TTNHABBridge

Project TTNHABBridge
A software bridge between TheThingsNetwork and the UKHAS high-balloon network
Status	Completed
Contact	bertrik
Last Update	2020-08-15

Status

This software is a bridge application that accepts high altitude balloon telemetry data from TheThingsNetwork and forwards it to the habhub tracker website.

It has been tested in practice with the 'koppelting1' payload on 2017-8-26. If you look at the habhub map and see a receiver called EUI-xxxxxxxx then that one is probably generated by this software.

May 2019

I've updated the parsing for telemetry data encoded using 'cayenne LPP': it now simply places *all* cayenne fields in the UKHAS sentence, in the order they appear in the cayenne messages, like this:

 $$name,sequence,time,field,field,...,field*checksum\n

This way you have some control over what is sent to UKHAS. I recommend to place the GPS field first in the Cayenne LPP message.

June 2020

I finally fixed the problem where no more messages would come in from TTN after a transient netwerk issue. The application now uses the built-in reconnect feature of the mqtt library and explicitly resubscribes to the MQTT topic.

Introduction

This application uses TheThingsNetwork as a receiver for high-altitude balloon telemetry and forwards it to habhub.org, so it can be shown on their overview map. The habhub map shows the current position of the balloon, but also shows a prediction (based on weather model) about where the balloon will burst, where it will land, which receivers are currently in range, etc.

In the current situation, high altitude balloons send telemetry on the 434 MHz band using RTTY modulation. This is received by a set of dedicated receivers. The operator of each receiver (typically radio amateurs) 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.

In the new situation, the existing infrastructure of TheThingsNetwork is used to receive low-bitrate telemetry packets and forward them on the internet. It uses LoRa as a modulation scheme which allows for very small transmission power and is much less susceptible to slight tuning errors than the currently used RTTY modulation. Packets are encoded using the LoRaWAN protocol. Telemetry packets can be received completely autonomously by TTN gateways, there is no need for a radio operator to make adjustments etc.

In short, the idea is:

• you attach a LoRaWAN transmitter to the balloon, which been 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 which forward into the TTN infrastructure
• the bridge software listens for packets received by the TTN and decodes the payload data into an id, latitude, longitude, altitude of the balloon
• the bridge software converts each telemetry packet according to 'UKHAS' conventions and forwards it to the habitat.habhub.org server
• the bridge software also forwards the TTN gateway locations, so they also appear on the map, along with their name / EUI and antenna altitude
• the habitat.habhub.org 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.

Building the software

Steps (on Debian Linux):

• install a java-11 JDK

 sudo apt install openjdk-11-jdk

• clone the software from github

 git clone https://github.com/bertrik/ttnhabbridge

• enter directory ttnhabbridge/gradle

 cd ttnhabbridge
 cd gradle

• run the gradle script to build the executable

 ./gradlew assemble (Linux)
 gradlew assemble (Windows)

• the executable is now available at ttnhabbridge/build/distributions/ttnhabbridge.tar and ttnhabbridge.zip

Running the software

• extract either the tar (Linux) or zip (Windows) that was generated in the build step
• run it once to generate the initial configuration file, this file is called ttnhabbridge.properties

 cd ttnhabbridge
 bin/ttnhabbridge

• stop it, using CTRL-C
• edit the ttnhabbridge.properties configuration file with a text editor, enter your application name, keys, payload encoding, etc.
• run it again and verify that the program starts correctly with the new properties file

Design

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 TTN gateway that received the data:
    • send a listener info document and a listener telemetry document to the habhub server
    • send the payload telemetry (ASCII sentence) to the habhub server

Example MQTT data, 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

The habitat interface is accessed as a REST service.

payload upload

The application communicates with the habhub server using 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, 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>

This is implemented 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 allows the following encodings for the telemetry data, configurable in the application properties:

• 'cayenne' (recommended): "cayenne" encoding
• 'sodaqone': SodaqOne universal tracker conventions.
• 'json': a JSON encoded format

See the table and example below to know which fields are required to construct the habitat ASCII sentence from the LoRa data payload sent by your tracker.

The habitat ASCII sentence typically looks like:

 $$ttntest1,64,20:41:10,52.022100,4.693100,20.0,14.0,3.88*1C8A

Property mapping

Field value	SodaqOne raw	JSON	Cayenne	Remark
ttntest1	TTN node name	TTN node name	TTN node name	TTN node name
64	LoRaWAN FCNT	LoRaWAN FCNT	LoRaWAN FCNT	Frame count
20:41:10	field 0	TTN metadata time	TTN metadata time	TTN metadata time
52.022100	field 3	"lat"	see below	latitude (degrees)
4.693100	field 4	"lon"	see below	longitude (degrees)
20.0	field 5	"gpsalt"	see below	altitude (meters)
14.0	field 2	"temp"	see below	temperature (degrees Celcius)
3.88	field 1	"vcc"	see below	battery voltage (volts)

For Cayenne encoding, the UKHAS sentence starts with the node name, sequence number and time. The fields following after that are simply all fields as they appear in the Cayenne message. I recommend to put the GPS position *first* in the cayenne message. Typically you would put in the Cayenne message: GPS position (latitude/longitude/altitude), temperature, battery voltage.

The field 1C8A is the CCITT-CRC16 over the characters between $$ and *, interpreted as bytes using US-ASCII encoding

Listener cache

The cache keeps track of when listener information / telemetry was sent last. This makes it possible to avoid sending the listener information / telemetry with each payload telemetry document, reducing the load on the server.

Listener information / telemetry is sent only:

• when this is the first time we are sending something for this listener
• when it has been more than X minutes ago that we sent listener information / telemetry for this listener, where X is typically 10 minutes or so.

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

JSON examples

TTN data as received over MQTT

Over MQTT, the TTN data 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?

Make up your mind about which binary payload format you are going to use: I recommend the 'cayenne' payload format. It's relatively flexible and is an actual standard. It is supported by ttnhabbridge without needing customisation. You need to put in at least the GPS coordinate in your cayenne message. If you have it available, I would also put in the battery voltage and temperature in it.

Create the TTN application and register your tracker with TTN:

• Create an account on TTN, https://account.thethingsnetwork.org/
• Go to the TTN console and create a new application, or select an existing application you want to add the device to.
• If you use Cayenne as the payload format, you can configure this under 'Payload Formats', this allows you to view the raw payload as a kind of JSON format in the TTN console
• create a new node/device, the name 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 GPS: 'gps=1'
  • set GPS fix interval: 'fi=1'
  • set GPS fix timeout: 'gft=30'
  • set num coords to upload: 'num=1'
  • set min GPS satellite count: 'sat=1'
  • enable ABP mode (disable OTAA): 'otaa=0'
  • disable ADR: 'adr=0'
  • set spreading factor 7: 'sf=7'

Creating a payload configuration document

The payload configuration document allows habhub to decode the UKHAS sentence back into meaningful data items, like which fields contain latitude, longitude, altitude, etc.

Steps:

• Go to the habhub habitat website and create new payload document
• under 'payload name' fill in the callsign (e.g. 'ttntest1'), this has to match exactly with the TTN node name
  • add a parser configuration, using the new format wizard
  • enter the following example string: $$ttntest1,64,20:41:10,52.022100,4.693100,20.0,14.0,3.88*1C8A
• the fields are the following:
  • sentence id
  • time
  • all of the Cayenne fields in the order they appear in the Cayenne message.
• click save

So, this contains the basic position information, plus two fields for temperature and battery level.

Configuring the TTN-HAB bridge software

• get the release zip/tar 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), typically starts with "ttn-account-v2."
  • ttn.payload.encoding with the type of payload encoding you use (I recommend Cayenne)
  • 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)
• if you use Linux with systemd, you can use the ttnhabbridge.service file to run it as a service, see instructions inside this file