Time was created by people when they wanted to put boundaries on what we now call a day, the length of time it takes the earth to make one rotation on its polar axis. Possibly it was the Egyptians that created a 24-hour day. The positions of the stars in the sky helped measure time as well, as the night was divided into 12 hours.
Well, time has come a long way since those many thousands of years ago. Now, in the 21st century, we have GPS with 27 flying clocks in orbit around the earth.
Physics.org explains some details of time in orbit like this: The time differences of GPS satellites due to relativity are measured on the scale of nanoseconds — one nanosecond being just one billionth of a second — however the difference is enough to be measured and enough to matter.
GPS satellites travel at approximately 8,700 mph (14,000 km/h) with respect to Earth. This means time runs 7,200 nanoseconds per day slower for a satellite relative to us on Earth as described by special relativity.
However, using general relativity it is possible to calculate that time goes faster for a GPS satellite by 45,900 nanoseconds per day, due to the satellite being 19,000km above the Earth (therefore in weaker gravity). This means overall time runs 38,700 (45,900-7,200) nanoseconds faster per day for a GPS satellite relative to us stationary on Earth.
GPS receivers in communication infrastructure
GPS receivers provide base stations with the precise time and frequency to synchronize their time so that when a cell phone moves between different base stations, we have a seamless hand-off of calls, streaming video with the quality we expect, accurate car navigation, and E911 service in the US.
GPS jamming and GPS spoofing (when a GPS signal appears to the receivers to be a valid signal but the frequency, position, and/or time-of-day content is altered) can and does occur. There are easy-to-get, low cost devices available in some electronics supply stores that can disrupt the GPS system and wreak havoc in a widely covered area.
One possible alternative solution to prevent this type of disruption might be a new, highly integrated packet-based primary reference source from Symmetricom called TimeProvider 1500. It provides a viable alternative to protect from GPS vulnerabilities.
Here's a bit more background information on Symmetricom's secure time products as they pertain to the smart grid (an intelligent power distribution grid):
The SyncServer SGC-1500 comes with a built-in IEEE 1588 v2 Telecom Profile input option. This enables the Smart Grid Clock to derive time from the communications wide area network (WAN), thus eliminating the need to have GPS at every substation and PMU. The rubidium atomic clock option offers holdover capability in the event of GPS disruption. These options result in a highly cost effective and resilient solution for power utilities.
Join us on Integration Nation on November 20 at 11:00 a.m. EST (16:00 GMT/UTC) for a chat session on clock integration ICs. Ian Dobson from IDT will be our resident expert in a text-based, lively, and educational discussion. IDT has a great integrated product called a Programmable Universal Frequency Translator that relates to this chat. See the article on EDN for a synopsis of this highly integrated, versatile device that reduces clock tree footprint.
Have you done any design work on ultra-high accuracy time sources such as these? What problems did you encounter?
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