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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@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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