[time-nuts] GPSDO Component Selection

[Bob Camp]

Hi

Indeed true for most non-geodetic gps units. Put another way - true unless you have a lot of money.

Bob

On Sep 9, 2012, at 7:25 PM, bg at lysator.liu.se wrote:

> Hi Bob,
> 
> Probably true for Motorola Oncores. Not very true for geodetic receivers.
> 
> Until you have a receiver clock that is on par with the satellite clocks
> AND you are short on visable satellites. This might be true if you can
> load up a modern cesium in your vehicle, and go for a downtown "urban
> valley" type of scenario.
> 
> On a stationary site, your expensive clock will not matter to much, since
> your solution is already pretty over-determined with some 60 measurements
> on each epoch. (9 GPS +6 Glonass)*2(L1/L2)*2 (code + phase)
> 
> --
>    Björn
> 
>> Hi
>> 
>> Position accuracy and timing accuracy are two very different things.
>> Firmware is optimized to improve either one. "Position" firmware is often
>> pretty poor for timing.
>> 
>> Bob
>> 
>> On Sep 9, 2012, at 5:05 PM, Chris Albertson <albertson.chris at gmail.com>
>> wrote:
>> 
>>> On Sun, Sep 9, 2012 at 1:14 PM,  <bg at lysator.liu.se> wrote:
>>> 
>>>> True for a cheap oem navigation receiver. Not true for a geodetic
>>>> quality
>>>> receiver, who usually have some options (external frequency input,
>>>> PPS_in)
>>>> to make them the best timing receivers available. However they are much
>>>> more expensive than the typical single frequency timing reciver.
>>> 
>>> I looked at every link and can't see where they give a timing accuracy
>>> spec on the PPS with respect to UTC.   Possition accurracy is very
>>> good and we might assume the timing is as good.  But they don't say it
>>> is.  What's interesting is these GPSes will accept an accurate clock
>>> input in order to give better location data.   That is the opposite of
>>> a timing GPS where you tell it accurate location data so that it can
>>> get better timing.   Cutting down the unknown in one lets you do
>>> better in the other.   I assume these all cost well over $50.  You can
>>> get a pretty good timing GPS for $30 and it WILL have the PPS error
>>> specified.
>>> 
>>> To the OP.  None of this matters a lot because PPS is a standard input
>>> signal.  It is easy to swap out a GPS receiver later.  Same with the
>>> OCXO.  From a control point of view they are all pretty much the same.
>>> You can swap them out later
>>> 
>>> 
>>> Chris Albertson
>>> Redondo Beach, California
>>> 
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>> 
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> 
> 
> 
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