I just read on Yahoo! News which published a Reuters news story about a leap second being added to 2008 (http://news.yahoo.com/s/nm/20081228/ts_nm/us_time_seconds). I thought in the past that GPS clocks did not add these leap seconds and our GPS clocks are actually several seconds "ahead" of true time. One sentence in this story infers that the GPS will be adding one second as well. Does anyone know if this is true?
I'm sure it will be, as time is extremely important in GPS. The fact that the signals from the satellites are not instantaneous (speed of light) the lag time must be included in the position figuring. Otherwise your GPSr would be telling you where you were, not where you are.
And the faster you are going, the more "were" you are. While the updates are often faster than you notice, your GPS continues to show you your last plotted position as you continue to move.
The time was added at the Atomic Clock, then adjusted at the satellites constellation level.
The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time. Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ...
Time is how the whole system works!
Now my dash clock and my gps finally match and I can get some sleep tonight.
Don't sleep to well just yet, you have 3 years to wait for this:
The next-generation GPS system that will be operational by 2012. The receivers will be able to combine the signals from 30 Galileo and 28 GPS satellites to greatly increase the accuracy.
The best known satellite navigation system is the United States' Global Positioning System (GPS), and as of 2006 the GPS is the only fully functional satellite navigation system. This consists of 24 to 27 satellites that orbit in six different planes. The exact number of satellites varies as satellites are replenished when older ones are retired. They orbit at an altitude of approximately 20,000 km with an inclination of 55 degrees. The satellites are tracked by a world-wide network of monitor stations. The tracking data is sent to a master control station that continuously updates position and clock estimates for each satellite. The updated data is then uplinked to the satellite via one of several ground antennas.
The European Union and European Space Agency agreed on March 2002 to introduce their own alternative to GPS, called Galileo. At a cost of about £2.4 billion, the required satellites will be launched between 2006 and 2008 and the system will be working, under civilian control, from 2010 (Two years later than originally anticipated). The first satellite was actually launched on 28 December 2005. Galileo is expected to be compatible with the next-generation GPS system that will be operational by 2012. The receivers will be able to combine the signals from 30 Galileo and 28 GPS satellites to greatly increase the accuracy.
OK, thanks for the previous replies. I just found the US Naval Observatory website (http://tycho.usno.navy.mil/leapsec.html). According to that, GPS is synchronized with the UTC and no leap seconds are added. It said "The Global Positioning System (GPS) epoch is January 6, 1980 and is synchronized to UTC. GPS is NOT adjusted for leap seconds.
BEFORE THE LEAP SECOND: GPS-UTC IS 14 (GPS IS AHEAD OF UTC BY 14 SECONDS)
AFTER THE LEAP SECOND: GPS-UTC WILL BE 15 (GPS WILL BE AHEAD OF UTC BY 15 SECONDS)"
I think I understand this who time thing, but there is no way I can explain it.
I wondered why my timing was off --- I thought it was my drinking
Please, Stop already I'm getting a headache.
Interesting. According to the link to the US Naval Observatory, it's OK to have a difference between UTC and civil time, as long as it is less than 0.9 seconds, however, after the adjustment, GPS will be ahead of UTC by 15 seconds (it is ahead 14 already). I guess we still don't have vehicles that travel fast enough to require to compensate for the time differences.
So everyone, when you ring in the new year, make sure you count 'one Mississippi' after the ball drops, in order to be 'safe'
Did not know any of this. I would have made a guess that the GPS was updated by the UTC the Navy.
Interesting. I will have to compare the clocks on my cell phone, "atomic clock", and GPS to see if they are indeed off by 14 seconds.
I lined up a Garmin Nüvi 310, 760, StreetPilot C510 and a PC (the latter synched to within 5mS of UTC).
They all change to the next minute at the same time, (as measured with a Mark 1 eyeball).
Maybe the Garmin software 'knows' about this 14 second difference - or information about it is present in the gps data stream?
(I could swear that when I first tried this, 2 years ago, I saw the (then) 13 second difference.)
I compared the World Clock on my Garmin nuvi 255 and the World Clock on my Palm Centro, which updates the clock from the cell towers, and they are both in sync to the second.
I just checked my nuvi, WWV and a national time server on the Internet. The nuvi seems to be dead on with UCT, not off, and I believe it is even up to date with the latest leap second.
Did anybody else have fun with the countdown on New Years? Since the leap second was added right before midnight, I enjoyed counting down from ten and repeating the number one before saying happy New Year.
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