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Re: [gpsd-dev] GPS week wraparound

From: Magnus Danielson
Subject: Re: [gpsd-dev] GPS week wraparound
Date: Sat, 28 Sep 2013 08:24:38 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130827 Icedove/17.0.8

On 09/27/2013 11:50 PM, Hal Murray wrote:
> address@hidden said:
>> Notice that the leap second insertion occurs only the last minute of the UTC
>> month, with preference for end of each half year. You can do a hold-down the
>> last 15 min of each month where new devices is not trusted. 
> Are on a wild goose chase?  Let me try again.
> The leap second offset is in the almanac.  It takes 15 minutes to download a 
> new almanac.  You have to have an almanac before you can get a sensible 
> solution, but old almanacs are good enough.
> In this context, old means hours or days.  The leap offset gets updated 
> months before the leap happens, so an old almanac is good enough and we won't 
> have a problem.
I know.
> Does a GPS receiver need an almanac to receive and decode a signal?  Can it 
> work with a partial almanac?
No. It can start tracking signals and even get position prior to having
almanac. It just helps with the scheduling. Each bird has a precision
orbit so once tracked that part of the almanac can be derived.

Even if you only have a rough idea about UTC time, for a particular
point in time you don't know if a new receiver have received enough info
to give coherrent and correct view on the seconds reading due to the
start-up effect. So, if the receiver is outside of +/- 500 ms of current
system time, hold it down for 15 min before accepting it's time should
handle the concern.


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