DARPA Unattended Ground Sensor systems - Analysis of communication systems for internetting UGS systems

During May 1997 - October 1998, SPAWARSYSCEN Code D746, under the sponsorship of Defense Advanced Research Projects Agency (DARPA), conducted several field tests using off-the-shelf(OTS) 900 MHz wireless modems in different operating environments typical of those targeted by Unattended Ground Sensor (UGS) applications. Using ground-based 12 " (mounted on 3 " base) 3dB omni-directional antennas on both communication ends, the performance of three OTS modems was evaluated. By varying path lengths, baud rates, file sizes, RF environments, and degrees of line of sight (LOS), a comprehensive profile of each modem's performance was established. Because of the nature of ground-level communications (varied LOS) and the many environmental variables which can dramatically affect performance, this paper does not attempt to model performance or draw conclusions as to which modem is best suited for all UGS applications. However, based on the results of these field tests, the OTS modem using Frequency Hopping Spread Spectrum (FHSS) performed considerably better than the two other OTS modems using Direct Sequence Spread Spectrum. This certainly could be attributed to the poor quality of the two DSSS modems used in the field tests, and a more comprehensive side-by-side comparison is recommended. In addition, the paper does offer insight into the limitations of the OTS wireless modems, and demonstrates how greater degrees of LOS can tremendously boost their range and reliability.