Radio gateway time synchronization... Suggestions

LPWAN Developer Gateways / Concentrators RAK7243
1

Hello guys

So, once I have figured it out how to obtain nanosecond on timestamps, I did a test.

So, on an open space for tests, with 03 RAKs connected to TTN, I programed a lorawan mote to send some payload to TTN periodically… And obviusly, I expected to receive the messages through the 03 RAKs in order to check the respective timestamps (that each radio gateway imparts) on TTN App GUI.

Once I checked them, I noticed that the timestamps differ up to microseconds… (10us at most). If we do a simple multiplication d=c.t (c=speed of ligth, t = timestamps difference) we can figure out that the distance between the radio gateways is supposed to be hundreds or even thousends of kilometers, which is false.

So, this is obviusly a sync problem between the RAKs. From my POV, I Just can figure out that maybe:

  1. RAKs are obtaining their time sync from different satellites. Or,
  2. RAKs are obtaining their time sync from same satellite but at different times and therefore timestamping differs.

Can you please help me out to figure out a clue to what is happening?

Thanks in avance for your support.

2

2019-08-20%2009%3A01%3A07

2019-08-20%2009%3A00%3A50

Which is your RAK7243?
The first picture has an LTE module, and the second one has no LTE module.

3

Hi ZhuQI

Yes, I have 03 RAKs 7243 with LTE modelue, altough I haven’t use them yet. Does LTE module supports 3g and 2g network too? Because on quectel datasheet it mentions wcdma and gprs bands.

Thanks

4

Regarding the bands of LTE module support, you can see it

5

Hi @researcher
Can you give us a ssh screens on all 3 gateways receiving at same time? Please give us screens from raw frames timestamp in ttn (not ttn timestamp).

Regards
Todor Velev

6

Hello Velev, thanks for your response.

This is a LoRaWAN packet received at TTN,

application_console_TTN
application_console_TTN.jpg1406×1313 249 KB

The following correspond to the LoRaWAN packet on each RAK (command: tail -f /var/log/syslog | grep rxpk)

RAK1

gateway1
gateway1.png1285×235 266 KB

RAK2

gateway2
gateway2.png1265×198 224 KB

RAK3

gateway3
gateway3.png1398×194 213 KB

global_conf.json (GPS config only) on each RAK (nothing modified on local_conf.json)
image

And this is very repetitive on the obtained samples.
Based on this, the question arise:

How can people synchronize their RAKs? I’ve read some “LoRa geolocation based on TDoA” thesis but they never mention that they struggle with timestamping … so I assumed that this is an easy issue but apparently it is not. As you can see, timestamps show different times … Is this normal? Any suggestions on how can I synchronize the RAKs? Thanks so much for your attention

7

Hello everyone …
Is there any suggestions on how we can synchronize better the 03 RAKs?
I have many doubts about if it is possible to synchronize the 03 RAKs only with the GPS at a level of micro-seconds. …

Best regards, thanks