Terahertz wireless communication

The goal of this thesis is to explore Terahertz (THz) wireless communication technology. More specifically the objective is to develop and characterize several THz communication systems and study the effect of atmosphere propagation through fog droplets and dust particles on THz communications. For demonstration, a THz continuous wave (CW) photomixing system is designed. Terahertz signals are phase encoded with both analog ramp signals and pseudorandom binary data, transmitted over a short distance, and detected. The limitation of transmission bandwidth, low single to noise ratio, vibration effects are also analyzed. In order to study and compare propagation features of THz links with infrared (IR) links under different weather conditions, a THz and IR communications lab setup with a maximum data rate of 2.5 Gb/s at 625 GHz carrier frequency and 1.5 gm wavelength, have been developed respectively. A usual non return-to-zero (NRZ) format is applied to modulate the IR channel but a duobinary coding technique is used for driving the multiplier chain-based 625 GHz source, which enables signaling at high data rate and higher output power. The bit-error rate (BER), signal-to-noise ratio (SNR) and power on the receiver side have been measured, which describe the signal performance. Since weather conditions such as fog and dust exhibit a spectral dependence in the atmospheric attenuation, the corresponding impact on THz in comparison with IR communications is not equivalent. Simulation results of attenuation by fog and dust in the millimeter and sub-millimeter waveband (from 0.1 to 1 THz) and infrared waveband (1.5 µm) are presented and compared. Experimentally, after THz and IR beams propagated through the same weather conditions (fog), performance of both channels are analyzed and compared. The attenuation levels for the IR beam are typically several orders of magnitude higher than those for the THz beam. Mie scattering theory was used to study the attenuation of THz and IR radiation due to the dust particle. Different amounts of dust are loaded in the chamber to generate a variety of concentration for beam propagation. As the dust loading becomes heavier, the measured attenuation becomes more severe. Under identical dust concentrations, IR wavelengths are strongly attenuated while THz shows almost no impact

The development of a stepped frequency microwave radiometer and its application to remote sensing of the Earth

The design, development, application, and capabilities of a variable frequency microwave radiometer are described. This radiometer demonstrated the versatility, accuracy, and stability required to provide contributions to the geophysical understanding of ocean and ice processes. A closed-loop feedback method was used, whereby noise pulses were added to the received electromagnetic radiation to achieve a null balance in a Dicke switched radiometer. Stability was achieved through the use of a constant temperature enclosure around the low loss microwave front end. The Dicke reference temperature was maintained to an absolute accuracy of 0.1 K using a closed-loop proportional temperature controller. A microprocessor based digital controller operates the radiometer and records the data on computer compatible tapes. This radiometer exhibits an absolute accuracy of better than 0.5 K when the sensitivity is 0.1 K. The sensitivity varies between 0.0125 K and 1.25 K depending upon the bandwidth and integration time selected by the digital controller. Remote sensing experiments were conducted from an aircraft platform and the first radiometeric mapping of an ocean polar front; exploratory experiments to measure the thickness of lake ice; first discrimination between first year and multiyear ice below 10 GHz; and the first known measurements of frequency sensitive characteristics of sea ice

Millimetre wave remote sensing of the atmosphere

Recent advances in millimetre wave technology has opened up a new region of the spectrum to remote sensing from artificial satellites. The main part of this work involves a millimetre wave proving experiment for a satellite borne millimetre wave active sounder to measure surface pressure over the oceans. The Microwave Pressure Sounder is a6 channel, low power radar operating in the spectral range from 24 to 75 GHz. The strength of the return echoes from the sea surface determines the amount of oxygen in the path which can be directly related to the surface pressure to an accuracy of 1 mb, when corrected for sea surface reflectivity and atmospheric temperature and water content by this multichannel instrument. Measurements of atmospheric attenuation along a horizontal path were related to atmospheric pressure changes by a millimetre wave instrument built at Heriot-Watt University. The transmissometer measured the differential absorption between two frequencies (54 and 58 GHz) over a 650 metre path. The deduced atmospheric pressure was found to compare with the barometric pressure with a standard deviation of two millibars for the best data set. These results demonstrate that atmospheric attenuation can be measured with sufficient precision for a satellite borne instrument to determine the surface atmospheric pressure over the oceans to an accuracy of approximately one millibar. This accuracy would lead to significant improvements in the modelling of the atmosphere and weather forecasting. Various other techniques to remotely sense surface atmospheric pressure are reviewed. Recently, increased awareness of the sensitivity of the environment and evidence of the effects of man-made pollutants has given rise to an increased awareness in the health of the Earth and led to several instruments being developed to monitor our planet. One of these instruments, the Microwave Limb Sounder to be flown on the Upper Atmosphere Research Satellite (launch October 1991) is described. This instrument uses millimetre wave radiometers at 63 GHz, 183 GHz and 205 GHz to measure the amount of chlorine oxide, ozone and water vapour in the upper atmosphere. These gases are important in understanding the photochemistry of the mesosphere. Global distributions of the gases will be produced and changes in concentration will be monitored during the three year mission

Laser Based Altimetry for Unmanned Aerial Vehicle Hovering Over a Snow Surface

A microwave radar for non-invasive snow stratigraphy measurements has been developed. Results were promising, but it failed to detect light powder snow in the air-snowpack interface. The aim of this thesis is to find and verify a system for estimating altitude on centimeter scale over a snow surface, independent of snow conditions. Also, relative pitch and roll angle estimation between the UAV and local surface should be resolved, to help directing the radar beam perpendicularly to the surface. After a variety of technical solutions were examined, we propose a system of three time-of-flight near-infrared altimeters pointing at different directions towards the surface. Experimental results showed RMS error of 1.39 cm for range estimation averaged over the most common snow types, and 2.81 cm for wet snow, which was the least reflective medium. An experiment conducted for an array of two altimeters scanning over a snow surface, showed that the local, relative surface tilt was found to be accurate within ±2° given that it was sufficiently planar. Further, the altitude RMS error was estimated to 1.57 cm. We conclude that the chosen altimeter was within the requirements, and that an array of three altimeters would give acceptable relative tilt estimation in to planes on the snow surface. The system should be subject to flight testing and implemented on UAV platform such that it can aid the microwave radar system during snow scanning

Shuttle imaging radar-C science plan

The Shuttle Imaging Radar-C (SIR-C) mission will yield new and advanced scientific studies of the Earth. SIR-C will be the first instrument to simultaneously acquire images at L-band and C-band with HH, VV, HV, or VH polarizations, as well as images of the phase difference between HH and VV polarizations. These data will be digitally encoded and recorded using onboard high-density digital tape recorders and will later be digitally processed into images using the JPL Advanced Digital SAR Processor. SIR-C geologic studies include cold-region geomorphology, fluvial geomorphology, rock weathering and erosional processes, tectonics and geologic boundaries, geobotany, and radar stereogrammetry. Hydrology investigations cover arid, humid, wetland, snow-covered, and high-latitude regions. Additionally, SIR-C will provide the data to identify and map vegetation types, interpret landscape patterns and processes, assess the biophysical properties of plant canopies, and determine the degree of radar penetration of plant canopies. In oceanography, SIR-C will provide the information necessary to: forecast ocean directional wave spectra; better understand internal wave-current interactions; study the relationship of ocean-bottom features to surface expressions and the correlation of wind signatures to radar backscatter; and detect current-system boundaries, oceanic fronts, and mesoscale eddies. And, as the first spaceborne SAR with multi-frequency, multipolarization imaging capabilities, whole new areas of glaciology will be opened for study when SIR-C is flown in a polar orbit

Airborne Wind Shear Detection and Warning Systems. Second Combined Manufacturers' and Technologists' Conference, part 1

The Second Combined Manufacturers' and Technologists' Conference hosted jointly by NASA Langley (LaRC) and the Federal Aviation Administration (FAA) was held in Williamsburg, Virginia, on October 18 to 20, 1988. The purpose of the meeting was to transfer significant, ongoing results gained during the second year of the joint NASA/FAA Airborne Wind Shear Program to the technical industry and to pose problems of current concern to the combined group. It also provided a forum for manufacturers to review forward-look technology concepts and for technologists to gain an understanding of the problems encountered by the manufacturers during the development of airborne equipment and the FAA certification requirements

Analytical evaluation of ILM sensors, volume 1

The functional requirements and operating environment constraints are defined for an independent landing monitor ILM which provides the flight crew with an independent assessment of the operation of the primary automatic landing system. The capabilities of radars, TV, forward looking infrared radiometers, multilateration, microwave radiometers, interferometers, and nuclear sensing concepts to meet the ILM conditions are analyzed. The most critical need for the ILM appears in the landing sequence from 1000 to 2000 meters from threshold through rollout. Of the sensing concepts analyzed, the following show potential of becoming feasible ILM's: redundant microwave landings systems, precision approach radar, airborne triangulation radar, multilateration with radar altimetry, and nuclear sensing

The Development of a Stepped Frequency Microwave Radiometer and Its Application to Remote Sensing of the Earth

The design, development, application, and capabilities of a variable frequency microwave radiometer are described. This radiometer has demonstrated the versatility, accuracy, and stability required to provide contributions to the geophysical understanding of ocean and ice processes. The design technique utilized a closed-loop feedback method, whereby noise pulses were added to the received electromagnetic radiation to achieve a null balance in a Dicke switched radiometer. Stability was achieved through the use of a constant temperature enclosure around the low loss microwave front end. The Dicke reference temperature was maintained to an absolute accuracy of 0.1 K using a closed-loop proportional temperature controller. Versatility was achieved by developing a microprocessor based digital controller which operates the radiometer and records the data on computer compatible tapes. Accuracy analysis has shown that this radiometer exhibits an absolute accuracy of better than 0.5 K when the sensitivity is 0.1 K. The sensitivity varies between 0.0125 K and 1.25 K depending upon the bandwidth and integration time selected by the digital controller. Computational techniques were developd to (1) predict the radiometric brightness temperature at the input to the radiometer antenna as a function of the geophysical parameters, (2) compute the required input radiometric brightness temperature as a function of the radiometer output using a mathematical model of the radiometer, (3) achieve computational efficiency through a simplified algorithm to determine the expected radiometric brightness temperature, and (4) calculate the emissivity of a layered dielectric media such as ice over water. The effects of atmospheric absorption due to oxygen, water vapor, nonprecipitating clouds have been included. Correction factors for the finite antenna beamwidth, surface roughness, and wind induced foam were employed in these computations. Remote sensing experiments were conducted from an aircraft platform using this radiometer. The purpose of these experiments was to demonstrate that the accuracy and versatility of this instrument had been achieved in actual field experiments. Four significant scientific observations were accomplished during these experiments. These observations consisted of the first radiometric mapping of an ocean polar front, exploratory experiments to measure the thickness of lake ice, first discrimination between first year and multiyear ice below 10 GHz, and the first known measurements of frequency sensitive characteristics of sea ice

Tropical oceanic precipitation frequency from Nimbus 5 microwave data

Microwave brightness temperature data from the Nimbus 5 satellite are analyzed, using threshold brightness temperatures, to yield tropical oceanic precipitation frequencies for several classes of rainfall rates during the season December 1972 through February 1973. Data taken near local noon and near local midnight are analyzed. The overall results are consistent with both climatological precipitation frequency and with concurrent satellite-derived frequency of highly reflective clouds. The difference between the local noon and local midnight frequencies is small, but the heavier rainfall rates tend to occur more frequently near local noon. The ratios of the frequencies of light, moderate, and heavy rain were observed to be relatively constant over the tropical oceans. Passive microwave measurements from space seem to be an important step toward accurate measurement of oceanic precipitation

The Second Spaceborne Imaging Radar Symposium

Summaries of the papers presented at the Second Spaceborne Imaging Radar Symposium are presented. The purpose of the symposium was to present an overwiew of recent developments in the different scientific and technological fields related to spaceborne imaging radars and to present future international plans