TTNHABBridge

Project TTNHABBridge
A software bridge between TTN and the HAB network
Status	In progress
Contact	bertrik
Last Update	2017-08-28

Status

The bridge application has been implemented in Java and works.

Next steps:

• investigate "cayenne" encoding for the payload. This also appears to be a setting in the Sodaq tracker firmware.
• think about a way to specify the payload encoding (autodetect, specify in config file?)
• discuss/review with the people at habhub/ukhas/TTN

Issues:

• consider merging all of the payload telemetry in a single document: put more than one receiver in field "receivers"
  • if I try this, I get a "500 internal server error", so I won't try this anymore
• we have no way yet to tell what TTN payload format is used

Introduction

This idea is about using the-things-network as a receiver for amateur balloon telemetry.

Receiving telemetry from amateur balloons is currently typically done on the 434 MHz band using RTTY modulation, with dedicated receivers listening for RTTY-modulated ASCII strings. The operator of each receiver has to prepare his radio setup for receiving the telemetry, by tuning to the correct frequency at the correct time, setting up a dedicated software client that decodes the RTTY modulation and forwards the data to a central system over the internet. The central system accepts data from many such receivers, performs de-duplication, keeps track of who received what and updates a nice graphical map of where each balloon is and where the receivers are. It can do a burst prediction, landing prediction based on a weather model

A network like the-things-network can help a lot, it has a lot of gateways already (in the Netherlands at least..), already performs deduplication. It uses LoRa as a modulation scheme which is much more sensitive and much less susceptible to slight tuning errors than RTTY.

In short, the idea is:

• you attach a LoRaWAN transmitter to the balloon
• the LoRaWAN transmitter is pre-configured with a set of keys generated by the TTN
• the balloon broadcasts its telemetry every once in a while (say a few times per minute) and this is picked up by one or more TTN gateways
• the bridge software listens for packets received by the TTN and decodes the payload data into an id, latitude, longitude, altitude of the balloon
• for each packet, we know which gateways received it and where they are. So we can "fake" a client for each gateway and construct an ASCII sentence according to the HAB server conventions
• the HAB server still sees the same messages like it would if there were many traditional receivers, so doesn't need any modification!

This way, the entire things network can be used to receive balloon telemetry! There is no longer a need for radio operators to be present at their receiver at the exact time the balloon is launched, making manual adjustments, etc. The Netherlands is already covered by many TTN gateways, greatly increasing the chance the balloon telemetry will be picked up.

Software

Source code

Source code is available at: https://github.com/bertrik/ttnhabbridge

The README explains the tool chain setup.

Using this source code, the basic functionality works.

There is no user guide yet, but the .properties settings file has a one-line comment explaining what each setting does.

Tasks

Stuff to do:

• come up with a simple but flexible way to encode telemetry in a binary packet, to be transmitted over TTN. Perhaps borrow some ideas from how it's done with [1]
• figure out how to receive data from TTN, this is an MQTT stream -> pick an easy-to-use Java MQTT client library. Perhaps this one: https://github.com/fusesource/mqtt-client ?
• figure out the protocol between dl-fldigi and the HAB server -> look into https://github.com/ukhas/habitat-cpp-connector , reverse engineer it and create a Java version. It looks like a couchdb-specific database connection, sending JSON messages! -> it seems we can access this interface as if it were a REST service!
• implement this (in Java for example) and publish it on github!

Modules

The software consists of the following modules.

Main process

The main process of the bridge is something like this:

• listen to the MQTT stream of the HAB application
• once we get data:
  • decode the payload into latitude/longitude/altitude, and encode it into a habhub ASCII sentence with correct CRC, see https://ukhas.org.uk/communication:protocol
  • for each gateway that received the data:
    • send a listener info and a listener telemetry document to the habhub server
    • send the payload telemetry (ASCII sentence) to the habhub server

MQTT listener

Implemented using the Eclipse paho MQTT client. We can listen on a certain application and catch *all* traffic for all nodes registered to that application.

Example invocation, using mosquitto_sub:

 bertrik@zenbook:~$ mosquitto_sub -h eu.thethings.network -t '+/devices/+/up' -u 'ttnmapper' -P 'ttn-account-v2.Xc8BFRKeBK5nUhc9ikDcR-sbelgSMdHKnOQKMAiwpgI' -v

Actual result:

 ttnmapper/devices/mapper2/up {"app_id":"ttnmapper","dev_id":"mapper2","hardware_serial":"0004A30B001ADBC5","port":1,"counter":1,"payload_raw":"d4WaWXATAAAAAAAAAAAAAAAAAAD/","metadata":{"time":"2017-08-21T07:02:14.842855925Z","frequency":868.1,"modulation":"LORA","data_rate":"SF7BW125","coding_rate":"4/5","gateways":[{"gtw_id":"eui-008000000000b8b6","timestamp":865620531,"time":"2017-08-21T07:02:14.846095Z","channel":0,"rssi":-119,"snr":-3.8,"rf_chain":1,"latitude":52.0182,"longitude":4.70844,"altitude":27}]}}
 ttnmapper/devices/mapper2/up {"app_id":"ttnmapper","dev_id":"mapper2","hardware_serial":"0004A30B001ADBC5","port":1,"counter":4,"payload_raw":"loeaWW4T2+8BHzYZzAIeAA8A/QUS","metadata":{"time":"2017-08-21T07:11:18.313946438Z","frequency":868.3,"modulation":"LORA","data_rate":"SF7BW125","coding_rate":"4/5","gateways":[{"gtw_id":"eui-008000000000b8b6","timestamp":1409115451,"time":"2017-08-21T07:11:18.338662Z","channel":1,"rssi":-114,"snr":-0.2,"rf_chain":1,"latitude":52.0182,"longitude":4.70844,"altitude":27}]}}

Habitat uploader

payload upload

This uses a HTTP interface, performing a PUT to a certain URL with a certain content:

• the URL is http://habitat.habhub.org/habitat/_design/payload_telemetry/_update/add_listener/<doc_id>
• the <doc_id> is created from the telemetry sentence by doing ASCII to BASE64 conversion, then hashing using SHA-256 and encoding as ASCII-hex (as LOWER case!).
• the HTTP method is a PUT with the following headers:
  • "Accept: application/json"
  • "Content-Type: application/json"
  • "charsets: utf-8"
• The contents of the PUT is the following JSON structure

{
  "data": {
    "_raw": "[base64 of telemetry sentence]"
  },
  "receivers": {
    "[receiver_id]": {
      "time_created": "[timestamp]",
      "time_uploaded": "[timestamp]"
    }
  }
}

The "[receiver_id]" part can be repeated as many times as there are gateways that received the data.

listener upload

To figure out how to upload information about the receiver station, I captured some traffic between dl-fldigi and habitat, it seems the following happens:

• a GET is done on http://habitat.habhub.org/_uuids?count=100, this returns a list of 100 UUIDs, where a UUID is the lower-case ascii hex representation of 16 bytes.
• a PUT is done with a "listener_information" doc to /habitat/<the first UUID>
• a PUT is done with a "listener_telemetry" doc to /habitat/<the second UUID>

Perhaps implement this with an expiring cache, so listener information/telemetry is uploaded regularly (say every 20 minutes) but not with *every* payload telemetry.

Payload encoder/decoder

This part decodes the binary payload received from the TTN into a standard habitat sentence.

The bridge application currently assumes that the binary payload is encoded according to SodaqOne universal tracker conventions.

The habitat ASCII sentence typically looks like:

 $$hadie,181,10:42:10,54.422829,-6.741293,27799.3,1:10*002A

Mapping:

• payload callsign = name of TTN device
• payload counter = LoRaWAN frame count
• payload time = current date/time of the application (TODO?)
• payload lat/lon/alt = as decoded from the TTN binary payload
• payload checksum = CCITT-CRC16

Gateway cache

We don't actually need this, it seems the required gateway data (id/lat/lon/alt) is already provided by the MQTT stream.

Helpful links

From a conversation on #highaltitude:

20:24 < adamgreig> there's a) a python library that's a lot easier to read
20:24 < adamgreig> but b) basically the gist is you just PUT to http://habitat.habhub.org/habitat/_design/payload_telemetry/_update/add_listener/<id> with some stuff
20:24 < adamgreig> http://habitat.readthedocs.io/en/latest/habitat/habitat/habitat/habitat.views.payload_telemetry.html#module-habitat.views.payload_telemetry
20:24 < adamgreig> so you have a new string, you take the sha256 hex digest of the base64 encoded raw data
20:25 < adamgreig> you PUT to that URL with that ID
20:25 < adamgreig> and you include that JSON struture with your callsign/details

C implementation of the interface between the client and the habitat server

list of habitat JSON schemas

The habitat interface is accessed as a REST service, using a combination of Jetty/Jersey/Jackson Java libraries.

JSON examples

MQTT data

Over MQTT, it looks like this:

{
  "app_id": "ttnmapper",
  "dev_id": "mapper2",
  "hardware_serial": "0004A30B001ADBC5",
  "port": 1,
  "counter": 587,
  "payload_raw": "g3ybWXkTef8BH1EOzAK1\/wEA5wQT",
  "metadata": {
    "time": "2017-08-22T00:36:18.674804288Z",
    "frequency": 868.1,
    "modulation": "LORA",
    "data_rate": "SF7BW125",
    "coding_rate": "4\/5",
    "gateways": [
      {
        "gtw_id": "eui-008000000000b8b6",
        "timestamp": 3979529499,
        "time": "2017-08-22T00:36:18.345616Z",
        "channel": 0,
        "rssi": -120,
        "snr": -7,
        "rf_chain": 1,
        "latitude": 52.0182,
        "longitude": 4.70844,
        "altitude": 27
      }
    ]
  }
}

Device setup on TTN

Steps:

• Go to the TTN console, to the application you want to add the device to.
• In the device settings screen:
  • under 'Device EUI', fill in the tracker EUI (LoRaWAN hardware address) as shown in the serial debug output at startup of the tracker
  • under 'Activation method' choose ABP
  • disable option 'Frame Counter Checks'
  • click Save
• in the SodaqOne serial debug console, type the following:
  • copy the Network Session Key from the TTN console and type in the SodaqOne serial debug console 'key=<key>'
  • copy the App Session Key from the TTN console and type in the SodaqOne serial debug console 'app=<key>'
  • enable ABP mode (disable OTAA), by typing 'otaa=0'
  • set spreading factor 7: 'sf=7'
  • enable GPS: 'gps=1'
  • set GPS fix interval: 'fi=1'
  • set GPS fix timeout: 'gft=30'
  • num=1
• On the habitat website, create a new payload document
  • under 'payload name' fill in callsign (e.g. 'ttntest1')
  • ...
  • click save
• In the bridge configuration properties file, fill in the following
  • ...
  • restart the bridge software after changing the configuration