Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Chris Albertson]

On Mon, Aug 1, 2016 at 9:48 AM, Bob Camp  wrote:

>
> The serial string is fine for an "eyeball clock". It’s not so fine for a
> GPSDO.
>
>
That is a good way to say it.  NMEA was designed for marine navigation.
Driving ships and boats across an ocean.So the NMEA that my water speed
sensor produced was good enough it is output at 1Hz. The NMEA is that
the "data needs to be valid within a second of when it is written".
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Nick Sayer via time-nuts]

> On Aug 1, 2016, at 9:33 AM, Chris Albertson  wrote:
> 
> 
> 
> In fact that would be a good experiment:  Put two clocks up on a large
> computer monitor and make one always tick some random number of
> milliseconds away from system time and the other always thick on the
> system time.  Then you click on the one you think is correct.  Can you
> do better than a 50/50 guess. Keep incl=reasing the error until the
> guesses are about 90% correct.   I bet you find you eyes are really
> bad.   You ears are a little better and you might notice 40 mS by
> listening to the "tick" sound
> 

I’ve done something akin to this with my Crazy Clock movements.

The Vetinari clock works by stealing 100 ms from a fraction of the ticks until 
it’s gathered up enough to do a “double tick.” I wrote the firmware and I can’t 
tell which seconds are 100 ms off.

There’s also the “Whacky” firmware. It ticks on a random tenth of a second 
within each second. In practice, it’s far more subtle than I thought it would 
be. It’s only really obvious when it picks values far apart from their 
neighbors.

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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Gary E. Miller]

Yo Mark!

On Sat, 30 Jul 2016 20:14:02 +
Mark Sims  wrote:

> A couple of people have asked about the poor message arrival time
> performance of the popular Adafruit Ultimate GPS receiver. 

I have several months of Adafruit graphs.  I also find that is is usually
as accurate as you would expect.  But often it just seems to have
huge swings in Lat/Lon even while stationary.

Here is are two 24 hour scatter plots:

https://pi2.rellim.com/pi2.png
https://pi2.rellim.com/pi2-2.png

RGDS
GARY
---
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@rellim.com  Tel:+1 541 382 8588


pgpJmQaMv4tGe.pgp
Description: OpenPGP digital signature
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Bob Camp]

HI

You very much are not *done* when you get your point at a million seconds. 
That’s
just where you get to *start* ….

Since you are talking about nearly two weeks per sample, there are a *lot* of 
things 
that could happen. If your loop needs ten samples to do much with, it will be 4 
months
before anything at all happens. In that period, you need to wonder if the power 
will stay on 
for the whole time …. (thunderstorms in the area at the moment …)

Even if you back off to “I only need 1 ppm”, you still have a really slow 
process.

Bob

> On Aug 1, 2016, at 1:35 PM, Hal Murray  wrote:
> 
> 
> kb...@n1k.org said:
>> If I start with a serial string that is good to 10ms and my goal is 10 ppb,
>> I’m waiting for a  million seconds per sample.  
> 
> That also assumes that the temperature isn't changing while you are 
> collecting the data.
> 
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[David J Taylor]

From: Mark Sims

Well,  the whole point of the exercise is to see how well you can do if you 
DON'T have an internet connection, a 1PPS signal,  or a stratum 1 time 
server available...  only the humble messages coming from a 10 dollar GPS 
receiver.  Try getting a net connection in the middle of the Gobi desert 
(where one user uses Lady Heather's time sync feature to keep their system 
clocks reasonably accurate).



If you can put the receiver into a binary message mode, you can usually do 
better than NMEA (but, surprisingly, usually not by much).  And by selecting 
a receiver that has known good / stable message timing you can do 
surprisingly well.



Well, yes, but if you choose a GPS receiver wisely it also has PPS, even on 
some of the cheapest eBay units.  Of course, what is best depends on what 
accuracy the user needs, and how much they are prepared to pay.


Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Bob Camp]

Hi

The issue with any of these approaches is how long it will take to converge.

If I start with a pps that is good to 10 ns and my goal is 10 ns or 10 ppb, I’m 
there in a second.

If I start with a serial string that is good to 10ms and my goal is 10 ppb, I’m 
waiting for a 
million seconds per sample. 

If I want to get an Rb to 1x10^-12, I need to wait for 10,000 seconds per 
sample with the pps.
To do the same thing with the serial string is 10 billion seconds for each 
independent sample. 

The serial string is fine for an "eyeball clock". It’s not so fine for a GPSDO. 

Bob


> On Aug 1, 2016, at 12:33 PM, Chris Albertson  
> wrote:
> 
> You are right.  NTP, even over a poor internet connection can
> typically do better then the tens milliseconds we see with some NEMA
> GPS'.
> 
> But you eyes and human perfection is still even worse.  You can't
> notice 40mS of error.
> 
> In fact that would be a good experiment:  Put two clocks up on a large
> computer monitor and make one always tick some random number of
> milliseconds away from system time and the other always thick on the
> system time.  Then you click on the one you think is correct.  Can you
> do better than a 50/50 guess. Keep incl=reasing the error until the
> guesses are about 90% correct.   I bet you find you eyes are really
> bad.   You ears are a little better and you might notice 40 mS by
> listening to the "tick" sound
> 
> Another thought experiment is to show that the randomness of NMEA
> jitter does not matter would be to try and build a GPSDO that uses
> NMEA data.  If you averaged over a long enough time it would work.
> Might be a way to set a Rb oscillator?
> 
> One reason NTP works so well even over poor Internet connections is
> that it can use 5, 7 or even more other NTP servers to get the time
> and all it needs is that a few of them are good.   I typically use
> five pool servers when I set up NTP
> 
> 
> 
> 
> 
> On Mon, Aug 1, 2016 at 12:01 AM, Hal Murray  wrote:
>> 
>> david-tay...@blueyonder.co.uk said:
>>> From your data and my own measurements, I feel that using the serial NMEA
>>> stream would, today, be a last resort, as an Internet sync would be
>>> considerably better.  Would you agree with that?
>> 
>> Depends on your internet connection and/or the specific GPS module you are
>> using.
>> 
>> I don't know of any good GPS modules that use NMEA.  I do know of really
>> crappy internet connections.  Bufferbloat is the buzzword.
>> 
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
>> ___
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> 
> 
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Chris Albertson]

As an exercise it might be fun to try to do the best you can with just
NMEA.  But practically speaking even my very  $10, 8-channel motorola
GPS receiver can output a PPS to about 50ns.  Better then needed for
NTP.   You friend in the Gobi desert would be better off my $10 GPS

That said, if you had a good local oscillator you could make a decent
GPSDO using NMEA only with a VERY long time constant

On Mon, Aug 1, 2016 at 9:23 AM, Mark Sims  wrote:
> Well,  the whole point of the exercise is to see how well you can do if you 
> DON'T have an internet connection, a 1PPS signal,  or a stratum 1 time server 
> available...  only the humble messages coming from a 10 dollar GPS receiver.  
> Try getting a net connection in the middle of the Gobi desert (where one user 
> uses Lady Heather's time sync feature to keep their system clocks reasonably 
> accurate).
>
>
> If you can put the receiver into a binary message mode, you can usually do 
> better than NMEA (but, surprisingly, usually not by much).  And by selecting 
> a receiver that has known good / stable message timing you can do 
> surprisingly well.
>
>
> --
>
>
>> I feel that using the serial NMEA stream would, today, be a last resort, as 
>> an Internet sync would be considerably better.  Would you agree with that?
>
>
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-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Chris Albertson]

You are right.  NTP, even over a poor internet connection can
typically do better then the tens milliseconds we see with some NEMA
GPS'.

But you eyes and human perfection is still even worse.  You can't
notice 40mS of error.

In fact that would be a good experiment:  Put two clocks up on a large
computer monitor and make one always tick some random number of
milliseconds away from system time and the other always thick on the
system time.  Then you click on the one you think is correct.  Can you
do better than a 50/50 guess. Keep incl=reasing the error until the
guesses are about 90% correct.   I bet you find you eyes are really
bad.   You ears are a little better and you might notice 40 mS by
listening to the "tick" sound

Another thought experiment is to show that the randomness of NMEA
jitter does not matter would be to try and build a GPSDO that uses
NMEA data.  If you averaged over a long enough time it would work.
Might be a way to set a Rb oscillator?

One reason NTP works so well even over poor Internet connections is
that it can use 5, 7 or even more other NTP servers to get the time
and all it needs is that a few of them are good.   I typically use
five pool servers when I set up NTP





On Mon, Aug 1, 2016 at 12:01 AM, Hal Murray  wrote:
>
> david-tay...@blueyonder.co.uk said:
>> From your data and my own measurements, I feel that using the serial NMEA
>> stream would, today, be a last resort, as an Internet sync would be
>> considerably better.  Would you agree with that?
>
> Depends on your internet connection and/or the specific GPS module you are
> using.
>
> I don't know of any good GPS modules that use NMEA.  I do know of really
> crappy internet connections.  Bufferbloat is the buzzword.
>
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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-- 

Chris Albertson
Redondo Beach, California
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[time-nuts] Adafruit Ultimate GPS timing message arrival times

[Mark Sims]

Well,  the whole point of the exercise is to see how well you can do if you 
DON'T have an internet connection, a 1PPS signal,  or a stratum 1 time server 
available...  only the humble messages coming from a 10 dollar GPS receiver.  
Try getting a net connection in the middle of the Gobi desert (where one user 
uses Lady Heather's time sync feature to keep their system clocks reasonably 
accurate).


If you can put the receiver into a binary message mode, you can usually do 
better than NMEA (but, surprisingly, usually not by much).  And by selecting a 
receiver that has known good / stable message timing you can do surprisingly 
well.


--


> I feel that using the serial NMEA stream would, today, be a last resort, as 
> an Internet sync would be considerably better.  Would you agree with that?


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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Paul]

On Mon, Aug 1, 2016 at 3:01 AM, Hal Murray  wrote:

> Depends on your internet connection and/or the specific GPS module you are
> using.
>
> I don't know of any good GPS modules that use NMEA.  I do know of really
> crappy internet connections.  Bufferbloat is the buzzword.
>

Bufferbloat is an issue but the NTP problem is unknown assymetric delays.

If you're a bit flexible regarding "module" you can do quite well using
NMEA.  Both points are shown in the output below (sent using a constant
width font).  FURY is a Fury, offset and jitter in milliseconds.

$ ntpq -p
 remote   refid  st t when poll reach   delay   offset
jitter
==
oPPS(0)  .GPPS.   0 l38  3770.000   -0.002
0.003
*GPS_NMEA(0) .FURY.   0 l28  3770.000   -0.010
0.018
 time-d.nist.gov .ACTS.   1 u  264  512  353  105.770   29.742
0.348
+nub .GPPS.   1 s4   16  3770.151   -0.025
0.022


Compared to the MTK3xxx in the Adafruit UGPS circa 2-3 years ago.

 remote   refid  st t when poll reach   delay   offset
jitter
==
oPPS(0)  .GPPS.   0 l48  3770.0000.002
0.004
*GPS_NMEA(0) .ADAU.   0 l38  3770.000  -78.472
25.623
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Hal Murray]

david-tay...@blueyonder.co.uk said:
> From your data and my own measurements, I feel that using the serial NMEA
> stream would, today, be a last resort, as an Internet sync would be
> considerably better.  Would you agree with that? 

Depends on your internet connection and/or the specific GPS module you are 
using.

I don't know of any good GPS modules that use NMEA.  I do know of really 
crappy internet connections.  Bufferbloat is the buzzword.



-- 
These are my opinions.  I hate spam.



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[time-nuts] Adafruit Ultimate GPS timing message arrival times

[Mark Sims]

Yes,  I find it confusing also.   I've been reporting the value that Lady 
Heather uses to do most of her evil internal message off machinations with...   
 For those,  the negative offset value is the "natural" polarity.  I'm probably 
going to change it around to something humans (including me) find more natural 
to interpret.


-


> Thanks, Mark. I'd missed that in the post. I still find it confusing to plot 
> positive times as negative ones.

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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[David J Taylor]

From: Mark Sims

As mentioned in the post the times reported are the time stamp in the 
receiver packet  minus the system clock time when it was received... 
negative value indicate the message arrives after the PPS.  The polarity of 
the reported value is consistent with how Lady Heather makes use of the 
value...

===

Thanks, Mark.  I'd missed that in the post.  I still find it confusing to 
plot positive times as negative ones.


From your data and my own measurements, I feel that using the serial NMEA 
stream would, today, be a last resort, as an Internet sync would be 
considerably better.  Would you agree with that?


Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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[time-nuts] Adafruit Ultimate GPS timing message arrival times

[Mark Sims]

As mentioned in the post the times reported are the time stamp in the receiver 
packet  minus the system clock time when it was received...  negative value 
indicate the message arrives after the PPS.  The polarity of the reported value 
is consistent with how Lady Heather makes use of the value...


---


>  Are you sure that the arrival time is negative? In my experience, the NMEA 
> message is /after/ the PPS, and not before it.
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[David J Taylor]

From: Mark Sims

A couple of people have asked about the poor message arrival time 
performance of the popular Adafruit Ultimate GPS receiver.   I modified Lady 
Heather to analyze the message arrival times using a histogram instead of a 
simple average.  When I looked at the histogram data (.01 msec resolution), 
I was rather shocked...  With an hour of data,  most receivers have maybe a 
couple dozen bins hit,  with the peak bin several hundred counts above the 
next lower peak.   The Adafruit had over 1800 bins hit, with the peak bin 
having six hits.   Attached is the histogram...  you probably don't want to 
use this receiver to drive a clock based upon message arrival times...

==

Mark,

Are you sure that the arrival time is negative?  In my experience, the NMEA 
message is /after/ the PPS, and not before it.


Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Nick Sayer via time-nuts]

I’ve used the PA6H so far for all of my GPSDOs. The two reasons I am 
considering the Venus838LPx-T to replace it are that it has a sawtooth 
correction message and it has a survey and static solution mode to allow it to 
tolerate poorer reception.

I’m driven to want to experiment with a static mode GPS receiver from this page 
on Atilla Kinali’s blog: 
https://attila.kinali.ch/blog/2016/02/07/gps-disciplined-oscillator, which 
references some e-mail here from some time ago. My take-away was that 
navigation mode receivers could achieve good results as long as they had ideal 
reception, but with poor reception they would do much worse than a receiver in 
static mode that’s done a good survey.

As for the sawtooth correction, I’m less convinced that it’s required given 
that I’m using averaging on the phase detector output. I can’t detect hanging 
bridges with the current setup, but since I haven’t actually looked for them, 
ignorance is bliss. Tom, if you still have the data, can you speak to whether 
you saw anything of the sort? In any event, it’s a box I’d like to check if for 
no other reason than to learn something.

> On Jul 30, 2016, at 2:32 PM, Tom Van Baak <t...@leapsecond.com> wrote:
> 
> Hi Brooke,
> 
> That's a reasonable assumption. I haven't ever tried mine at 10 Hz. But note 
> that fast update rates is more meant for navigation and positioning than it 
> is for timing.
> 
> Just in case we have some newcomers to the thread I'd like to point out that 
> this recent series of measurements of RS232 / NMEA have no bearing at all on 
> the quality of the timing output. Timing NMEA is more of a curiosity; 
> something to measure at the hundreds or tens of millisecond level. As anyone 
> knows, the real timing output of these receiver is the 1PPS pulse itself, 
> which is good to the tens of nanoseconds level. So a factor of a million 
> different.
> 
> I actually like the Adafruit GPS receiver, and would recommend it to anyone. 
> I use it for projects around the house more than any other receiver. It's so 
> simple to use -- no configuration needed, no sawtooth correction needed, no 
> survey required, fast acquisition, works fixed or mobile, sensitive antenna 
> included on board, small and low power, just give it 5 volts and out comes a 
> UTC 1PPS.
> 
> Its 1PPS output is superb. Attached is the ADEV/MDEV of the raw 1PPS of the 
> Adafruit GPS board.
> 
> /tvb
> 
> - Original Message - 
> From: "Brooke Clarke" <bro...@pacific.net>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts@febo.com>
> Sent: Saturday, July 30, 2016 1:50 PM
> Subject: Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times
> 
> 
>> Hi Mark:
>> 
>> Isn't this the receiver that hears a very large number of GNSS satellites 
>> and also has a 10 Hz update rate?
>> If so, I'd expect that there would a large variation in message lengths.  
>> How stable  is the 10 PPS or 1 PPS output?
>> 
>> -- 
>> Have Fun,
>> 
>> Brooke Clarke
>> http://www.PRC68.com
>> http://www.end2partygovernment.com/2012Issues.html
>> The lesser of evils is still evil.
>> 
>>  Original Message 
>>> A couple of people have asked about the poor message arrival time 
>>> performance of the popular Adafruit Ultimate GPS receiver.   I modified 
>>> Lady Heather to analyze the message arrival times using a histogram instead 
>>> of a simple average.  When I looked at the histogram data (.01 msec 
>>> resolution), I was rather shocked...  With an hour of data,  most receivers 
>>> have maybe a couple dozen bins hit,  with the peak bin several hundred 
>>> counts above the next lower peak.   The Adafruit had over 1800 bins hit, 
>>> with the peak bin having six hits. Attached is the histogram...  you 
>>> probably don't want to use this receiver to drive a clock based upon 
>>> message arrival times...
>>> 
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Hal Murray]

hol...@hotmail.com said:
> A couple of people have asked about the poor message arrival time
> performance of the popular Adafruit Ultimate GPS receiver.   I modified
> Lady Heather to analyze the message arrival times using a histogram
> instead of a simple average.  When I looked at the histogram data
> (.01 msec resolution), I was rather shocked...  With an hour of data,
> most receivers have maybe a couple dozen bins hit,  with the peak bin
> several hundred counts above the next lower peak.   The Adafruit had
> over 1800 bins hit, with the peak bin having six hits. ...

Have you tried bigger bins?  10 microseconds was too small for me.  100 
worked well.

The SiRF chips are famous for horrible timing.  The arrival times were spread 
over 100 ms, but it was a slow wander rather than random jitter.  The pattern 
was a sawtooth, but the period was hours to many hours.  Something like that 
could lead to what you are seeing.

The old Garmin GPS-18 was reasonably good.  The new GPS-18x has the same 
horrible pattern as the SiRF.




-- 
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Hal Murray]

t...@leapsecond.com said:
> Just in case we have some newcomers to the thread I'd like to point out
> that this recent series of measurements of RS232 / NMEA have no bearing at
> all on the quality of the timing output. Timing NMEA is more of a
> curiosity; something to measure at the hundreds or tens of millisecond
> level. As anyone knows, the real timing output of these receiver is the
> 1PPS pulse itself, which is good to the tens of nanoseconds level. So a
> factor of a million different. 

I think that depends on how far down the nutty scale you are operating.

I think all the investigations of timing over serial ports are useful.  You 
can get sub millisecond timing over RS-232.  That is interesting for things 
like NTP if you don't have a PPS connection, so it helps to know which chips 
are good and which ones to avoid,

Serial ports are getting rare.  Most USB to RS-232 chips are slow with a 1 ms 
polling rate.  I think AdaFruit has one that polls 8x faster.  That should 
get sub-ms timing if coupled with a good GPS chip.


-- 
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Bob Stewart]

I liked the Adafruit too when I was using it a couple of years ago, but it did 
have a bad habit of going walkabout.  Yeah, a lot of that was my antenna, but 
it's still worth mentioning.
Bob -
AE6RV.com

GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info

  From: Tom Van Baak <t...@leapsecond.com>
 To: Discussion of precise time and frequency measurement <time-nuts@febo.com> 
 Sent: Saturday, July 30, 2016 4:32 PM
 Subject: Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times
   
Hi Brooke,

That's a reasonable assumption. I haven't ever tried mine at 10 Hz. But note 
that fast update rates is more meant for navigation and positioning than it is 
for timing.

Just in case we have some newcomers to the thread I'd like to point out that 
this recent series of measurements of RS232 / NMEA have no bearing at all on 
the quality of the timing output. Timing NMEA is more of a curiosity; something 
to measure at the hundreds or tens of millisecond level. As anyone knows, the 
real timing output of these receiver is the 1PPS pulse itself, which is good to 
the tens of nanoseconds level. So a factor of a million different.

I actually like the Adafruit GPS receiver, and would recommend it to anyone. I 
use it for projects around the house more than any other receiver. It's so 
simple to use -- no configuration needed, no sawtooth correction needed, no 
survey required, fast acquisition, works fixed or mobile, sensitive antenna 
included on board, small and low power, just give it 5 volts and out comes a 
UTC 1PPS.

Its 1PPS output is superb. Attached is the ADEV/MDEV of the raw 1PPS of the 
Adafruit GPS board.

/tvb

- Original Message - 
From: "Brooke Clarke" <bro...@pacific.net>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Sent: Saturday, July 30, 2016 1:50 PM
Subject: Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times


> Hi Mark:
> 
> Isn't this the receiver that hears a very large number of GNSS satellites and 
> also has a 10 Hz update rate?
> If so, I'd expect that there would a large variation in message lengths.  How 
> stable  is the 10 PPS or 1 PPS output?
> 
> -- 
> Have Fun,
> 
> Brooke Clarke
> http://www.PRC68.com
> http://www.end2partygovernment.com/2012Issues.html
> The lesser of evils is still evil.
> 
>  Original Message 
>> A couple of people have asked about the poor message arrival time 
>> performance of the popular Adafruit Ultimate GPS receiver.  I modified Lady 
>> Heather to analyze the message arrival times using a histogram instead of a 
>> simple average.  When I looked at the histogram data (.01 msec resolution), 
>> I was rather shocked...  With an hour of data,  most receivers have maybe a 
>> couple dozen bins hit,  with the peak bin several hundred counts above the 
>> next lower peak.  The Adafruit had over 1800 bins hit, with the peak bin 
>> having six hits.  Attached is the histogram...  you probably don't want to 
>> use this receiver to drive a clock based upon message arrival times...
>>
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Tom Van Baak]

Hi Brooke,

That's a reasonable assumption. I haven't ever tried mine at 10 Hz. But note 
that fast update rates is more meant for navigation and positioning than it is 
for timing.

Just in case we have some newcomers to the thread I'd like to point out that 
this recent series of measurements of RS232 / NMEA have no bearing at all on 
the quality of the timing output. Timing NMEA is more of a curiosity; something 
to measure at the hundreds or tens of millisecond level. As anyone knows, the 
real timing output of these receiver is the 1PPS pulse itself, which is good to 
the tens of nanoseconds level. So a factor of a million different.

I actually like the Adafruit GPS receiver, and would recommend it to anyone. I 
use it for projects around the house more than any other receiver. It's so 
simple to use -- no configuration needed, no sawtooth correction needed, no 
survey required, fast acquisition, works fixed or mobile, sensitive antenna 
included on board, small and low power, just give it 5 volts and out comes a 
UTC 1PPS.

Its 1PPS output is superb. Attached is the ADEV/MDEV of the raw 1PPS of the 
Adafruit GPS board.

/tvb

- Original Message - 
From: "Brooke Clarke" <bro...@pacific.net>
To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com>
Sent: Saturday, July 30, 2016 1:50 PM
Subject: Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times


> Hi Mark:
> 
> Isn't this the receiver that hears a very large number of GNSS satellites and 
> also has a 10 Hz update rate?
> If so, I'd expect that there would a large variation in message lengths.  How 
> stable  is the 10 PPS or 1 PPS output?
> 
> -- 
> Have Fun,
> 
> Brooke Clarke
> http://www.PRC68.com
> http://www.end2partygovernment.com/2012Issues.html
> The lesser of evils is still evil.
> 
>  Original Message 
>> A couple of people have asked about the poor message arrival time 
>> performance of the popular Adafruit Ultimate GPS receiver.   I modified Lady 
>> Heather to analyze the message arrival times using a histogram instead of a 
>> simple average.  When I looked at the histogram data (.01 msec resolution), 
>> I was rather shocked...  With an hour of data,  most receivers have maybe a 
>> couple dozen bins hit,  with the peak bin several hundred counts above the 
>> next lower peak.   The Adafruit had over 1800 bins hit, with the peak bin 
>> having six hits.   Attached is the histogram...  you probably don't want to 
>> use this receiver to drive a clock based upon message arrival times...
>>
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[time-nuts] Adafruit Ultimate GPS timing message arrival times

[Mark Sims]

It is only tracking the GPS sats,  running at 1Hz rate.   Almost every GPS 
receiver that can run faster than 1Hz has warnings that the 1PPS output is only 
valid/stable if the device is configured for 1Hz output.



My Ublox 8 receiver tracks GPS/Glonass/Beidou/SBAS sats.   I have seen it 
tracking over 24 sats, but its NMEA and binary messages timings are very 
stable.  Remarkable considering all the NMEA sentences it take to report all 
those sats.



I'm currently collecting histogram data from the Trimble Resolution-T SMT (TEP) 
receiver... it has one of the worse standard deviations of all the receivers 
that I tested (the TEP version can run in either TSIP and Motorola emulation 
modes, both had poor message timings).   The original Resolution-T had very 
stable message timing performance.


-


> Isn't this the receiver that hears a very large number of GNSS satellites and 
> also has a 10 Hz update rate? If so, I'd expect that there would a large 
> variation in message lengths. How stable is the 10 PPS or 1 PPS output?
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Re: [time-nuts] Adafruit Ultimate GPS timing message arrival times

[Brooke Clarke]

Hi Mark:

Isn't this the receiver that hears a very large number of GNSS satellites and 
also has a 10 Hz update rate?
If so, I'd expect that there would a large variation in message lengths.  How 
stable  is the 10 PPS or 1 PPS output?

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
The lesser of evils is still evil.

 Original Message 

A couple of people have asked about the poor message arrival time performance 
of the popular Adafruit Ultimate GPS receiver.   I modified Lady Heather to 
analyze the message arrival times using a histogram instead of a simple 
average.  When I looked at the histogram data (.01 msec resolution), I was 
rather shocked...  With an hour of data,  most receivers have maybe a couple 
dozen bins hit,  with the peak bin several hundred counts above the next lower 
peak.   The Adafruit had over 1800 bins hit, with the peak bin having six hits. 
  Attached is the histogram...  you probably don't want to use this receiver to 
drive a clock based upon message arrival times...


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[time-nuts] Adafruit Ultimate GPS timing message arrival times

[Mark Sims]

A couple of people have asked about the poor message arrival time performance 
of the popular Adafruit Ultimate GPS receiver.   I modified Lady Heather to 
analyze the message arrival times using a histogram instead of a simple 
average.  When I looked at the histogram data (.01 msec resolution), I was 
rather shocked...  With an hour of data,  most receivers have maybe a couple 
dozen bins hit,  with the peak bin several hundred counts above the next lower 
peak.   The Adafruit had over 1800 bins hit, with the peak bin having six hits. 
  Attached is the histogram...  you probably don't want to use this receiver to 
drive a clock based upon message arrival times...
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