[time-nuts] Old NMEA GPS units with week rollover bug

Bob kb8tq kb8tq at n1k.org
Sat Apr 22 07:12:54 EDT 2017


If you drive them with a simulator, essentially *all* GPS devices have the “rollover bug”. 
Put another way, take a LEA-(any)T. Cold start it. Feed it with a date ( = week number) 20 
years past when it was made. It will go back and show you a date prior to today. Note - 
the cold start each time is a key part of the process. 

For example only, assume that you have a device that was designed in 2009 and that it the 
basis for it’s firmware date calculation.  The magic week would be around 512. Feed it weeks 512 to 1024 and 
it will show dates from 2009 to 2019. Feed it weeks from 0 to 511 and it will show dates
from 2019 through 2029. Give it week 512 again and it will give you 2009, just like it always

The previously mentioned GSS-6100 single satellite simulators are quite adequate for this testing.
You will not get a PPS with only one sat in view, but the date will come up. As far as I can see, the
date you get this way is indeed the same date you would get with multiple sat’s in view. 

There are (needless to say) a number of flakey ways that GPS modules behave. This may
or may not be the “bug" you are after …..


> On Apr 22, 2017, at 3:30 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> I'm looking for one for testing software.  Does anybody know a model that has 
> this problem and/or know where I can get one?
> PS, Cartoon:
>  https://xkcd.com/1825/
> -- 
> These are my opinions.  I hate spam.
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