A transceiver has been designed for an Unattended Ground Sensor (UGS) system which meets several key requirements
that are shared by numerous sensor networks. The communication subsystem can be configured quickly through the
use of secure, low-power RF links. An efficient media access layer provides low latencies during high traffic scenarios.
A robust networking algorithm allows the network to adapt to changes in the RF environment, including remote
jamming. A low power consumption hardware design allows each unit to achieve long mission durations while
operating off of limited battery resources. A highly efficient RF front end allows for several kilometer link ranges to be
achieved with ground level antennas. The transceiver hardware conforms to a small form factor designed for high
volume production with low per-unit cost. While the transceiver was specifically designed for a particular system, the
hardware platform can easily be configured to suit a variety of sensor applications. Because waveform modulation and
demodulation are executed via digital signal processing, changes to modulation technique and data rate can be
accommodated. The DSP and PLD based digital architecture provides a software definable radio platform that serves as
a mature and tested alternative to the developmental JTRS Cluster 5 radio.
Unatttended Ground Sensor (UGS) systems are adversely affected by the physics of RF propagation at low elevations. Units are often located at or below ground level in an effort to reduce the visual signature. Two key elements of the link budget work are compromised by the ground level antenna height: path loss and antenna gain.
A two-ray reflected path model predicts that path loss increases by R4 with distance separating units and is inversely
related to the height of the antennas. Strict adherence to this model indicates that infinite path loss is incurred by
antennas located at the ground (height = 0). Ground wave propagation allows for a minimum effective antenna height to be assigned thereby eliminating this theoretical anomaly. Regardless, ground reflections for low elevation RF propagation result in up to 40 dB or more of extra path losses when compared to the free space model. Antenna modeling programs used to predict antenna patterns also paint a grim picture for UGS units. Monopole antennas have a null at the horizon and can be predicted to have antenna gains less than -20 dBi at low takeoff angles. Combining path loss and antenna gain to obtain an overall picture of the decrease in signal level is not straightforward. Simply combining the two sources of signal loss results in predicted performance that is much less than what is
measured with real hardware. This paper examines the interaction of path loss and antenna gain and presents a reasonable approach for accounting for both in the link budget. Experimental results are presented to support the models.
Unattended Ground Sensors (UGS) have proven to be invaluable in various military missions. Specifically, UGS systems add significantly to the capability and security of reconnaissance and surveillance units during military operations by monitoring the battlefield. Recent initiatives for Homeland Defense target the use of DoD technologies for use in the public sector for Offices under the Department of Homeland Defense. UGS systems can be utilized for Homeland Defense for perimeter security, surveillance, tracking, and intrusion detection. This paper depicts the use of present UGS technologies for use in Homeland Defense applications.
The networked communications requirements for programs such as Future Combat Systems and others have spawned numerous developments in the area of low profile, low cost, yet high performance transceivers. A primary objective for these next-generation unattended devices is maximum mission life, hence the radios employ not only low power circuit designs, but also power-efficient routing protocols and fast acquisition waveforms to support duty cycling.
The network architecture of the systems employing these transceivers is similarly optimized. In numerous scenarios, low power (< 1 watt transmitted output) transceivers compose the local network that interconnects relatively closely spaced nodes, typically front line sensors. A typically higher-powered, and higher data rate transceiver within the network provides the longer link (tens of kilometers) to a Command and Control Station.
Operational considerations specific to each system, such as the number of nodes, anticipated traffic volume, latency requirements, forward error correction, encryption, etc., are used to determine the data rate for both the local and long haul links. Additional requirements for low probability of detection, low probability of intercept, and anti-jam protection provide the final input to the process of waveform selection or design.
In many cases, unique transceivers are designed to satisfy the requirements for each of the two links. However, judicious trade-offs between the two can yield a single dual-mode device capable of operating in a low power, low rate mode for sensor interoperability while also offering higher layer communications. This paper outlines the design considerations for networked sensor system transceivers and presents performance data for prototype systems.