Occasional large offsets with GPS signal

This might be slightly off-topic but I don=E2=80=99t know a better place to =
ask.

For the past two years I have been running chrony on a Raspberry Pi=20
which is connected to an Adafruit Ultimate GPS GNSS module via USB and=20
PPS. The Pi acts as time server for my router and its WDS nodes (all of=20
them running chrony, too). I also poll other NTP servers but for=20
information only. I monitor last offset and frequency for all devices=20
via InfluxDB, telegraf and grafana. The setup is usually running w/o any=20
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=20
the evening hours the offset jumps to 200 ms. After about 4-5 hours the=20
distortion disappears and the values reduce to the usual level. The=20
current episode started 13 days ago. The previous occured in November=20
2024. I thought about GPS jamming and checked the map on the=20
flightradar24 website. The affected areas are at least 100 km away and=20
have been there for months.

Have you encountered a similar issue? Do you have any suggestions how I=20
could find the reason for the distortion?

Thanks
Torsten

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