The critical component that is becoming more apparent as a necessity for cubesatellite platforms is the GPS Disciplined Oscillator, GPSDO for short. GPSDOs comprise of a GPS receiver, Oven Controlled Crystal Oscillator (OCXO), and a Phased Locked Loop (PLL). The Global Navigation Satellite System (GNSS), in geo-stationary orbit, provides a GPS signal that many use today on the ground for navigation. With a GPS receiver (along with a GPS antenna) in place on board the satellite, a very precise 1 pulse per second signal can be generated from the signal transmitted from the GNSS (10-12 depending on how well the GPS receiver was designed). With the 1PPS signal from the GPS receiver, the OCXO can be steered using a phased locked loop. The PLL essentially compares a PPS signal derived from the OCXO to the 1PPS signal generated by the GPS receiver and a control circuit adjusts the OCXO frequency to synchronize both signals at the same timing offset (or phase). The GPS receiver is a great solution for long-term stability because OCXOs degrade over time with respect to stability, known as aging (i.e. increase in deviations over time from the desired frequency), and over temperature as well. This degradation is being compensated over time with the control circuitry in the PLL which that adjusts the frequency of the OCXO as the PLL is locking the derived PPS signal from the OCXO to the GPS 1PPS signal. However, short-term stability on the 1PPS signal from the GPS receiver is quite noisy due to atmospheric fluctuations, jitter within the GPS receiver, and other noise contributors. The use of a very stable OCXO for the GPSDO remedies this issue (ultra-stable quartz OCXOs can have short-term precision on the order of 10-12 or 10-13). The GPSDO design enables for a great short-term stability (from the OCXO) and long-term stability solution (from the 1PPS GPS signal). A system block diagram is provided below:
Figure 1: GPSDO System Block Diagram
There are also instances where a jamming signal may be present which causes the GPS receiver to lose lock to the GPS signal and the GPSDO enables the satellite to continue to operate under these types of interruptions. The GPSDO typically continues to output accurate and precise timing with the use of a holdover function. When lock is lost with the GNSS signal, a holdover mode allows the GPSDO to utilize the performance of the oscillator alone.
Commercial small satellites operating in LEO orbit, at altitudes significantly lower than the GEO belt, can use this method as a means for timing coordination in applications mentioned previously. Again, long-term stability is achieved via the PLL locking the derived 1PPS signal from the OCXO onto the 1PPS signal and short-term stability is achieved by using a very stable quartz OCXO. In certain applications such as radar imaging or phased array applications for geolocation, if the timing is not precise, the data captured from the sensors wind up being inaccurate or junk leading to an unsuccessful mission.
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