Class B Downlinks not scheduled for right time in chirpstack

Question

Hi Everyone, @brocaar I have been trying to work with lorawan network for class B, So basically when i am scheduling downlink for class B using chirpstack so it is sent to the gateway with tmms (consider 1447146208.530) and packet forwarder calculates concentrator_time(consider 788596175) as per this tmms value and consider it to transmit at that concentrator time but calculated concentrator_time for transmission is around 0.2 ms to 0.9ms / 1sec behind the current concentrator time(consider 78950066) as i tried printing these in packet forwarder.
So can you pls help me in figuring out the solution for this?
Hoping best for the response.

Thanks

john · Answer

hi,
I was able to fix the problem by some slight modfication in semtech packet forwarder source code like calculating GPS time from UTC time and class B works fine for my case with the quectel module itself.

Thanks

Orne Brocaar · Answer

Please use the Add comment next time, as the Post your answer for answering your original question, not for providing more context.

To hopefully answer your question:

The Waveshare concentrator module does not have a uBlox GNSS module, it has a Qualtec GNSS module. The Semtech HAL expects a uBlox module, as it uses a proprietary / binary payload for time synchronization. This might be the cause of your issue.

The warning in your logs might be related to this:

WARNING: [gps] GPS out of sync, keeping previous time reference

Class B Downlinks not scheduled for right time in chirpstack

2 Answers