and other long-term effects. A GPSDO aims to utilize the best of both sources, combining the short-term stability performance of the oscillator with the long-term stability of the GPS signals to give a reference source with excellent overall stability characteristics. GPSDOs typically phase-align the internal flywheel oscillator to the GPS signal by using dividers to generate a 1PPS signal from the reference oscillator, then phase comparing this 1PPS signal to the GPS-generated 1PPS signal and using the phase differences to control the local oscillator frequency in small adjustments via the tracking loop. This differentiates GPSDOs from their cousins NCOs (numerically controlled oscillator). Rather than disciplining an oscillator via frequency adjustments, NCOs typically use a free-running, low-cost crystal oscillator and adjust the output phase by digitally lengthening or shortening the output phase many times per second in large phase steps assuring that on average the number of phase transitions per second is aligned to the GPS receiver reference source. This guarantees frequency accuracy at the expense of high phase noise and jitter, a degradation that true GPSDOs do not suffer. When the GPS signal becomes unavailable, the GPSDO goes into a state of holdover, where it tries to maintain accurate timing using only the internal oscillator. Sophisticated algorithms are used to compensate for the aging and temperature stability of the oscillator while the GPSDO is in holdover. The use of Selective Availability (SA) prior to May 2000 restricted the accuracy of GPS signals available for civilian use and in turn presented challenges to the accuracy of GPSDO derived timing. The turning off of SA resulted in a significant increase in the accuracy that GPSDOs can offer. GPSDOs are capable of generating frequency accuracies and stabilities on the order of parts per billion for even entry-level, low-cost units, to parts per trillion for more advanced units within minutes after power-on, and are thus one of the highest-accuracy physically-derived reference standards available. References Synchronization