GPSDOs for Cubesatellites!

Director of Business Development

The growth of small satellites has grown dramatically over the past decade. The initial surge has been attributed to the curiosity of research and development whether it is affiliated directly with universities, research centers, or government bodies such as NASA and NOAA. New technology development includes payloads such as science instrumentation, communications, radar applications, and sensors. Small satellites took on the platform of a cubesatellite.

Cubesatellites are essentially a form factor for a satellite design highly driven by the need for standardization to keep costs down for launch vehicle compatibility. The successes of this initial surge of smallsats has caused private industry to provide their undivided attention to the concept as a means for profitable business opportunities. With launch vehicle costs going down and large-scale manufacturability becoming common place for the cubesatellite platform, many startups have been receiving generous investments from venture capital firms for their business propositions. Big data has been the business plan for quite a few of these small startups; some examples are tracking maritime vessels, weather forecasting, tracking emissions to determine carbon footprints geographically, or obtaining world imagery in timely fashion. Although these startups have different missions, there is one item that remains constant which is the cubesatellite platform as a means to an end to support their value proposition within the big data industry. Connectivity is another application (not necessarily using cubesatellites as the platform) that has also been getting a lot of media outlet attention since global coverage requires a much larger LEO constellation size. This update is to highlight one of the critical components within the cubesatellite design that many of these startups are starting to realize as an instrumental portion to have successful and reliable missions. he concept of timing amongst satellites within a constellation has become a significant technical challenge. For example, for a very large LEO constellation providing connectivity, a satellite is constantly traversing in its orbital plane at fast speeds and as it leaves a particular coverage area, it needs to coordinate a handover to the next satellite approaching that very same coverage area.

There is a need for high precision timing so that this handover goes seamless without any data loss. Reliable connectivity service is one of the primary criteria for user experience, and precision timing is one of the key enablers for good user experience. Another example that is more applicable and prevalent for a cubesatellite platform is the need for time synchronization amongst multiple satellites to perform a mission task in concert such as tracking a maritime vessel, taking an image, or even relaying mission data through inter-satellite links to downlink the data to a ground station. 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 (10E-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 10E-12 or 10E-13). The GPSDO design enables for a great short-term stability (from the OCXO) and long-term stability solution (from the 1PPS GPS signal). 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. Bliley Technologies has been recognized in the frequency control industry as one of the premier suppliers for high quality OCXOs.

Bliley has been moving up the value chain from an RF system perspective. Within the last year, Bliley has developed a commercial LEO solution to provide a master reference oscillator which provides the functionality of a PLL and OCXO from the GPSDO design and is currently in production. Bliley recognizes that there is no definitive solution for the LEO cubesatellite market, and is currently in development of a solution in a form factor which is suitable for the cubesatellite platform by challenging size, weight, power, and ease of integration. It is to Bliley’s understanding that many cubesatellite missions have different needs whether it be superior phase noise, superior short-term Allen Deviation, or very low power consumption and thus Bliley is offering variants to cater to each of these needs.

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