Periodic spike in offset for pps

Martin Boissonneault dev at
Tue May 5 19:33:34 UTC 2020

Hi everybody,

On 2020-05-05 15:22, Doug Curtis via users wrote:
> On Tue, May 5, 2020 at 2:51 PM Paul Theodoropoulos via users 
> <users at <mailto:users at>> wrote:
> Only tangentially related  - since only two of the three are using the
>     Adafruit gps hat - are you regularly (weekly, biweekly)
>     downloading the
>     satellite ephemeris data from mediatek?
> I'm not familiar with that so I'll have to do some reading on it.

It should be useless for a receiver that is always on. The idea is to 
load the GNSSr with a database of what the birds are currently 
broadcasting to speed up acquisition. All that data is broadcast, and 
your GNSSr will fetch it in real time from the birds. What it hears is 
the current data, unlike a download database that does age. In short, 
useful for cold start, not useful after.

>     The most peculiar characteristic is the 'long wave' rise of positive
>     offset spikes followed later by negative offset spikes
>     (specifically the
>     'server offset SHM1' graph). The only thing that it vaguely
>     matches up
>     with is the local GPS long wave due to visibility. But again - spikes
>     would certainly not be an expected symptom of visibility!
>     Are you using a common power supply/source for all three pi's -
>     including
>     the ntpviz device?
> The ntpviz server is a separate Centos 7 server in my rack on a 
> different circuit.  Each pi has its own power supply and they are on 
> separate circuits.  I believe the pi3 and pi4 are using Canakit power 
> supplies.  Because of this testing, each antenna is on a different 
> side of the house (west, south, and east).  This does start me 
> wondering the cleaniness of my power at the house though.  Since the 
> spikes don't seem to happen on all of them at the same 34 minute 
> interval, I'm guessing it's probably not something noisy on my house 
> power.  I did just receive a pi3 with an office raspberry pi power 
> supply and once I solder the Adafruit breakout gps kit, I plan on 
> testing with it as well.

How about power interference due to motors/heating/cooling? Your graphs 
are very different from my spikes, btw

>     Pure shots in the dark. I've dealt with intermittent spiking in
>     the long
>     ago past, with a resolution that is very unlikely to be related
>     (turning
>     off the Monit monitoring application, which isn't part of the default
>     install of debian/raspbian)
> Thanks for the ideas.  I've just removed the avahi daemon and will 
> check on the results of that.  I plan to go through and eliminate 
> unneeded services one by one to try and narrow this down further.
> Doug
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