Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRSynchronizing Clocks on Moving Platforms
Building a netted radar system whose components are located on different aircraft is not that simple. This requires the clocks in the radar devices to be synchronized to within nanoseconds. At Fraunhofer FHR, methods for this task are being developed in the “iClock” project.
When two people meet at a café, it’s sufficient for their clocks to be roughly in sync – after all, one can wait a few minutes for the other. However, when radar devices located on two aircraft need to interconnect, utmost precision is essential. For such networking, the clocks in the radar devices must tick within nanoseconds, if not even picoseconds. But the "ticking" of the clocks depends on the respective flight path: the Earth's gravitational field causes clocks to run faster or slower depending on altitude. Other influences, such as time dilation in the troposphere and ionosphere, can also put the clocks out of sync by increasing measurement uncertainty. Moreover, the rate of change of distance between the platforms and simple motor or other drive vibrations disrupt the stability of the onboard clock.
Simulations Combined with Real Networked Clocks
In the “iClock” project, funded by the Fraunhofer Discover Program, Ferran Valdes Crespi from Fraunhofer FHR is developing a method to precisely synchronize the radar device clocks on flying platforms. The basis consists of simulations. “In these, I calculate the exact distances of the aircraft for various flight paths, such as figure eights, loops, curves at different altitudes, and the time delay that occurs between the clocks as a result,” explains Valdes Crespi. What impact does it have on the synchronization of the clocks and the measured time delay when the aircraft fly parallel for a short time, one hovering higher than the other, or when they make a turn? However, simulations alone are not sufficient to align real clocks on aircraft. After all, real clocks do not behave like the virtual clock models in the simulation; they are influenced by external conditions such as acceleration, temperature, and other factors during a time measurement. Therefore, the researcher employs a trick: he couples simulations and real hardware. More specifically, he imprints the results from the simulations onto two real clocks located in the laboratory that are connected via cable. Thus, the clocks operate as if they were indeed on the flying platforms following the simulated flight paths instead of being static and motionless in the lab. At the same time, the clocks measure each other through the cable. How must they adjust to tick simultaneously? The initial results are promising: the researcher has already demonstrated in several cases that synchronization can be achieved this way.