Out of This World Timing!
Posted: 9th January 2023
As current technology becomes more complicated, it is becoming increasingly difficult for a single person or business to handle the design and development of a new product on their own. To successfully develop a new product, engineers therefore approach external designers and engineers to collaborate on solutions.
The following article describes one case where IQD Frequency Products was closely involved in the development and solution of an acute problem encountered by its customer GMV to their mutual benefit.
GMV is a multinational company which services the space industry with systems and space applications such as simulators, navigation and control systems or on-board software. Applications include contributions to the Galileo system and other missions from the European Space Agency (ESA).
Challenges with product firmware to support sky-high results
In 2021, GMV required a disciplined OCXO for their latest product development and owing to its excellent holdover stability, good noise reduction and competitive pricing, GMV selected IQD’s IQCM-200 along with the IQCM evaluation board.
After several days of testing, GMV found that the product firmware did not meet the requirements for their special application on geostationary satellites: Their system requires an extremely precise and accurate time base to reduce synchronisation error between ground stations. Using the original supplied firmware would have affected the calculation of the positioning of these geostationary satellites. It is important to note that the IQCM-200 is designed to provide both 1 PPS (pulse per second) and 10 MHz output signals with a holdover stability of ±1.5 μs over a 24 hour period. This enables the IQCM-200 module to meet the requirements of telecommunications standards for applications such as LTE-TDD. However, GMV hoped to use the IQCM-200 as a type of filter to remove the interference generated by the GPS receiver in the 1-PPS signal. To be able to support this deviation of use, IQD and GMV collaborated closely to reach possible solutions.
Holdover defines the time deviation that a signal for network synchronisation may have when the source of the synchronisation is disturbed or temporarily unavailable
How far can we go for optimal synchronisation?
Following careful investigation in the IQD testing laboratory, the IQCM-200's 1 PPS signal showed less electronic noise than the GPS receiver itself while in lock status and over a short period of a few minutes. ‘In lock status’ means that the 1 PPS signal from the synchronisation source - in this case the GPS receiver - is still present and thus the IQCM-200's 1 PPS output signal is constantly synchronized to it. However, when viewed over an extended period of hours, large fluctuations in the IQCM-200's 1 PPS output signal were evident. Since the IQCM-200's primary function is performance during holdover, optimising performance ‘in lock status’ had never been a focus in the past.
After several months of development and testing, IQD found a solution for GMV: The firmware was customised by changing the frequency and phase lock time to optimise performance with noise reduction. The noise reduction gives GMV an improved, more accurate signal for its application.
Figure 1 shows the phase data of the GPS receiver recorded by IQD (blue line) and the phase data of the IQCM-200 when connected to this GPS receiver (lock status). The measurement shows that the old IQCM-200 firmware (yellow line) has a number of significant jumps in phase deviation. The new IQCM-200 firmware (red line), on the other hand, proves to be much more stable compared to both the GPS receiver and the original IQCM-200 firmware, over the measurement period. The IQCM-200 therefore now acts as a kind of noise filter for the input signal.
Figure 1: The 1 PPS output signals from a IQCM-200 with the new firmware compared to the original firmware and IQD’s GPS receiver as seen over a period of more than ten hours
The modification of the IQCM-200 firmware has enabled GMV to accurately record timing information at ground stations. In addition, the data protocol is now persistent, allowing accurate synchronisation between ground stations and ensuring the position of geostationary satellites.
Raúl Nieto Freire, part of the production team at GMV, said “The last use case for our timing receiver, the WANtime Rx, is its integration in a passive ranging architecture for geostationary satellites where the accuracy limits for the orbit determination need to be evaluated. In this system our timing receiver acts as a time base between two tracking ground stations. Here, the accuracy and precision of the timestamp source is critical to cross-correlate the data received at each of the stations.
The GMV raw 1 PPS signal from the GNSS receiver was a digital jitter of +/- 4 ns due to the limited speed of its internal oscillator. This can cause a synchronisation error of 8 ns between two tracking stations, in the worst case. By using the IQCM-200 this digital jitter is filtered out, which results in a better synchronisation between stations.”
An error of 8 ns in synchronisation corresponds to an error of 2.4 metres within three-dimensional space
It has been a great pleasure for IQD to work in partnership with GMV. This project has given IQD the opportunity to improve its products and helped define a new application for the IQCM-200 disciplined OCXO.