What is GPS Disciplined Oscillator (GPSDO) in Amateur Radio?

During a 40m chat a few days back, VU2XTO told me that as we go higher up in frequency, a GPS Disciplined Oscillator (GPSDO) is needed for good frequency stability. All of us homebrewers would be familiar with the drift in frequency which we had in our variable frequency oscillators in the yester years, making it difficult for the receiving station to track the signal exactly! GPSDO also known as GPS Clock, can be considered as a combination of a Global Positioning System receiver and a high quality stable oscillator whose output is controlled by the signals broadcast by GPS or other Global Navigation Satellite System satellites.

As navigation needs very accurate time signals from the satellites, signals are typically accurate at the nanosecond level. In amateur radio, GPSDO is essential for frequency stability in radios working at higher end of the radio spectrum typically at the GHz range. Popular example GHz range amateur radio communication in this region is the 2.4 GHz uplink and 10 GHz downlink of the QO 100 Geostationary Amateur Radio satellite. Abroad there are several hams working on GHz frequency, including Rain Scatter propagation. As you can see in earlier the image, GPSDO has a GPS antenna socket for receiving GPS signals.

When GPS signals are not being received, as can occur in certain locations, stability will depend on the intrinsic stability of the oscillator design. Important factors which can affect the stability are temparature and aging. Crystal oscillators enclosed in a temperature controlled chamber to prevent expansion and contraction of the crystal which could cause frequency fluctuation, is known as Oven Controlled Crystal Oscillator (OCXO).