Characterizing the performance of GPS disciplined oscillators with respect to UTC(NIT)

Global Positioning System Disciplined Oscillators (GPSDOs) are now the primary standard of time and frequency at many laboratories and calibration facilities. They are typically accepted as self-calibrating standards, and their users generally assume that they meet the manufacturer's specifications. To gain a better understanding of the actual performance of GPSDOs, this paper presents a method of characterizing both their long and short-term performance that uses the UTC(NIST) time scale as a reference. It then describes how this method is used to characterize four GPSDOs, including two that use an oven controlled quartz oscillator (OCXO) as their time base, and two equipped with a rubidium oscillator. All four devices were simultaneously tested using the same antenna over two 60 d measurement intervals. During the first 60 d measurement, a previously surveyed antenna position was used and the same coordinates were applied to all four devices. During the second 60 d measurement, each GPSDO performed an independent survey of the antenna's position and applied its own coordinates. Both the timing output (1 pulse per second) and the frequency output (10 MHz) of each GPSDO was measured during both 60 d intervals. A low-noise dual mixer time difference system was used to characterize the short-term frequency stability of each device's 10 MHz output, and all measurement results are presented and summarized.