[time-nuts] Best Chance GPS module
kb8tq at n1k.org
Thu Dec 1 22:06:28 EST 2016
> On Dec 1, 2016, at 9:47 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> People are mixing precision timing with NTP level or timing. That is way
> the conflict in the quotes below.
> If you care about nanoseconds then yes, location comes first. You first
> use the GPS to do the site survey to determine location from possibly HOURS
> of data collection from a fixed antenna. This gives a very good estimate
> of the antenna location. Then you place the GPS receiver in timing mode
> where youTELL the GPS the location and it computes the time. The GPS can
> give much more certain timing if there is little uncertainty in location.
> This only works for antenna that are bolted down to the top of a
> permanent mount. With a surveyed location the error bars on the time
> are smaller. So for precision time, it is a two step process
> But in the normal use case of a GPS that is turned on at some unknown
> location, yes location and time come together.
Ummm …. errrr …. not so much. If you are in a situation with < 4 sats, you
can get a degraded solution. I have not seen a timing receiver (= one that puts
out the PPS that you need) that will give you a pps without a proper nav
solution. There are *lots* of posts from people on this list who have run into
this problem again and again.
> There is a third mode used for marine navigation. You can set some GPSes
> to "sea level" and tell it the height of the antenna above sea level and
> then the GPS gives better location information because all of the
> uncertainty is taken out of one dimension.
> The people doing those site surveys are likely running precision
> oscillators and worried about errors less than one part in 10 to the 10th
Based on tearing apart a number of survey grade GPS boxes …. not so much. The
oscillator is hardly in the “super duper" category. The improvement in precision comes
from dual (or triple band) operation and post processing.
> On Thu, Dec 1, 2016 at 12:34 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>> kb8tq at n1k.org said:
>>> The navigation solution is something you must have before you can begin
>>> get a timing solution.
>> That sounds like a 2 step process: where, then when. Does it work that
>> I thought you got where and when at the same time - you couldn't get where
>> without also getting when.
>> These are my opinions. I hate spam.
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> Chris Albertson
> Redondo Beach, California
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