Richard K. Moore

Microwave Remote Sensing

The sections in this article are 1 Radiometers 2 Radar Scattering 3 Radar Scatterometers 4 Radar Altimeters 5 Ground-Penetrating Radars 6 Imaging Radars 7 Real-Aperture Radars 8 Synthetic-Aperture Radars

Scanning Spaceborne Synthetic Aperture Radar with Integrated Radiometer

Spaceborne synthetic aperture radar systems are severely constrained to a narrow swath by ambiguity limitations. Here a vertically scanned-beam synthetic aperture system (SCANSAR) is proposed as a solution to this problem. The potential length of synthetic aperture must be shared between beam positions, so the along-track resolution is poorer; a direct tradeoff exists between resolution and swath width. The length of the real aperture is independently traded against the number of scanning positions. Design curves and equations are presented for spaceborne SCANSARs for altitudes between 400 and 1400 km and inner angles of incidence between 20° and 40°. When the real antenna is approximately square, it may also be used for a microwave radiometer. The combined radiometer and synthetic-aperture (RADISAR) should be useful for those applications where the poorer resolution of the radiometer is useful for some purposes, but the finer resolution of the radar is needed for others.

An improved coherent radar depth sounder

The University of Kansas developed a coherent radar depth sounder during the 1980s. This system was originally developed for glacial ice-thickness measurements in the Antarctic. During the field tests in the Antarctic and Greenland, we found the system performance to be less than optimum. The field tests in Greenland were performed in 1993, as a part of the NASA Program for Arctic Climate Assessment (PARCA). We redesigned and rebuilt this system to improve the performance. The radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along-track resolution. It operates at a center frequency of 150 MHz with a radio frequency bandwidth of about 17 MHz, which gives a range resolution of about 5 m in ice. We have been operating it from a NASA P-3 aircraft for collecting ice-thickness data in conjunction with laser surface-elevation measurements over the Greenland ice sheet during the last 4 years. We have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice-thickness information over outlet glaciers and ice margins. In this paper we provide a brief survey of radar sounding of glacial ice, followed by a description of the system and subsystem design and performance. We also show sample results from the field experiments over the Greenland ice sheet and its outlet glacicrs.

Preliminary study of rain effects on radar scattering from water surfaces

Preliminary wave-tank results indicate that radar scatter from water surfaces is severely affected by rain at low but not at high wind speeds. The effect is governed by both the rain rate and droplet size. A simple experiment to check this phenomenon is described.

Active microwave measurement of soil water content

Abstract Measurements of radar backscatter from bare soil at 4.7, 5.9, and 7.1 GHz for incident angles of 0–70° have been analyzed to determine sensitivity to soil moisture. Because the effective depth of penetration of the radar signal is only about one skin depth, the observed signals were correlated with the moisture in a skin depth as characterized by the attenuation coefficient (reciprocal of skin depth). Since the attenuation coefficient is a monotonically increasing function of moisture density, it may also be used as a measure of moisture content over the distance involved, which varies with frequency and moisture content. The measurements show an approximately linear increase in scattering with attenuation coefficient of the soil at angles within 10° of vertical and all frequencies. At 4.7 GHz this increase continues relatively large out to 70° incidence, but by 7.1 GHz the sensitivity is much less even at 20° and practically gone at 50°. An inversion technique to determine how well the moisture content can be estimated from the scattered signal indicates good success for near-vertical angles and middle ranges of moisture density, with poorer success at smaller moisture densities and an anomaly in the data at the highest moisture density that must be resolved by further experimentation.

Four years of low-altitude sea ice broad-band backscatter measurements

The ability to use radar to discriminate Arctic Sea ice types has been investigated using surface-based and helicopter-borne scatterometer systems. The surface-based FM/CW radar operated at 1.5 GHz and at multiple frequencies in the 8-18-GHz region. Measurements were made at angles of 10\deg to 70\deg from nadir. The helicopter-based radar operated at the 8-18-GHz frequencies with incidence angles of 0\deg to 60\deg . Extensive surface-truth measurements were made at or near the time of backscattar measurement to describe the physical and electrical properties of the polar scene. Measurements in the 8-18-GHz region verify the ability to discriminate multiyear, thick first-year, thin first-year, and pressure-ridged sea ice and lake ice. The lowest frequency, 9 GHz, was found to provide the greatest contrast between these ice categories, with significant levels of separation existing between angles from 15\deg to 70\deg . The radar cross sections for like antenna polarizations, VV and HH, were very similar in absolute level and angular response. Cross-polarization, VH and HV, provided the greatest contrast between ice types, The 1.5-GHz measurements showed that thick first-year, thin first-year, and multiyear sea ice cannot be distinguished at 10\deg to 60\deg incidence angles with like polarization, VV, by backscatter alone; but that undeformed sea ice can be discriminated from pressure-ridged ice and lake ice. The effect of snow cover on the backscatter from thick first-year ice was also investigated. It contributes on the order of 0 to 4 dB, depending on frequency and incidence angle; the contribution of the snow layer increased with increasing frequency. Snow cover on smooth lake ice was found to be a major backscatter mechanism. Summer measurements demonstrate the inability to extend the knowledge of the backscatter from sea ice under spring conditions to all seasons.

Surface-Based Scatterometer Results of Arctic Sea Ice

Radar backscatter measurements were made of shorefast sea ice near Point Barrow, AK, in May 1977, with a surface-based FM-CW scatterometer that swept from 1-2 GHz and from 8.5-17.5 GHz. The 1-2 GHz measurements showed that thick first-year and multiyear ice cannot be distinguished at 10-70° incidence angles, but that undeformed sea ice can be discriminated from pressure ridges and lake ice. Results also indicate that frequencies between 8-18 GHz have the ability to discriminate between thick first-year, multiyear, and lake ice. Cross polarization was found to be a better discriminator than like polarization. In addition, at these latter frequencies the differential scattering cross section ¿° was found to have an approximately linearly increasing frequency response.

SIR-C/X-SAR observations of rain storms☆

Abstract The spaceborne imaging radar-C, X-band synthetic aperture radar observations of rain storms are the first multipolarization and multifrequency observations of precipitation from space. In addition to numerous, often dramatic images of severe weather systems obtained by forming a synthetic aperture in the usual side-looking attitude, several data takes were performed while the radar antennas were parallel to the ground and the radar beams were pointing at nadir. These opportunities coincided with the passage of the Shuttle over Tropical Cyclone Odille in the southern Indian Ocean during the first flight and over Typhoon Seth in the western Pacific during the second flight. The resulting observations, or, more appropriately, the resulting measurements, demonstrate for the first time the capability of a spaceborne multifrequency multipolarization microwave radar system to quantify precipitation rates, to detect hydrometeor phase, and to classify rain type.

Radio communication in the sea

Because of the electromagnetic properties of seawater, very-low-frequency communication systems are used, Surface-to-submarine, submarine-to-surface, and submarine-to-submarine propagation, as well as antennas and noise, are considered. It is shown that seawater is a good conductor and that atmospheric noise is generally more of a problem in sea communications than thermal noise. Communication ranges are limited by the effects of depth attenuation and atmospheric noise. Ranges in the tens of kilometers are possible for antennas within 5 meters of the surface; bandwidth must be less than 1 Hz and tens of kilowatts of power are required.

The Measurement of Precipitation with Synthetic Aperture Radar

Abstract The radar equation for the measurement of precipitation by SAR is identical to that for a conventional radar. The achievable synthetic beamwidth, βs, is proportional to σv/U, the ratio of the spread of the precipitation Doppler spectrum to the platform velocity. Thus, a small βs can be achieved only with small σv or from a fast-moving vehicle such as a spacecraft. Also, the along-track resolution is variable with σv and is not known. Nevertheless, the reflectivity is measured correctly. A possible approach to the measurement of σv is noted. The C-band SAR proposed for the Shuttle Imaging Radar-C (SIR-C) mission is capable of detecting a rain rate as small as 0.5 mm h−1 at nadir when the beam is filled. Because the cross-track beam dimension is about 20 km wide, we suggest use of a high-resolution microwave radiometer to correct for the unfilled beam and the variation of gain across it. Alternatively, the cross-track dimension should be decreased to no more than about 5 km by increasing the antenn...