[LEAPSECS] DCF 77

[Magnus Danielson]

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