[time-nuts] Stability vs. Accuracy

[Roy Phillips]

Bill

A very big thank you for what must be a near perfect (accurate) answer to a 
question that most of us without Degrees have pondered from time to time. I 
shall paste this into my "book of many answers"


Regards

Roy Phillips.


----- Original Message ----- 
From: "wje" <wje at quackers.net>
To: <ddabney01 at yahoo.com>; "Discussion of precise time and frequency 
measurement" <time-nuts at febo.com>
Sent: Tuesday, July 22, 2008 12:03 PM
Subject: Re: [time-nuts] Stability vs. Accuracy


> Simply put, stability is a measure of how much something varies from a
> constant value.
> A stable value doesn't mean an accurate value; accuracy is a measure of
> how close to some standard value a device emits (in this case) or
> measures, for a meter.
>
> So, you could have something that's perfectly stable but not accurate.
> You can also have something that's accurate but not stable.
>
> In theory, they can be perfectly (ok, nearly perfectly) accurate because
> their lack of stability is averaged out over a long period of time, and
> the average value is an accurate representation of the time kept by the
> satellites, which themselves provide an accurate representation of
> 'true', i.e., NIST time.
>
> In practice they wander around a bit, and that wandering is the
> stability measurement you see. What you can tell from the stability
> figures is that the unit is that close to perfect accuracy most of the
> time. The stability figure is really a measure of the statistical
> probability that at any given time the accuracy is within that bound.
>
> Why do they wander? Many factors contribute, ranging from the stability
> and accuracy of the local VCXO and its control loop to atmospheric
> propagation variations to variations in the satellite clocks themselves,
> etc.
>
> Bill Ezell
> ----------
> They said 'Windows or better'
> so I used Linux.
>
>
>
> Richard Dabney wrote:
>> I'm not a scientist or engineer but have a question to those of you who 
>> are.
>>
>> The many recent posts regarding the GPSDOs and comparisons between the 
>> various ones  have been comparing stability. How about accuracy compared 
>> with the national frequency standard?
>> Are stability and accuracy the same? Stability to E-13.5 with the 
>> Thunderbolt. How close to
>> perfect time and frequency?............  Thanks...Dick W5UFZ    FMT-nut
>>
>>
>>
>>
>>
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