Spaceborne radiometers operating near 1.4 GHz are the primary instrument for recent efforts to remotely sense nearsurface soil moisture around the globe. Generally, these instruments must contend with the effects of vegetation growing in the soil. However, an important first step is to model the measurements made by a radiometer that is viewing bare (vegetation-free) soil. The proposed model uses a matching layer and a random depolarizer to describe bare soil surface roughness and some aspects of antenna beamwidth. The model suggests that the effects of nearsurface soil moisture and roughness upon the radiometer measurement are more distinct than is currently thought. Furthermore, it appears that both moisture and roughness can be retrieved from a single set of radiometer measurements made at orthogonal linear polarizations. This retrieval precision is predicted to be poor at soil observation angles near nadir but improves for larger angles. At observation angles near 50°, the vertically polarized radiometer measurements are predicted to be nearly insensitive to roughness. A convenient parameterization of the model is provided and permits quick implementation.

The University of Waterloo scatterometer, which is a system developed for observation of snow and ice properties, is described. The system is composed of two frequency-modulated continuous-wave radars operating at center frequencies of 17.2 and 9.6 GHz. A field-deployable platform allows a rapid setup and observation at remote sites under harsh environmental conditions. A two-axis positioning system moves the radar beam across a user-programmable range of azimuth (±180°) and elevation angles (15°-105°). Typical azimuth scans of 60° angular width generate between 21 and 586 independent samples, depending on the wavelength and the elevation angle. The backscatter response of terrestrial snow in the Canadian Subarctic is demonstrated with two experiments conducted in Churchill, MB, Canada, between 2009 and 2011.

Microwave radiometer measurements of the Planck emission from soil can be used to estimate the near-surface soil moisture. A more straightforward and consistent model of this emission results if nonuniform, rather than uniform, plane waves are used. Adaptation of this new model to a layered medium representation for the soil is improved using a normalization that is based on the isothermal soil limit. The separate concepts of radiometer sensing depth and in situ sampling depth for soil moisture are examined and theory for the sensing depth is presented. Improved approximations (at 1413 MHz) to the full model for soil emission are developed since they are needed to construct algorithms that retrieve an estimate of soil moisture from the radiometer raw measurement. The error associated with the comparison of these remotely sense values against in situ measurements is calculated. Results suggest that this comparison error could be lowered if the soil moisture sampling depth was reduced to values less than 0.01 m from the currently used values near 0.02 m, although the effect of surface roughness has not yet been analyzed.

Methods described in the IEEE standards for frequency generator stability can be used to estimate the precision of brightness temperature measurements made by a microwave radiometer. The application of these methods and the inherent assumptions are reviewed. A simple statistic is proposed for use in estimating accuracy drift.

A portable surface-based polarimetric C-band scatterometer for field deployment over sea ice is presented. The scatterometer system, its calibration, signal processing, and near-field correction are described. The near-field correction is shown to be effective for both linear polarized and polarimetric backscatter. Field methods for the scatterometer are described. Sample linear polarized and polarimetric backscatter results are presented for snow-covered first-year sea ice (FYI), multiyear hummock ice, and rough melt pond water on FYI. The magnitude of backscatter signature variability due to system effects is presented, providing the necessary basis for quantitative analysis of field data.

The advent of fast computers and digital signal processing permits many aspects of radiometer operation to be decided in real time so that precision and accuracy are maximized. Closed-form expressions for precision, optimal antenna/load dwell times, and maximum chopping period are derived for a class of radiometers that uses two loads to calibrate its measurements of antenna brightness temperature. The importance of load brightness temperature selection emphasizes the need to develop "cold" calibration loads that can operate at ambient temperature. A method of incorporating critical thermistor data into the radiometer brightness temperature calibration can be used to help stabilize measurements against drift and may permit radiometers to operate without thermal control in some low-accuracy applications.

FOPAIR, a FOcused Phased Array Imaging Radar, provides high-resolution X-band images of the ocean surface. The system is designed to provide high-speed imagery (up to 180 frames/s) for short range applications (50-400 m) from a fixed platform such as a pier or tower. FOPAIR employs a fast, sequentially sampled antenna array and uses a software-based beamforming technique to generate high resolution imagery without the need for multiple radar receivers or beamforming hardware typical of active phased arrays. A summary of the principles of operation and the design of the instrument is given, followed by examples of FOPAIR's imaging capability. To the authors' knowledge, these examples include the highest-resolution, highest-speed microwave images of the ocean surface produced to date. A brief comparison between FOPAIR and synthetic aperture radar techniques is also included.>

Millimeter-wave radars have been used since the early 1950s to study clouds and precipitation, but until recently these early instruments were limited to simple backscatter power measurements and were plagued by hardware problems. However, development of solid-state millimeter-wave componentry and high-power klystron amplifiers has spurred the evolution of reliable, coherent radars operating up to 95 GHz. In addition, advances in digital signal processing technology have resulted in single-card processors that can simultaneously execute algorithms to compute reflectivity, Doppler, and polarimetric quantities in real time. A review of the current state of the art in millimeter-wave cloud radars is presented, including a discussion of transmitters, antennas, and receiver components. Two radar systems built by the University of Massachusetts are described, including a mobile, dual-frequency (33- and 95-GHz) polarimetric radar, and an airborne 95-GHz polarimetric radar that was recently flown in a cooperative experiment with the University of Wyoming. Spaceborne applications are also discussed, especially the use of satellite-based 95-GHz radars for measuring the vertical distribution of clouds.>

Polarimetric radar measurements of foliage made in the 220-GHz transmission window are described. A noncoherent, 60-W-peak-power polarimetric radar was used to measure the target Mueller matrix directly by measuring the scattered polarization state for either four or six incident polarizations. Measurements of deciduous trees indicate that waves scattered from erectophil trees (vertically oriented leaves) are more highly polarized than waves scattered from planophil trees (horizontally oriented leaves) for near-grazing incidence angles. White pine trees, which have thin needles, where the least polarized of all trees studied. A comparison of the Mueller matrices of 11 independent footprints taken from seven white pine trees showed a high degree of consistency between measurements.>