detected localized enhancements in water vapor.

The Venus session was devoted exclusively to observations during the 1962 conjunction, including the Mariner radiometer results. F. T. Barath, A. H. Barrett, J. Copeland, D. E. Jones, and A. E. Lilley presented the results of the Mariner microwave measurements. Only the 19-mm data have been analyzed but they show definite and significant limb darkening, thus demonstrating that the source of the intense radio emission is the surface of Venus. After eliminating the limb darkening and dividing the resulting brightness temperature by an assumed surface emissivity, a fairly uniform actual surface temperature of about 700°K was obtained. T. Gold commented during the discussion that part of the limb darkening could be due to anisotropy in the surface emissivity. The authors agreed, and L. D. Kaplan pointed out that this would not invalidate the conclusion that a hot surface was the primary source of the cm emission.

The 20-micron brightness temperatures of the rings were determined using the 224-cm telescope of the Mauna Kea Observatory, and the standard University of Hawaii radiometer with a 17- to 25-micron filter. The observations were made on the nights of Aug. 20 and 21, and Sept. 26 and 27, 1972. The brightness temperatures of the A, B, and C rings are, respectively, 89 plus or minus 3 K, 94 plus or minus 2 K, and 89 plus or minus 4 K. A possible explanation of the relatively high temperature of the C ring is that Saturn has radiation belts and the inner ring is heated by particle bombardment.

Results of numerical experiments on model atmospheres obtained from radiosonde observations indicate that the contribution of the troposphere to the electrical path length at zenith is about 200 cm, and that this length can be inferred to an accuracy of 1 or 2 cm from ground-based microwave radiometric measurements.

Mariner 2 limb darkening measurements of Venus at microwave range, discussing peak brightness temperature and opacity models

Radiometric maps of brightness temperature contours on lunar disk from measurements of 3.3 mm wavelength thermal emission

Lunar thermal radiation measurements at 8.6 and 3.2 mm wavelengths, noting brightness temperature- time relationship in curves and as Fourier components

A statistical histogram method is developed to objectively determine sea-surface temperature from satellite high resolution window radiation measurements. The method involves inferring the distribution of surface radiances for the clear atmospheric case from observed histograms of generally cloud-contaminated radiances. The brightness temperature associated with the clear atmosphere modal peak radiance is the statistically most probable surface temperature. The reliability of the inferred surface temperature depends upon the number of cloud-free measurements available to define the clear mode. The method accounts for atmospheric attenuation and instrumental noise and also objectively discriminates cloud-free from cloud-contaminated observations. The statistical histogram method is applied to 3.8 micrometer window radiation data obtained by the High Resolution Infrared Radiometer flown on the Nimbus 2 and Nimbus 3 satellites. Examples of sea temperatures inferred over both small and large areas ar...

The infrared radiometer on Mariner 10 measured the thermal emission from the planet with a spatial resolution element as small as 40 kilometers in a broad wavelength band centered at 45 micrometers. The minimum brightness temperature (near local midnight) in these near-equatorial scans was 100 degrees K. Along the track observed, the temperature declined steadily from local sunset to near midnight, behaving as would be expected for a homogeneous, porous material with a thermal inertia of 0.0017 cal cm(-2) sec(-(1/2)) degrees K(-1), a value only slightly larger than that of the moon. From near midnight to dawn, however, the temperature fluctuated over a range of about 10 degrees K, implying the presence of regions having thermal inertia as high as 0.003 cal cm(-2) sec-(1/2) degrees K(-1).

The lunar radio observations and interpretations of Piddington and Minnett (1949) and Gibson (1958) show that the lunar brightness variation with phase at millimeter wave lengths can be used to determine the physical properties of the lunar surface. They found that the millimeter-wave brightness lagged the optical phase, and their interpretation was that the millimeter radiation originates below a surface layer that is a very good thermal insulator. The thickness of this layer could not be determined from observations at one frequency. Observations at different frequencies give different results because of the wave-length dependence of the radio absorption by the surface material. The attenuation in the material increases with decreasing wave length, and therefore it is possible, in principle, to determine surface layer thickness from radio observations at several wave lengths. For this reason, observations of lunar radiation were started at the Naval Research Laboratory at a wave length of 4.3 mm. This is half the wave length used by Gibson (1958) in his earlier studies. The radio telescope used for these observations has been described in detail in a previous publication (Coates 1958). The antenna is a parabolic reflector 10 feet in diameter, and it has a beam width of 6.7 minutes of arc at the wave length of 4.3 mm. This is about one-fifth the diameter of the moon. The receiver was a Dicke-type radiometer.