[time-nuts] GPSDO Component Selection

bg at lysator.liu.se bg at lysator.liu.se
Sun Sep 9 23:55:46 UTC 2012


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where it had been replaced by modern GPS/GLONASS receivers.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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