[LEAPSECS] DCF 77

[Warner Losh]

On 12/23/2010 13:15, Magnus Danielson wrote:

> 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.

The cached information isn't very useful when the GPS receiver has been 
off for a while.  Coming up on a cold-spare GPS receiver requires that 
you wait.

Warner