Gateway timestamps in uplink metadata - how calculated?

If a message is received by multiple gateways, where each one gives a timestamp, how is the timestamp calculated which is seen in the node’s uplink metadata?

Is it some kind of average?

It’s simply the server time:

// Time when the server received the message
"time": "1970-01-01T00:00:00Z"

Gateways don’t even need to know the true time (a.k.a. wall time); all LoRaWAN timing is based on relative timestamps that the gateway and server exchange in their messages. (The timestamp field is only seen in the gateways array entries, not on the parent level, which only shows time like above.)

In see the gateway timestamps, as well as a timestamp in the parent, assuming that this is the LoRaWAN network server time?

And another question: How are the receiving gateways sorted in the gateway array in the metadata set? What is the sort criteria?

I guess you’ll need to show an example of what you’re seeing then.

As an aside: even the server’s time will be different for each of the messages received from each gateway. So, maybe it’s the server time of the first message, maybe it’s the server time of the moment the uplink is forwarded to some handler, or maybe it’s something else altogether. :slight_smile: (The code that does this is open source, of course…)

I don’t know how the data is sorted, if it is even sorted explicitly. (But, also open source :slight_smile: )

1 Like

The time stamp at gateway level is the gateway wall time which might (not) be accurate.

Below is what i get:

  1. “time” -> from TTN LoRaWAN server?
  2. “time” -> from gateways
  3. “timestamp” -> from gateways

If assume:

  1. is from TTN LoRaWAN server and is UTC time without leap secs.
  2. is from the gateways and is UTC time, calculated from gps time.
  3. is from the gateways and a monotic counter, in sync with LoRaWAN network

Ist this correct?

    {
        "topic": "test/devices/test/up",
        "payload": {
            "app_id": "test",
            "dev_id": "test",
            "hardware_serial": <deleted>",
            "port": 9,
            "counter": 0,
            "payload_raw": [
                1
            ],
            "payload_fields": {
                "timesync_seqno": 1
            },
            "metadata": {
                "time": "2019-03-10T18:25:18.591541462Z",
                "frequency": 867.9,
                "modulation": "LORA",
                "data_rate": "SF9BW125",
                "airtime": 164864000,
                "coding_rate": "4/5",
                "gateways": [
                    {
                        "gtw_id": "eui-12<deleted>",
                        "timestamp": 1733761908,
                        "time": "2019-03-10T18:25:17.555305Z",
                        "channel": 7,
                        "rssi": -49,
                        "snr": 11.5,
                        "rf_chain": 0,
                        "latitude": 52.<deleted>,
                        "longitude": 13.<deleted>,
                        "altitude": 60
                    },
                    {
                        "gtw_id": "eui-34<deleted>",
                        "timestamp": 550320884,
                        "time": "2019-03-10T18:25:18.393047Z",
                        "channel": 0,
                        "rssi": -59,
                        "snr": 13.25,
                        "rf_chain": 0,
                        "latitude": 52.<deleted>,
                        "longitude": 13.<deleted>,
                        "location_source": "registry"
                    }
                ]
            }
        },
        "qos": 0,
        "retain": false,
        "_msgid": "51e32a21.e32eb4"
    }

Partially. Check my answer above your message. Gateway time is just a wall time, could be GPS based. Could be NTP based, could be manually entered and totally wrong.

Yes, so it will be reliable only, if one has control over the gateway.

But at least it can be probed against current TTN servertime (which i assume as being reliable) in the same dataset, to estimate if it is a valid, recent, gateway time.

I tried to find the logic how the gateway list is sorted in the repo, but no chance without knowledge of the code structure. Can you point me?

As far as I know the order is that in which packets arrive at the back-end. However I never felt the need to check the code. You can take a look at the Go sources for V2 of the back-end at GitHub.

I found the v2 stack sources in github already, but where to look? tons of code…

A few dozen examples would likely yield insight.

  • if you see things that are not in signal strength order, it can’t be that…

  • if you see things that are not in gateway wall-clock order, it can’t be that…

Granted, you could see a pattern in a limited data set which turns out not to be true overall. But at the same time, it would be fair to ask “why do you care?”.

If it matters to you, and it is not explicitly clear in the specifications then you should probably sort the list yourself, to avoid any assumptions on behavior outside the specification on which it would be unsafe to rely, as it’s fair for unspecified behavior to change in a new version.

Incidentally, if a gateway based on a typical Linux has had a loss of Internet backhaul, and especially rebooted while under such loss, it is entirely possible when it gets back online for some packets to be reported with absurdly wrong wall clock times before the system gets an NTP fix and starts using accurate ones, unless it has specific logic to prevent that. Given that GPS can take time to lock, I could image a similar issue on a sufficiently cold start there as well.

3 Likes

Well, you’re right, i should parse the complete list and put my own secret sauce on it, instead relying on a unknown one.

Fun fact: Can ONE gateway be listed TWICE in the TTN metadata.gateways array?

Including future roaming. :slight_smile:

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.

Hi all,

I am trying to create an application where different LoRa devices are synchronised based on a time from TTN server.

For example lets say my GW gives me the bellow information:

{
  ...
  "metadata": {
    "time": "2020-01-15T14:41:55.369687496Z",
    "frequency": 867.3,
    "modulation": "LORA",
    "data_rate": "SF7BW125",
    "airtime": 71936000,
    "coding_rate": "4/5",
    "gateways": [
      {
        "gtw_id": "-----",
        "timestamp": 1880628275,
        "time": "2020-01-15T14:41:55.337396Z",
        ...
      }
    ]
  }
}

For instance, the time from metadata where is it coming from?
How is it calculated?

Best,
Haris

The time field in the metadata is generated by the backend when the (first instance of the) packet arrived from a gateway.

Do you have any feeling how much time this is taking? tens or hundreds of msecs?

If you’re trying to make the nodes transmit at the same time: don’t. That will create network issues. Instead, the node should always apply some randomness when it wants to transmit something.

If you’re trying to make the nodes know the current time, then remember that a downlink is always RX1 or RX2 seconds after the uplink, which can be used to synchronize clocks with a time you create yourself, rather than relying on some timestamp generated by some network server. But beware that a downlink that is scheduled after some uplink might not be transmitted right away, so might be transmitted after the next uplink, which you need to detect by including some counter. For all this, see Time synchronisation of a Node.