• David Mitlyng

Weekly Takeaways-July 22, 2022

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How to Transfer Time Imagine you want to synchronize your clock to a master clock. As we mentioned previously, synchronizing clocks that are close to each other is easy. The real challenge is synchronization over long distances. For this, you need time transfer. GPS is currently the main source of time synchronization over global distances. Each GPS satellite knows its position and time, and broadcasts it to the user. This signal is also used to calculate the distance to the satellite (see below). You will need this information from multiple satellites to triangulate your position and time. But if you only care about time, then one GPS satellite will work. Instead of calculating distance, you can calculate the time of flight from the satellite, and add it to the satellite time. Other time transfer methods work in a similar fashion: a reference clock sends a stable RF, ethernet, or optical signal, which the receiver uses to calculate the travel time. The accuracy of the time transfer is impacted by the signal quality, among other things, which tend to degrade over long distances. Last Week's Theme: Commercial Space Steps Up


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How to Transfer Time Part II So how does time transfer from GPS work? The GPS signal contains a pseudorandom code (sequence of ones and zeros) that is sent to the receiver. The receiver knows this code and creates its own version. By comparing the offset from the satellite and the receiver code, you know the difference in time. This can be used to calculate distance using the speed of light, or the receiver time. Credit: Department of Geography, The Pennsylvania State University Other time transfer methods work in a similar fashion with added complexity to improve performance. But the accuracy is fundamentally limited by:

  • Time of travel changes through the medium of travel.

  • The ability to receive a good quality signal.

  • The quality of the transmitter and receiver.

  • The accuracy of the reference clock.

In addition, for high accuracy time transfer between moving platforms (say, from a satellite like GPS), Doppler and relativistic effects must also be carefully taken into account.