[LEAPSECS] DCF 77

[Warner Losh]

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.

The only down side to the almanac is that it doesn't give you historical 
leap second info, which may be needed sometimes.  But for most real-time 
operational systems, you won't need to know the full history, just the 
running sum of adjustments.

Warner