Frequently Asked Questions: ANTARIS FAQ

Topic

5	Leap Second: Receiver outputs valid time and position but time is off by a few seconds

Leap Second: Receiver outputs valid time and position but time is off by a few seconds

Question
My ANTARIS™ GPS receiver gets a valid time and position after a few seconds but the time is constantly off by a few seconds. After several minutes, this suddenly corrects itselves and the is correct. What's wrong?

Answer
The problem is a GPS inherent. GPS is based on the so called GPS Time, which is a uniform time scale that does not keep in step with the irregular rotation of the Earth. For practical purposes, another uniform scale has been defined (UTC), which differs from GPS Time by an integer number of seconds. To avoid the uniform scale diverging indefinitely from that of the Earth's rotation, a leap second is introduced in UTC whenever necessary. The choice of the dates and the announcement of the leap seconds is under the responsibility of the International Earth Rotation and Reference Systems Service (IERS). At the time of writing, the number of leap seoconds is 13 but this has changed in the past (see International Bureau of Weights and Measures) and will change in the future, the next time on January 1, 2006. The GPS satellites transmit the number of leap seoconds (the offset between GPS Time and UTC) in the navigation message once in 12.5 minutes.

After a cold start, a GPS receiver has no information about the number of leap seconds until this information is received from the GPS satellites, which can take up to 12.5 minutes. During this time, the receiver will have to make an assumption and output a default value. u-blox has decided to output 0s in this case. Other manufacturers output 13s (the number of leap seconds at the time being) by default. Though the latter approach seems to be an advantage as one doesn't see any time jump in case of cold starts right now, it is very questionable approach since a potential problem is hidden for a long time. As the number of leap seconds will change sooner or later (the earth rotation slows down), the problem might not occur when the units are already deployed in the field.

What can be done to overcome this problem?

If one attach a backup battery to the ANTARIS GPS receiver and let the receiver run for at least 12.5 minutes when powering it up the first time, the receiver will store the UTC offset in its battery backup RAM (BBR). After the next power cycle, the receiver will use the UTC offset from the BBR and output the number of leap seconds time immediately.
Alternatively, one could check whether the UTC offset (leap seconds) has been downloaded by observing the UBX-NAV-TIMEGPS message.
If this isn't feasible too, you could initialize the UTC offset right after a power up by sending the correct information with the UBX-AID-HUI message. Obviously, this requires an external CPU to store these data. Please be aware, u-center AE V3.0 has a small bug in the implementation of the UBX-AID-HUI message but the receiver firmware (V3.0 or higher) is okay. This means one cannot simply tests this with u-center, one will have to implement a trial in the host.

Further Information

ANTARIS FAQ

/customersupport/faq_antaris/L...
last modified: 28.10.2005

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