[time-nuts] GPSDO Component Selection

Bob Camp lists at rtty.us
Mon Sep 10 00:14:06 UTC 2012


Hi

Except that on the same basis, the non-timing GPS gear is in the $10 to $20 range…

Bob
On Sep 9, 2012, at 7:55 PM, bg at lysator.liu.se wrote:

> Hi Bob,
> 
> This argument has been done before on time-nuts... sorry for repeating.
> 
> There are geodetic quality GPS reveivers, like the Ashtech Z12-CORS (with
> external 5-20MHz input - not the true Z12 Metronome) available for a few
> hundred dollars occasionally. I got my Z12 CORS for free, from a site
> where it had been replaced by modern GPS/GLONASS receivers.
> 
> I also found three Novatel Millenium OEM3 for ca $100 a piece, which in
> their days were used by national time labs. Unfortunately two of them has
> developed a problem with a custom IC.
> 
> I am far from having the economic freedom to purchase a new H-Maser or
> Cesium. However I still have a HP5065A running in the basement. When new
> in the early 1970ties, you could probably have traded the rubidium for the
> house it is now running in.
> 
> Conclusion: used geodetic gps equipment are no more expensive, than many
> of the oscillators we play with. They might actually once have been used
> in the same national time lab... ;-)
> 
> --
> 
>   Björn
> 
> 
> 
>> 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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> 
> 
> 
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