A highly accurate monitoring solution for active network measurement is provided without the need for GPS, based on an alternative software clock for PC's running Unix. With respect to clock rate, it's performance exceeds common GPS and NTP synchronized software clock accuracy. It is based on the TSC register counting CPU cycles and offers a resolution of around 1ns, a rate stability of 0.1PPM equal to that of the underlying hardware, and a processing overhead well under 1s per timestamp. It is scalable and can be run in parallel with the usual clock. It is argued that accurate rate, and not synchronised offset, is the key requirement of a clock for network measurement. The clock requires an accurate estimation of the CPU cycle period. Two calibration methods which do not require a reference clock at the calibration point are given. To the TSC clock we add timestamping optimisations to create two high accuracy monitors, one based on Linux and the other on Real-Time Linux. The TSC-RT-Linux monitor has offset fluctuations of the order of 1s. The clock is ideally suited for high precision active measurement
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