[chrony-users] Occasional large offsets with GPS signal

["Torsten Wolf" <torsten.wolf@xxxxxxx>]

This might be slightly off-topic but I don’t know a better place to ask.

For the past two years I have been running chrony on a Raspberry Pi 
which is connected to an Adafruit Ultimate GPS GNSS module via USB and 
PPS. The Pi acts as time server for my router and its WDS nodes (all of 
them running chrony, too). I also poll other NTP servers but for 
information only. I monitor last offset and frequency for all devices 
via InfluxDB, telegraf and grafana. The setup is usually running w/o any 
issues with the offset on the Pi not exceeding 500 ns.

However, every now and then there is a series of days during which in 
the evening hours the offset jumps to 200 ms. After about 4-5 hours the 
distortion disappears and the values reduce to the usual level. The 
current episode started 13 days ago. The previous occured in November 
2024. I thought about GPS jamming and checked the map on the 
flightradar24 website. The affected areas are at least 100 km away and 
have been there for months.

Have you encountered a similar issue? Do you have any suggestions how I 
could find the reason for the distortion?

Thanks
Torsten

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