The “Holdover function” is the ability to maintain accurate output frequencies in the event that GNSS signal is lost. Typically, GNSSDO is used in applications in which operating through various interruptions are critical to its infrastructure. During interruptions, such as noise, jamming signals, or even lightning strike causing the GNSSDO to lose lock to the GNSS receiver, the GNSSDO must continue to output highly accurate and precise timing for a certain period time. This is called “Holdover status”. When the GNSS receiver is locked to the satellite signal, the GNSSDO will learn the frequency characteristic of its oscillator (aging, temperature). If there is a signal loss, Furuno’s GNSSDO Holdover Function will use this learned characteristic to maintain its timing performance. The holdover performance can be measured by how accurately the GNSSDO learns of its oscillator's frequency characteristic.