Question: In the Chartroom Chatter section in a recent issue, an article titled, “GPS will roll over sooner than Y2K,” includes the statement that, “GPS time is 13 seconds ahead of UTC since UTC accounts for leap seconds and GPS does not.”
I spoke about this with Harry Darby, a friend in the Charleston Power Squadron who lives on the Isle of Palms near Charleston, S.C. He stated that he believed that to be erroneous. “I have a watch that sets to UTC through the computer at the National Institute of Standards and Technology in Colorado,” Darby said. “Compared to my GPS there is only 1/2-second difference. Sights taken on the beach near my house get good results using either UTC or GPS.”John Sikes Charleston, S.C.Answer: The GPS system time scale began at midnight on 6/7 January 1980, and runs continuously. GPS time is “steered” to match Coordinated Universal Time (UTC) to very high accuracy, but has no leap seconds. It is ahead of UTC by the 13 leap seconds inserted in UTC since January 1980. The navigation message that GPS satellites transmit includes data for the receiver to extract GPS time and convert it UTC. How, or even if, a receiver provides UTC varies widely among manufacturers. Some GPS units make the conversion and some do not.
For most of history, time has been based upon the rotation of the Earth. Celestial navigation still uses a time scale based on the rotation of the Earth. It is called UT1 and is essentially Greenwich Mean Time (GMT). UT1 matches the Earth’s rotation reasonably well, and is essential to determining longitude accurately. Our ways of counting that “ever rolling stream” differ, depending on the person or system doing the timing.
There is also International Atomic Time or TAI, derived from a large number of atomic clocks in many nations.
The technological advance of “atomic time” revealed that the Earth’s rotation has some very slight variations, and in fact is slowing down. A day should be exactly 86,400 seconds long. However, the time to complete one rotation with respect to the mean sun is now about 86,400.002 seconds of UTC, TAI, or GPS time. This doesn’t sound like much, but over a year UT1 gets slower than UTC by about three-quarters of a second. The Naval Observatory keeps UTC within 0.9 seconds of UT1 by inserting “leap seconds” periodically. It’s odd, but the more accurate time base, UTC, has to be adjusted to fit the slightly uneven one, UT1.
There was a leap second in UTC at midnight of December 31, 1998. As of January 1, 1999; TAI is ahead of UTC by 32 seconds. TAI is ahead of GPS by 19 seconds (always true); GPS is ahead of UTC by 13 seconds.
Now to the question: Where is GPS time to be found? Each GPS satellite operates on space vehicle (SV) time, using cesium or rubidium clocks. The satellites each broadcast a navigation message to give their orbital data and other factors, allowing the receivers to compute positions from the time of arrival of the signals. Essential parts of the message allow the receiver to correct SV time to determine GPS time at the satellite and at the receiver. (The difference is due to the range to the satellite.)
One frame of the message includes data for computing UTC from GPS time. Receivers handle this information in various ways. Some of the earliest units ignored it and gave times quite a few seconds ahead of UTC. Others entered a fixed correction based on the date of manufacture. It remained correct until the next leap second, and gradually got ahead by one, two, or more seconds. Others make corrections of varying sophistication.
It takes a specialized receiver, preferably at a fixed location, to extract accurate time from GPS signals. The overall system time is highly accurate, but the Department of Defense uses a technique of degrading the time accuracy of the satellites as part of selective availability, reducing the horizontal accuracy to about ±100 meters, 95% of the time.
There is another complexity. A navigator using celestial sights needs UT1. You can’t find this from GPS time or from the UTC extracted from GPS. The time signal broadcasts from WWV and WWVH are in UTC, and include a code to determine UT1 from UTC. These time signals mark the beginning of a minute with a long tone. The next 10-second ticks are coded to give the correction from UTC to UT1. If the first ticks are double-ticks, add the correction. If the last ticks are double-ticks, subtract it.
For example, WWV sends a tone to indicate the beginning of 19 hours, 23 minutes Coordinated Universal Time. The next four ticks are double-ticks. This is a correction of plus 0.4 seconds, so UT1 is 0.4 seconds ahead of UTC. Since UTC can differ from UT1 by as much as 0.9 seconds, celestial navigators enter the corrections to ensure that they are navigating with UT1.
