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    Re: Choice of timepiece
    From: Lu Abel
    Date: 2009 Nov 13, 19:50 -0800

    Doug:
    
    Without access to both the design specifications and the actual circuits 
    of your Magellen, it's hard to answer the question.
    
    But I can venture a few comments:
    
    First of all, while there are GPS receivers designed to behave as 
    accurate time receivers, consumer-grade receivers are designed to give 
    position.   Yes, one must determine time to nanosecond precision to 
    determine position, I suspect actually displaying highly accurate time 
    information was simply not in the design specs for your "good old" receiver.
    
    Consider the implications of being a second or two late in a display:   
    A delay in time is quite noticeable if one has another source of 
    accurate time to compare to the GPS.   But being a second or two late 
    with a display of position?   Unless you're in a supersonic aircraft, it 
    hardly matters.   And let's not forget that your Trimble was built in 
    the days of Selective Availability, where positional data had a 100 
    meter inaccuracy built into them
    
    Secondly, comparing (unfavorably) the performance of your Trimble to 
    today's receivers is unfair.  It's entirely like comparing the 
    performance of a 20 year old PC with today's PCs.   I paid over $2,000 
    for a 80486-based PC 18 years ago.   Last year I bought a PC that is 100 
    times faster and has 100 times as much memory and 100 times as much disk 
    -- for $200!  I'm not at all surprised that your Trimble "takes a break" 
    to catch up on its calculations every now and then -- the microprocessor 
    in it is probably no more powerful than the one that's found in hotel 
    electronic locks these days.
    
    Lu Abel
    
    
    
    douglas.denny{at}btopenworld.com wrote:
    > 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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