GPS clock is wrong. . .

The GPS time comes from the GPS system. Assuming you have a good satellite fix, the time will be correct to about a thousandth of a second. That should be good enough for most ordinary uses.

Trip time is calculated by the selected route and your estimated speed settings. If you set funky speeds for your estimated speeds you will get odd looking trip times.

The estimated time of arrival is based from the current time plus the estimated time en route, and will be displayed based on your time zone settings.

Tom

 

From the internet:

GPS works by measuring tiny time differences between the signals from various satellites based on the fact that the speed of light is constant. They measure time in nanoseconds:
Each satellite carries with it an atomic clock that "ticks" with an accuracy of 1 nanosecond (1 billionth of a second). A GPS receiver in an airplane determines its current position and heading by comparing the time signals it receives from a number of the GPS satellites (usually 6 to 12) and triangulating on the known positions of each satellite. The precision is phenomenal: even a simple hand-held GPS receiver can determine your absolute position on the surface of the Earth to within 5 to 10 meters in only a few seconds (with differential techiques that compare two nearby receivers, precisions of order centimeters or millimeters in relative position are often obtained in under an hour or so). A GPS receiver in a car can give accurate readings of position, speed, and heading in real-time! To achieve this level of precision, the clock ticks from the GPS satellites must be known to an accuracy of 20-30 nanoseconds. However, because the satellites are constantly moving relative to observers on the Earth, effects predicted by the Special and General theories of Relativity must be taken into account to achieve the desired 20-30 nanosecond accuracy.

 

Uh, yeah, I could tell that the clock on my GPS was off by 30 nanoseconds (rolling eyes).

 

This is true, but the 20-30 nanosecond accuracy is only used internally for position calculations. Most of the newer GPS chips provide an external time signal that is good to a thousandth of a second, which should be good enough for setting your watch.

Tom

 

Not entirely responsive to the initial question, but the GPS time in the Prius IS incorrect. Not enough to matter for time at your destination, but definitely off by a large number of seconds. If you have an '04 or '05, you can access the GPS raw data. (See RIKLBLOG - A Valuable Safety Feature ) If you do, and you have set your watch correctly or use another GPS, you'll see the difference. It's many trillions of nanoseconds.

I suspect the reason for this is that the Prius navigation system uses "GPS time" instead of the UTC (a.k.a., more or less, GMT). There is an increasing number of seconds difference because GPS is strictly atomic time and UTC is synchronized to earth's rotation. Whether this is an oversight that pretends to get you to your destination a bit earlier or later or is corrected later in the software I have no idea. Keep your Prius for a century or two while the error accumulates and it will be easier to tell.

Another nerd heard from,

Richard

 

Aha. 30 nanoseconds is only 30 billionths of a second... It would be pretty difficult to notice, since the light from the GPS takes around ten nanoseconds just to reach your eyes! So the 30 nanoseconds of "error" can't be the problem.

 

We need to pre-compensate for the light travel time. Perhaps the GPS could have an "eye distance" setup field.

Tom

 

Interestingly, the first commercial GPS product (circa 1980) was a box made by Trimble Navigation that was intended for calibrating atomic clocks. (This was before all the initial NAVSTAR satellites had been launched.) It made it possible for labs to do their own periodic instrument re-calibration in-situ. Until the product was available, U.S labs had the hassle of sending their atomic clocks to Boulder periodically, in order to have them calibrated by NIST. An additional hassle was that atomic clocks that were shipped back from NIST by jet had to be tweaked when they got home to accommodate the relativistic effects caused by their travel. The Trimble Model 5000A cost $15,000.