Synchronization over LTE/NB-IoT - SNTP time accuracy

[mjaszczykowski]

Hi,

We are developing an application that requires synchronization between two or more nRF9160-based devices. We have the following requirements and constraints:

• Required time synchronization accuracy: < 1 ms
• GNSS is not available — devices are deployed underground without access to an external antenna
• Both devices connect via LTE-M / NB-IoT

We have the following questions:

1. What is the expected accuracy of SNTP time synchronization over LTE on the nRF9160?
2. What is the expected accuracy of LTE network time received via NITZ or SIB16 (configurable via AT%XNETTIME)?
3. Are either of these solutions capable of meeting our < 1 ms accuracy requirement?
4. Are there any other Nordic-supported time synchronization solutions that could meet our accuracy requirement given our constraints?

Any guidance or alternative approaches would be greatly appreciated.

Thank you.

[tesc]

Hi,

What is the expected accuracy of SNTP time synchronization over LTE on the nRF9160?

While NTP can achieve sub-ms resolution on cabled networks, I would not expect too much from SNTP over an LTE network. Maybe around 100 ms could be achievable, but highly dependent on the network operator.

• What is the expected accuracy of LTE network time received via NITZ or SIB16 (configurable via AT%XNETTIME)?
• Are either of these solutions capable of meeting our < 1 ms accuracy requirement?
• Are there any other Nordic-supported time synchronization solutions that could meet our accuracy requirement given our constraints?

I will need to look further into this, and expect to have more answers within a couple of days.

In the meantime, I highly encourage the community here on DevZone to weigh in.

Regards,
Terje

[Simon D-M]

Hi,

tesc said:

• What is the expected accuracy of LTE network time received via NITZ or SIB16 (configurable via AT%XNETTIME)?
• Are either of these solutions capable of meeting our < 1 ms accuracy requirement?
• Are there any other Nordic-supported time synchronization solutions that could meet our accuracy requirement given our constraints?

I will need to look further into this, and expect to have more answers within a couple of days.

Unfortunately, the intended way of acheiving such precision with the nRF9160 is to use the 1PPS GPS signal.

I think it should be possible to get close to 1ms precision for time synchronization between devices if you handle the SFN counter by yourself. But this is not a " properly supported" way of doing it.

Best regards,

Simon 

[mjaszczykowski]

Thank you.

Could you give me any hint on how to access SFN counter?

[Simon D-M]

Hi,

The SFN counter is available in the PRS reception (link).

Best regards,

Simon

[mjaszczykowski]

Thank you. Just two additional question if I may.

1. Is there any additional piece of the documentation?
2. Is this available only for nRF9151? Am I reading it correctly?