Roland W. Lawrence

The fabrication and electrical properties of carbon nanofibre?polystyrene composites

Carbon nanofibre?polystyrene (PS) composites were fabricated by ultrasonic dispersion in a solution followed by spraying to cast films. These films were then hot-pressed to form thicker structures. Direct current conductivity measurement results show that the conductivity of the composite reaches 2.6 ? 10?5?S?m?1 at 3?wt% carbon nanofibre loading, which is an increase of ten orders of magnitude over the pure PS matrix, indicating that the composite is electrically conductive at a very low nanofibre loading. The dielectric properties of carbon nanofibre?PS composites were investigated at room temperature within the Ku-band (frequency range: 12.4?18?GHz). The results reveal that the dielectric constants of the composites are slightly dependent on the frequency but increase rapidly with increasing carbon nanofibre loading in the composite. The experimental data show that the dielectric constant of carbon nanofibre?PS composite can reach more than 80 at a frequency of 15?GHz.

Electrical Conductivity and Electromagnetic Interference Shielding of Multi-walled Carbon Nanotube Filled Polymer Composites

Multi-walled carbon nanotube (MWNT) filled polystyrene (PS) composites were synthesized for electromagnetic interference (EMI) shielding applications. SEM images of composites showed the formation of the conducting networks through MWNTs within the PS matrix. The measured DC conductivity of composites increased with increasing MWNT loading, showing a typical percolation behavior. EMI shielding characteristics of MWNT-PS composites were investigated in the frequency range of 8.2–12.4 GHz (X-band). It was observed that the shielding effectiveness (SE) of such composite increased with the increase of MWNT loading. The SE of the composite containing 7 wt% MWNTs could reach more than 26 dB in the measured frequency region.

Radiometric characterization of mesh reflector material for deployable real aperture remote sensing applications

The simultaneous requirement for long wavelength and high spatial resolution for important geophysical remote sensing application, such as soil moisture and salinity, suggest the use of large to moderate sized deployable reflectors as a possible solution. Many studies have recognized the potential advantages of mesh reflectors. However, the emissivity of the reflector and its stability are major concerns and must the carefully considered to enable the advantages of deployable reflector technology for remote sensing applications. This paper will describe an advanced measurement system to evaluate the emissivity of reflector materials for these applications.

Microwave radiometer sensor technology research for Earth science measurements

A research program has been initiated at NASA Langley Research Center to investigate the critical technologies for developing advanced microwave radiometers suitable for Earth science observations. A significant objective of this research is to enable microwave measurements with adequate spatial resolutions for a number of Earth science parameters, such as sea ice, precipitation, soil moisture, sea surface temperature, and wind speed over oceans. High spatial resolution microwave sensing from space with reasonable swath widths and revisit times favor large real aperture radiometer systems. However, the size requirements for such systems are in conflict with the need to emphasize small launch vehicles. This paper describes a tradeoff between the science requirements, basic operational parameters, test configurations, and expected sensor performance for a satellite radiometer concept. The preliminary designs of real aperture systems utilizing novel light-weight compact-packaging techniques are used as a means of demonstrating this technology.

Measurement of calibration stability of radiometer sensor systems

The desire for passive microwave measurements of improved spatial resolution and the development of radiometer and antenna technologies have resulted in several studies to investigate large aperture radiometer systems. These systems may utilize aperture synthesis, phased array, or reflectors and phased array feeds. The in-flight calibration and thus the stability of these systems is an important consideration in any realistic study. Thus, estimates of stability and calibration requirements for these complex radiometer systems are of interest. A statistical method to characterized the stability of microwave radiometer components and subsystems which could aid in the development of numerical models to predict the stability of these radiometer systems is presented in this paper. Preliminary measurements results to demonstrate the utility and limitations of the approach are also presented.

Development Of Microwave Wiometer Sensor Technology For Geostationary Earth Science Platforms

A new research and technology program has been initiated at the Langley Research Center of the National Aeronautics and Space Administration (NASA) for developing advanced, High Resolution Microwave Radiometer (HI-RES) sensors suitable for Mission to Planet Earth (MPE) remote sensing applications. This research program is sponsored by the NASA Office of Aeronautics, Exploration, and Technology (OAET) with a goal to provide the technology needed to enable and enhance the long-term observations, documentation, and understanding of the Earth as a system. In order for the technology to be relevant and useful, the program objectives and scheduled milestones are in support of the science and technology needs advocated by the Office of Space Science and Applications (OSSA). BACKGROUND

Prototype cryospheric experimental synthetic aperture radiometer (CESAR)

Present satellite microwave radiometers typically have a coarse spatial resolution of several kilometers or more. This is only adequate only over homogenous areas. Significantly enhanced spatial resolution is critically important to reduce the uncertainty of estimated cryospheric parameters in heterogeneous and climatically-sensitive areas. Examples include: (1) dynamic sea ice areas with frequent lead and polynya developments and variable ice thicknesses, (2) mountainous areas that require improved retrieval of snow water equivalent, and (3) melting outlet glacier or ice shelf areas along the coast of Greenland and Antarctica. For these situations and many others, an Earth surface spot size of no more than 100 m is necessary to retrieve the information needed for significant new scientific progress, including the synthesis of field observations with satellite observations with high confidence. At Goddard Space Flight Center, active/passive microwave remote sensing calibration and validation programs have resulted in instrumentation that uses the underside of the fuselage and wing space of Uninhabited Aerial Vehicles (UAVs) as small remote sensing platforms. This research takes advantage of prototype antennas developed for an L-Band frequency survey (nanosat), instrumentation developed for soil moisture and salinity measurements (RadSTAR), and finally a mission concept to study sea ice topography and its interaction with snow layers. The Cryospheric Experimental Synthetic Aperture Radiometer (CESAR). CESAR is a NASA proposal to fly K-Band and Ka-Band thinned arrays on an Uninhabited Aerial Vehicle (UAV) in order to measure at 100 meter ground resolution. Prior to the flight of CESAR, a prototype CESAR will be tested with elements that will fly at lower altitudes to begin the system level testing of the synthetic array with commercial-off-the-shelf (COTS) components. The prototype 4-element CESAR will precede the 10 element CESAR, and an L-Band version (Little CESAR) will precede that by using Ultrastable Radiometer (USR) components and techniques from the previous SBIR, and RadSTAR research. The K-Band and Ka-Band COTS prototype receivers will be augmented with monolithic microwave integrated circuits (MMICs) and multi-chip module receivers to be developed at Colorado State University. These miniaturized MMIC-based receivers have been designed to be installed inside the wing of the CESAR UAV, a specialized vehicle developed specifically for this high spatial resolution research. The antenna element positions along the wing will be monitored to allow for correction in software of deviations from a planar collecting array. In addition to the MMIC-based receivers, CESAR will employ a correlator chip developed for the Lightweight Rainfall Radiometer (LRR). This rad-hardened low power and low mass system was developed for Synthetic Thinned Array Radiometer (STAR) systems where mass and power are minimized to result in the largest collecting aperture possible on a given platform.

Mission concept for the remote sensing of the cryosphere using autonomous aerial observation systems

Improving the understanding of the Cryosphere and its impact on global hydrology is an important element of NASA’s Earth Science Enterprise (ESE). A Cold Land Processes Working Group (CLPWG) was formed by the NASA Terrestrial Hydrology Program to identify important science objectives necessary to address ESE priorities. These measurement objectives included Snow Water Equivalent (SWE), snow wetness, and freeze/thaw status of underlying soil. The spatial resolution requirement identified by the CLPWG was 100 m to 5000 m. Microwave sensors are well suited to measure these and other properties of interests to the study of the terrestrial cryosphere. It is well known that the EM properties of snow and soil at microwave frequencies are a strong function of the phase of water, i.e. ice/water. Further, both active and passive microwave sensors have demonstrated sensitivity to important properties of snowpack including, depth, density, wetness, crystal size, ice crust layer structure, and surface roughness. These sensors are also sensitive to the underlying soil state (frozen or thawed). Multiple microwave measurements including both active and passive sensors will likely be required to invert the effects of various snowpack characteristics, vegetation, and underlying soil properties to provide the desired characterization of the surface and meet the science needs required by the ESE. A major technology driver with respect to fully meeting these measurement needs is the 100 to 5000 m spatial resolution requirement. Meeting the threshold requirement of 5000 m at microwave frequencies from Low Earth Orbit is a technology challenge. The emerging capabilities of unmanned aircraft and particularly the system perspective of the Autonomous Aerial Observation Systems (AAOS) may provide high-fidelity/high-resolution measurements on regional scales or larger that could greatly improve our measurement capability. This paper explores a vehicle/sensor concept that could augment satellite measurements to enhance our understanding of the Cryosphere. The measurement performance and technology issues related to the sensor and aircraft will be assessed. Finally, specific technology needs and research necessary to enable this AAOS concept will be discussed.

Multidimensional analysis of autonomous aerial observation systems (AAOS) for scientific, civil, and defense applications

Better knowledge of the atmosphere, ocean and land are needed by a wide range of users spanning the scientific, civil and defense communities. Observations to provide this knowledge will require aerial systems with greater operational flexibility and lower life-cycle costs than are currently available. Persistent monitoring of severe storms, sampling and measurements of the Earth’s carbon cycle, wildfire monitoring/management, crop assessments, ozone and polar ice changes, and natural disaster response (communications and surveillance) are but a few applications where autonomous aerial observations can effectively augment existing measurement systems. User driven capabilities include high altitude, long range, long-loiter (days/weeks), smaller deployable sensor-ships for in-situ sampling, and sensors providing data with spectral bandwidth and high temporal and three-dimensional spatial resolution. Starting with user needs and considering all elements and activities required to acquire the needed observations leads to the definition of autonomous aerial observation systems (AAOS) that can significantly complement and extend the current Earth observation capability. In this approach, UAVs are viewed as only one, albeit important, element in a mission system and overall cost and performance for the user are the critical success factors. To better understand and meet the challenges of developing such AAOSs, a systems oriented multi-dimensional analysis has been performed that illuminates the enabling and high payoff investments that best address the needs of scientific, civil, and defense users of Earth observations. The analysis further identifies technology gaps and serves to illustrate how investments in a range of mission subsystems together can enable a new class of Earth observations.