On 12/23/2010 08:50 PM, Warner Losh wrote:
On 12/23/2010 12:26, Tom Van Baak wrote:
GPS's model for handling of leap seconds is better: you
get both a UTC offset and a date when the leap second
is/was to be applied. Thus it is possible for you to obtain
TAI, GPS, or UTC out of a GPS receiver. One downside
is that you have to wait up to 12.5 seconds for the leap
second information to show up, which can cause timing
issues with cold-start receivers.
Isn't it more like 12.5 minutes since the NAV data is clocked out at
only 50Hz? And I know some older M12 firmware had issues that meant
you'd have to wait 2x that long since it waited for the start of the
almanac to start getting the data, which meant if you just missed the
first bit, it waited for the whole thing to go by twice.
TAI and GPS time are always available after you acquire satellites.
Caching the last leap second value/time means that sometimes you can
start up more quickly if you assume semi-annual leap second possibilities.
Just as the almanac info, the leap second info can be cached in the
battery backed up memory. Many GPS receivers do have the feature, but
not always is the battery installed. However, just because the receiver
has a backup battery it does not mean the leap second info is placed there.
Cheers.
Magnus
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