J.D. Eastment

Cloudnet: Continuous Evaluation of Cloud Profiles in Seven Operational Models Using Ground-Based Observations

Cloud fraction, liquid and ice water contents derived from long-term radar, lidar and microwave radiometer data are systematically compared to models to quantify and improve their performance.

Radar Scattering from Ice Aggregates Using the Horizontally Aligned Oblate Spheroid Approximation

AbstractThe assumed relationship between ice particle mass and size is profoundly important in radar retrievals of ice clouds, but, for millimeter-wave radars, shape and preferred orientation are important as well. In this paper the authors first examine the consequences of the fact that the widely used “Brown and Francis” mass–size relationship has often been applied to maximum particle dimension observed by aircraft Dmax rather than to the mean of the particle dimensions in two orthogonal directions Dmean, which was originally used by Brown and Francis. Analysis of particle images reveals that Dmax ≃ 1.25Dmean, and therefore, for clouds for which this mass–size relationship holds, the consequences are overestimates of ice water content by around 53% and of Rayleigh-scattering radar reflectivity factor by 3.7 dB. Simultaneous radar and aircraft measurements demonstrate that much better agreement in reflectivity factor is provided by using this mass–size relationship with Dmean. The authors then examine t...

Self-consistent measurements of differential phase and differential reflectivity in rain

The addition of polarisation capabilities to meteorological radars has considerably increased their potential applications. However, such radars still require careful calibration of absolute power to achieve their full potential. This paper describes a technique which utilises a combination of differential reflectivity and differential phase measurements in rain to achieve a self-consistent calibration. The method is illustrated using data obtained with the 3 GHz Chilbolton radar, which is capable of measuring five parameters: reflectivity, differential reflectivity, linear depolarisation ratio, Doppler velocity and differential phase. The ability of the calibrated radar to measure rain fall rates is demonstrated by comparison with rain gauge and distrometer data, while accurate microwave attenuation estimates are shown with reference to comparisons with 12 GHz and 20 GHz beacon measurements made with the Olympus communications satellite.<<ETX>>

A vertically-pointing Doppler radar to measure precipitation characteristics in the tropics

Describes the design and preliminary results from a vertically-pointing pulsed S-band Doppler radar, capable of measuring reflectivity (dBZ), linear depolarization ratio (LDR), and the Doppler spectrum. The data reported were acquired during a short experimental campaign while the radar was sited in Southern UK. The system has now been sent to Papua New Guinea for a 12 month programme of statistical measurements on the nature of tropical rainfall. The results from this study will be applicable to communications system engineering in the tropics, and in providing ground truth for satellite-based rainfall measurements for global climatic modeling.

Interference Mitigation for Multi Spot Beam Satellite Communication Systems Incorporating Spread Spectrum

Nonlinear precoding techniques have robust transmit power stability and achieve superior interference suppression when compared to their linear counterparts. Tomlinson-Harashima Precoding (THP) is a suboptimal version of Costa’s well-known work on writing on dirty paper (DPC). Implementing these precoding techniques in a multi spot beam satellite communications system that employs frequency reuse can significantly reduce co-channel interference (CCI). In this paper, we investigate and compare the performance of linear and nonlinear precoding techniques on the forward link of a multiple spot beam satellite link. In addition, we examine the potential benefits of integrating the novel spread spectrum (SS) technique with the existing precoding techniques. The new system’s performance is evaluated and compared with that of standard precoding techniques, and the benefits of incorporating SS are weighed against the extra bandwidth requirements.

Ground-based aviation weather radar research at the Rutherford Appleton Laboratory and University College London

We present an overview of recent collaborative research, involving RAL and UCL, in the field of ground-based meteorological radar systems for weather assessment at, and in the terminal manoeuvring area (TMA) surrounding, civil airports. Three projects are briefly described. Firstly, we present the results from a design study intended to develop the specifications for a dedicated C-band weather surveillance radar covering an airport TMA. Secondly, we discuss the design of, and present some measured data from, a recently installed S-band weather radar system located at St. Andrews in Scotland. This low-cost radar uses a small antenna and low transmitted power, but is capable of providing a high-resolution display of significant precipitation within a coverage area extending to 60 km range. High sensitivity is achieved by coherent signal processing techniques. This system demonstrates that relatively inexpensive TMA weather surveillance using radar techniques is a practical proposition. Finally, we illustrate the potential of a vertically pointing millimetre-wave radar to remotely sense the height-distribution of cloud above an airport runway. Design details of a 94 GHz cloud-profiling radar system are presented, and examples of measured data from this instrument are used to show its capabilities for automatic detection of cloud-base height and cloud depth.

Melting-layer detection in tropical precipitation using polarimetric and Doppler radar signatures

Accurate mathematical models of microwave propagation over Earth-space paths are essential for successful design of satellite communications schemes. Whilst reliable models exist for temperate climates, little data are available to validate prediction procedures for the tropics. As satcomms terminals are rapidly proliferating in these regions, there is an urgent requirement to test current ITU-R recommendations against actual measurements. The most significant propagation impairments are attenuation, depolarisation and scattering, causing reduced circuit availability, and potentially resulting in cochannel interference. Though these effects are experienced at temperate latitudes, they are particularly severe in the tropics, due to the intensity of precipitation. We present design considerations for, and initial observations from, a polarimetric-Doppler radar deployed in Papua New Guinea for research on propagation effects caused by intense tropical precipitation. (5 pages)

A vertically-pointing Doppler radar to study the precipitation characteristics in the tropics

In order to study the precipitation characteristics in tropical regions, the Rutherford Appleton Laboratory (RAL) is currently engaged in an experimental programme to develop a 3 GHz vertically-pointing pulsed Doppler radar, capable of measuring the co-polar reflectivity (2), cross-polar reflectivity (LDR) and the full Doppler spectrum. The magnetron-based radar has been designed and built and is now undergoing tests to investigate the optimum signal processing algorithms for fast and efficient processing of pulse-to-pulse data.

An S-band Doppler radar to measure precipitation characteristics in the tropics

Describes the design and preliminary results from a vertically-pointing pulsed S-band Doppler radar, capable of measuring reflectivity (Z), linear depolarisation ratio (LDR), and the Doppler spectrum. The data reported were acquired during a short experimental campaign while the radar was sited at Rutherford Appleton Laboratory. In 1995, the system will be transported to Papua New Guinea for a 12 month programme of statistical measurements on the nature of tropical rainfall. The results from that study will be applicable to communications system engineering in the tropics, and in providing ground truth for satellite-based rainfall measurements for global climatic modelling. (6 pages)