Both the military and consumer sectors are pursuing distributed networked systems and sensors. A major stumbling
block to deployment of these sensors will be the radio frequency (RF) propagation environment within a few
wavelengths of the earth. Increasing transmit power (battery consumption) is not a practical solution to the problem. This
paper will discuss some of the physical phenomena related to the near earth propagation (NEP) problem. When radiating
near the earth the communications link is subjected to a list of physical impairments. On the list are the expected Fresnel
region encroachment and multipath reflections. Additionally, radiation pattern changes and near earth boundary layer
perturbations exist. A significant amount of data has been collected on NEP. Disturbances in the NEP atmosphere can
have a time varying attenuation related to the time of day and these discoveries will be discussed. Solutions, or
workarounds, to the near earth propagation problem hinge on dynamic adaptive RF elements. Adaptive RF elements will
allow the distributed sensor to direct energy, beam form, impedance correct, increase communication efficiency, and
decrease battery consumption. Small electrically controllable elements are under development to enable antenna
impedance matching in a dynamic environment. Additionally, small dynamic beam forming arrays are under
development to focus RF energy in the direction of need. With an increased understanding of the near earth propagation
problem, distributed autonomous networked sensors can become a reality within a few centimeters of the earth.
Daylight readability of hand-held displays has been an ongoing issue for both commercial and military applications. In an effort to reduce the effects of ambient light on the readability of military displays, the Naval Research Laboratory (NRL) began investigating and developing advanced hand-held displays. Analysis and research of display technologies with consideration for vulnerability to environmental conditions resulted in the complete design and fabrication of the hand-held Immersive Input Display Device (I2D2) monocular. The I2D2 combines an Organic Light Emitting Diode (OLED) SVGA+ micro-display developed by eMagin Corporation with an optics configuration inside a cylindrical housing. A rubber pressure-eyecup allows view ability only when the eyecup is depressed, eliminating light from both entering and leaving the device. This feature allows the I2D2 to be used during the day, while not allowing ambient light to affect the readability. It simultaneously controls light leakage, effectively eliminating the illumination, and thus preserving the tactical position, of the user in the dark. This paper will examine the characteristics and introduce the design of the I2D2.
Both the military and consumer sectors are driving towards distributed networked sensors. A major
stumbling block to deployment of these sensors is the radio frequency (RF) propagation environment
within a few wavelengths of the earth. Increasing transmit power (battery consumption) is not the practical
solution to the problem. This paper will discuss some aspects of the near earth propagation (NEP) problem
and provide a few solutions. When radiating near the earth the communications link is subjected to a list of
physical impairments. On the list are the expected Fresnel region encroachment and multipath reflections
along with the intriguing radiation pattern changes and near earth boundary layer perturbations. A
significant amount of data has been collected on NEP. Disturbances in the NEP atmosphere have a time
varying attenuation related to the solar radiation (insolation). Solutions, or workarounds, to the near earth
propagation problem hinge on dynamic adaptive RF elements. Adaptive RF elements will allow the
distributed sensor to direct energy, beam form, impedance correct, increase communication efficiency, and
decrease battery consumption. Small electrically controllable elements are under development to enable
antenna impedance matching in a dynamic environment. Additionally, small dynamic beam forming
antennas will be developed to focus RF energy in the direction of need. By creating provisions for
decreasing the output RF power to the level required, battery consumption can be reduced. With the
addition of adaptive RF elements, distributed autonomous networked sensors can become a reality within a
few centimeters of the earth.
Networks of small ground sensors and other near earth devices deployed in the battlefield are postulated to be of considerable value to the future warfighter. The radio frequency (RF) link between devices will dictate the resilience of the network in communicating critical information in the battlespace. A prior knowledge of the RF environment inches above the ground is required to properly design the sensor network. Signal strength was measured with antennas at 4, 7, and 120 inches above the ground over a range of 10 to 400 feet. The source consisted of a 1780 MHz, 1/4 watt transmitter feeding a quarter wave vertical monopole. The receive equipment consisted of a corner reflector monopole, spectrum analyzer and data logger program. Data points were taken at 10-foot increments over the 400-foot range. The received signal, at heights of 4 and 7 inches, were compared to the measurements taken at a height of 120 inches (close to “free space”). It was found that there is a significant increase in path loss as the antenna approached the ground. There was a 15 dB increase in path loss from when the antennas were at 120 inches to 7 inches off the ground and 18 dB increase in path loss with the antenna 4 inches off the ground. Variations in path loss (10 dB) over time (seconds) were also noted.
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