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Interference with other sources and receivers is further reduced by using directional antennas. The antenna design shown is highly directional. When penetrating the ground, we wish to eliminate as much of the multipath signal as possible. The directional antennas reduce the multipath signals detected to those that are relatively inline with the wave path, and eliminate much of the multipath signal that returns at odd angles. Impulse radar uses low power inherently because the transmissions occur in pulses separated by periods of no transmission. The power of the pulses is offset by the dead time between the pulses. The average output of the current system is about 300 MICROwatts. THE LOW AVERAGE POWER OF AN IMPULSE SYSTEM EFFECTIVELY HIDES THE TRANSMISSIONS FROM CONVENTIONAL RECEIVERS. Interference can be further reduced in an impulse system by using random interval spacing. As long as the transmit and receive antennas are in sync, the period between pulses can be varied to prevent aliasing with other continuous- or pulse-transmission systems that might be operating in the same locale. Furthermore, if an impulse system is being used to transmit data, varying the intervals between pulses prevents other impulse systems from locking onto the signal. Patriot Scientific's current GPR system does not use random interval spacing. Up to Contents -107-