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    Re: Choice of timepiece
    From: Douglas Denny
    Date: 2009 Nov 13, 17:47 -0800

    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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