[time-nuts] Warped back to 1993

Hal Murray hmurray at megapathdsl.net
Sun Aug 11 05:07:47 UTC 2013


> With the wrong date and time, the GPS should not find almanac data,  so will
> not lock. 

I don't think that's the right way of describing the problem.

The satellites broadcast on a known frequency, but that gets shifted all over 
the place by Doppler.   ("All over" means a big shift relative to the 
bandwidth of the signal.)

If you have a recent almanac and you know the date/time and location, then 
you can compute the Doppler and look in the right frequency and find the 
satellites quickly.  In this context, "find" means hearing a signal at an 
expected frequency.  If you don't hear anything where you expect it, then you 
get to check nearby frequencies.  If you don't find anything nearby, you get 
to give up and start searching the whole Doppler range.  It's the difference 
between warm start and cold start.

Once you do find several satellites, you can figure out the date/time and 
location and after a while get a new almanac.

Assume you have done all that.  You still don't really know the date.  It's 
like looking at a clock on the wall.  It tells you the time but not the date. 
 (Or looking at a digital display that tells you the month and day but not 
the year.)  The GPS signal tells you the date within a 1024 week epoch, but 
it doesn't tell you which epoch you are in.  Telling it the date has the side 
effect of telling it the epoch.

Real early GPS gear punted this problem.  There is no way to tell them the 
epoch.  I don't remember any details, but there have been various discussions 
about gear that is now useless.  (With some simple post processing, you could 
fix that.  That's assuming you have a serial interface rather than a 
7-segment display.)



-- 
These are my opinions.  I hate spam.






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