TTNHABBridge

Project TTNHABBridge
A software bridge between TTN and the HAB network
Status	In progress
Contact	bertrik
Last Update	2017-09-09

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. See the 'cayenne' project in the github software archive.
• 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
• the Java program uses a mix of old style 'Date' objects and new Java8 style Date/Time objects. The new objects can make formatting of dates/times easier. Not really an issue but a bit weird.

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.

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. Eh, no, this results in a 500 internal error from habitat!

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 as received in the TTN MQTT stream
• payload lat/lon/alt = as decoded from the TTN binary payload
• payload checksum = CCITT-CRC16 over the characters between $$ and *, interpreted as bytes using US-ASCII encoding

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
      }
    ]
  }
}

Tracker configuration for the TTN-HAB bridge software

Roughly the following configuration is needed for your tracker to use the TTN-HAB bridge:

• setup your LoRaWAN device for use with TheThingsNetwork
• create a payload configuration document on the habhub.org webpage
• download, configure and run the ttnhabbridge software

Setting up the tracker for TTN

Find out the LoRaWAN EUI of your tracker:

• specifically for the SodaqOne, the EUI (LoRaWAN hardware address) is shown at startup of the tracker
• perhaps just make up your own, if you don't use an RN2483?

Create the TTN application and register your tracker with TTN:

• Create an account on TTN
• Go to the TTN console and create a new application, or select an existing application you want to add the device to.
• create a new node/device, this needs to be equal to the habhub payload name (but cannot use uppercase characters)
• In the device settings screen:
  • under 'Device EUI', fill in the EUI of your LoraWAN tracker
  • under 'Activation method' choose ABP
  • disable option 'Frame Counter Checks'
  • click Save

Copy the network credentials back to your LoRaWAN tracker:

• this means the "device address", "network key" and "application key"
• in case you use a 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>'
  • copy the device id from the TTN console and type in the SodaqOne serial debug console 'dev=<deviceid>' (is this right?)
• configure the following additional settings in the SodaqOne:
  • 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'
  • disable ADR: 'adr=0'
  • num=1

Creating a payload configuration document

Steps:

• Go to the habhub habitat website and create a new payload document
• under 'payload name' fill in the callsign (e.g. 'ttntest1'), this has to match exactly with the TTN device/node name
  • add a parser configuration, using the 'new format wizard'
  • enter the following example string: TODO
• click save

Configuring the TTN-HAB bridge software

• get the release zip file and unzip it in some place
• make sure you have Java 8 installed
• In the bridge configuration properties file (ttnhabbridge.properties), fill in the following
  • ttn.app.id with the name of your TTN application
  • ttn.app.key with the key of your TTN application (you can find this in the TTN console)
  • ttn.payload.encoding with the type of payload encoding you use ('sodaqone' for their tracker software)
  • you can probably leave the other settings at their default value
• start the bridge software from the command line, either using the .bat file (for windows) or the .sh file (for Linux)