Absolute time of travel is the time required for the signal to travel between the transmitter and the receiver. Since the signal may travel different paths as it reflects off different surfaces, the absolute time of travel will depend on the path length of each multipath component. The time of travel is measured by triggering the acquisition of data at the receiver simultaneously with the onset of each new PN code generated at the transmitter. This is accomplished by clocking the PN code generator (at the transmitter) and a trigger device (at the receiver) with two highly stable and synchronized oscillators. Prior to measurements, the oscillators are observed on a phase/gain meter and adjusted to give less than a specified phase drift over a period of time – depending on the requirements of the measurement. The transmitter and receiver are then placed back–to–back and connected with a reset line. The PN code generator and trigger device are simultaneously reset to synchronize triggering with the beginning of each PN code. The triggering device is set to produce pulses spaced with an interval equal to the time length of the PN code. Additional circuit logic is used to determine which pulses are used to trigger the acquisition of data, allowing for a specified time interval between acquisitions (a multiple of the time length of the PN code). After synchronization the transmitter and receiver are separated for measurement. When the signal is received, digitized, and correlated to give an impulse response, the time of travel of each delay component can then be easily determined by observing the time between the beginning of the impulse response and the specified delay component.