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Hello, Forgive me, I'm not sure who I am addressing.

Yes I am aware the time to update the display is the reason for a delay in GPS 
receivers; and also aware the GPS engine has all the information to give UTC 
down to a few nanoseconds for some (and at least a  microsecond for other 
lesser beasts) - but that does not exempt the designers from allowing for the 
delays to the display from becoming significant, or flagging up on the 
display when unsuitable.
These are just excuses for a serious design flaw.

My lovely Transpak is indeed a dinosaur compared to modern GPS units which I 
note are now possible within a wristwatch (even if as watches they are a bit 
thick and chunky), but it is actually better than some modern receivers it 
seems from previous posts where someone was complaining their GPS display was 
several seconds 'out'. 

The display of the Transpak can 'hang' for some seconds every now and then too 
whilst it is calculating, but it does at least give the exact time of the 
update as found at calculation for the display update edge - which is of 
course the one second following calculation and sending it to the display, so 
it is always only one second behind the last update time and can be seen as 
such. If it 'hangs'  it flags up "calculating" so you know the time is the 
last update.

The Transpak also has an output socket and  one second pulse output too 
incidentally so I can use it for a laboratory standard, something my friend 
Peter Martinez does for digital radio signal processing.

I can understand too that modern GPS engines are going to calculate faster and 
so give less delay - but nevertheless there is no excuse whatever that the 
display time can, with no effort needed by the designer, be _exact_ and 
displayed as such or flagged as U/S until correct.  It is really rather silly 
to have a piece of kit which receives signals of time to within a few 
nanoseconds, knows it,  and cannot display it accurately.
------

"....If one is half way around the world and 
listening to one of the SW time broadcasts you suggest, it takes almost 
a full second for the broadcast tick to reach the receiver!..."

I could make a correction for the path length for the radio signal from WWV of 
course knowing my geographical position (as one does with Loran effetively) 
and know the time pips to within a fraction of a second - which is what GPS 
does too down to a microsecond or better... but still gets it wrong on the 
display ......  :-) 

Douglas Denny.
Chichester. England.

==============

Original Posting:-

Douglas:

GPS time displays can be "off" for one of two reasons:

1.  The actual time used by the GPS system is not compensated for 
leap-seconds.  Every GPS receiver built in the last 15 or 20 years knows 
the difference and can be set to display correct UTC.   In fact, the 
default settings on GPS receivers are for UTC and not GPS time.   But 
you can change that, perhaps inadvertently.

2.  The time it takes the receiver to display the time (!).   While this 
was significant in older GPS receivers, I've personally not heard a 
complaint by someone owning a more recent receiver.   Regardless, 
though, this should be a consistent delay and we can simply view it as 
watch error and compensate for it as any good navigator ought.   I find 
references to the Trimble Transpak dating back at least to 1993, so it's 
an antique and it's not fair to define the performance of current 
receivers by comparing them to the Transpak.

Let's not forget that that old tried and true standby, radio time 
broadcasts, can also be off.  If one is half way around the world and 
listening to one of the SW time broadcasts you suggest, it takes almost 
a full second for the broadcast tick to reach the receiver!



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