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MQTT payloads {#json}

Greenwich publishes messages containing payloads to particular topics. The topic branch is user-configurable by setting the publish and clientID settings. Assuming the former is set to owntracks/acme and the latter to van17, the base topic becomes owntracks/acme/van17, and that is where location messages (in JSON) are published to. (All publishes are typically retained, unless you configure retain=0.)

Location messages are published in either of two configurable formats:

  1. JSON, if set payload=json (default) has been configured
  2. CSV, if set payload=csv has been configured.


A JSON message as published by Greenwich could look like this^JSON elements marked with an asterisk (*) may or may not be present, depending on the configured fields.:

  "_type": "location",         // type
  "t": "t",                    // trigger type (see table)
  "tst": "1408810440",         // UNIX epoch timestamp
  "tid": "V7",                 // tracker-ID (configurable)
  "lat": "48.858334",          // latitude
  "lon": "2.295134",           // longitude
  "alt": 732,                  // altitude (*)
  "vel": 46,                   // velocity (speed) (*)
  "batt": "12.4",              // external battery level (*)
  "cog": 283,                  // course over ground (*)
  "dist": 569,                 // distance in meters since last publish (*)
  "trip": 8441                 // trip distance in meters since last reboot (*)
                               //  so trip(n+1) = trip(n) + dist(n+1)

In particular, these elements need explanation

  • trip is counted incrementally based on the GPS messages reported per/second, whereas
  • dist is the distance travelled in meters since last publish.


Messages published as CSV (Comma Separated Values) are shorter and thus easier on the volume of data incurred by Greenwich devices. If you are on a tight data budget, CSV is well suited.

In interest of saving as much as possible data, the CSV messages published by the device are as follows.

CSV format

The field names in the diagram correspond to the JSON fields described above. However, the values can be slightly different.

  • tst is the hexadecimal (base 16) representation of the UNIX epoch timestamp.
  • lat and lon are integer representations of the latitude and longitude respectively. To determine the exact decimal positions, divide both of these values by 1 million.
  • cog must be multiplied by 10 to arrive at the original approximate course
  • alt is the original altitude divided by 10.
  • trip is not in meters (as in JSON) but in Km. Multiply this value by 1000 to obtain the value published in JSON.


In the messages above, we mention the word trigger. This describes why a particular location message was published. The following is a list of triggers:

Trigger Reason

f     First publish after reboot.

c     Device started by alarm (c)lock (`AT^CALA`). Alive signal without
      ignition. Sent instead of `f`.  This message is published
      periodically (even without external power) after `sleep` seconds.

a     The Movement Sensor or _accelerometer_ is activated when the software shuts down,
      and is monitored even without external power supply (ignition off).
      Its sensitivity is controlled by the `motion` parameters. It wakes up
      the device when moved. Default for `motion` is 4, which is *very*
      sensitive. I set it to 63 of 255. When I shake it, it wakes up. This
      triggers an `a` (alarm) publish which is published to `../alarm`.

k     When transitioning from _move_ to _stationary_ mode an additional
      publish is sent marked with trigger `k` (park).

L     Last recorded position upon graceful shutdown .

l     GPS signal lost (NMEA `A` transitions to `V`). Even though GPS signal
      has gone (e.g. driven into tunnel) we may still have a GPRS signal, so we
      can publish the `l`ast known position.  It is likely that a `t` will
      follow suit.

m     For manually requested locations (e.g. by publishing to `/cmd`).

t     (time) for location published because device is moving.

T     (Time) Vehicle is immobile and `maxInterval` has elapsed.

v     Move. One `v` trigger is sent on transition from park or `t` to move.

Table: Triggers emitted in location publishes

In addition to location messages as shown above, the Greenwich will also publish additional messages. Recall that the base topic is constructed from the publish setting with clientID appended to it, for example owntracks/acme/van17.

+------------------+-------------------------------------------------------------------------------+ |Topic |Content | +==================+===============================================================================+ |../status |Connection status of the Greenwich +------------------+-------------------------------------------------------------------------------+ | |* 1 Greenwich sets this status immediately after connect | |* 0 MQTT broker sets this status when it looses the connection to the Greenwich client using the LWT mechanism | |* -1 Greenwich disconnects intentionally (on user request, ignition-off, etc.) +------------------+-------------------------------------------------------------------------------+ |../start | IMEI midlet-version date/time | | For example: | | 123456789012345 0.7.38 2014-09-01 08:37:04 +------------------+-------------------------------------------------------------------------------+ |../voltage/batt | Built-in battery voltage is published when voltage | changes "significantly", as configured with dBattVoltage. | (4.4) +------------------+-------------------------------------------------------------------------------+ |../voltage/ext | External power supply voltage is published when voltage | changes "significantly", as configured with dExtVoltage. | (12.2) +------------------+-------------------------------------------------------------------------------+ |../gpio/n | Status of the three GPIO pins (1, 3, 7) is published as 0 or 1. +------------------+-------------------------------------------------------------------------------+ |../cmd/out | Output of commands sent to the device (e.g. login, set): +------------------+-------------------------------------------------------------------------------+ |../alarm | A location payload with a trigger type a is published (non-retained) | when a switched-off device is moved. | {"_type": "location", "t": "a", ...} +------------------+-------------------------------------------------------------------------------+

: Topics published to