[LEAPSECS] it's WP7A week in Geneva

[Magnus Danielson]

Tom Van Baak wrote:

>> Almost every GPS receiver also applies the UTC corrections to produce 

>> a UTC time. Infact, many of them can't externally produce any other 

>> time than UTC. Some has the ability to deliver GPS time or UTC time. 

>> Only a fraction of the receivers made only use GPS time, but then for 

>> some specific use.

> 

> It seems a lot of telecom applications use GPS time, avoiding

> UTC altogether. Do you have any more information on this?


To the best of my knowledge that is very rare. Few GPS clocks I have 
seen uses their own GPS receiver. I can ask around.


> Your comments about GPS receivers and UTC need to include

> the disclaimer. "after the receiver is powered up and after it has

> achieved lock and after it has acquired the current UTC offset

> from page 18 in subframe 4, which could be up to 12.5 minutes

> later", then, yes, the GPS receiver will be able to produce UTC

> reliably. But until that point, what the receiver actually produces

> varies from model to model and also depends on how recently

> the receiver was last used, etc.

> 


My point was that most receivers can only output UTC time rather than 
UTC or GPS time, even if it has knowledge of both time scales. The 
navigation core uses GPS time, but ECEF coordinates gets converted into 
normal long, lat, height and UTC. Additional settings can adjust to 
local datums such as RT90 and UTC+1h/UTC+2h-

Of course it cannot output a correct UTC solution until it has received 
page 18 subframe 4, but it can store the leapsecond offset in 
non-volatile RAM since last lock and for most of times that would give 
correct UTC from first lock. How many if any receiver uses that, I do 
not know, but it is technically possible, just as you store the almenac 
data and last known position in order to accelerate those first 12,5 
min. Recall, for this strategy to fail, the receiver must have been 
turned off over a leap-second event. Most times it is turned off is not 
a leap-second event.

Cheers,
Magnus