Friday, October 28, 2016

Live GPS Tracking for Club Regattas (Step 1)

Live GPS tracking is already a standard feature at major sailing events.

 Each contestant is provided with a GPS tracker that communicates wirelessly with some remote station. Three technologies are being used for the transmission of data: satellite, GPRS (cellular) and UHF radio.

  • Satellite communication is used for offshore racing and will not be discussed here.

  • GPRS requires cellular coverage in the race area, and individual SIM card and mobile phone plan for each tracker. It is extensively used in a wide range of sporting events. An exemple of GPRS in sailing is the TracTrac offering. Here is a picture of their tracker and charging units:

  • UHF radio links are more flexible, but apply to shorter ranges, typical of what we find in local club events. This is what has been used in the recent Rio Olympics sailing events, with Swiss Timing as the technology provider. Here is picture of their tracker and charging units:

Application to club sailing events

I have been looking at a way to bring GPS tracking to local sailing clubs, along the following parameters.

- all boats are always within 1.5 nautical mile of the committee boat
- the committee boat is within 5 nautical miles of the clubhouse
- each boat reports its position, speed and heading at short intervals (1 second typical)

For the last parameter, the data (GPS position, SOG and COG) can all be incoded in 9 bytes. For example, the following NMEA sentence from the GPS:

can be sent as 4 integers:  598755 (Longitude, 3 bytes), 584321 (Latitude, 3 bytes), 120 (SOG, 1 byte) and 2700 (COG, 2 bytes).

In this post, we look at the communication between the committee boat and the contestant boats, in order to provide a live display of the race in the committee boat. Communication with the clubhouse and the live broadcasting of the event on the internet will be discussed later. 

Preferred technology: LoRa radios in the 915 MHz band

“Long Range (LoRa)” packet radios appear well suited to meet the objectives of this kind of project. For range tests, I have used 2 Adafruit Feather M0 RFM95 radios, featuring a Semtech LoRa transceiver.

Technical details can be found in the following in Semtech’s LoRa Modem Designer’s Guide.

Semtech also provides a useful LoRa Calculator Tool, under the tab ‘Docs & Resources’.

The Semtech tool can be used to calculate the ‘Time on Air’ required to transmitting the 9 bytes packets. In the following example, the LoRa radio is configured with the following settings:
- Spreading Factor : 6
- Bandwidth : 125 kHz
- Coding Rate : 4/5
- No Explicit Header

For these conditions, the ‘Time on Air’ is 18.04 ms, with an equivalent bitrate of 9375 bps. This means that up to 50 boats could be sampled each second for their position, COG and SOG from a single transceiver on the committee boat.

LoRa range tests

Two prototype transceivers have been assembled to test the communication range over water. The Adafruit modules have been programmed with the RadioHead software recommended on the Adafruit site.

With a 3 dBi half-wave dipole antenna in each module (as shown above), reliable communications have been obtained up to 1.75 nautical mile, with spotty communications up to 1.9 nautical mile. With simple wire antennas, the reliable range is reduced to 0.75 nautical mile. With a higher gain antenna on the committee boat transceiver (like 6 dBi), the range could be further extended, or the antenna gain on the trackers could maybe be relaxed, allowing smaller antennas.

These tests thus confirm the feasibility of the LoRa approach.

Further steps

The next steps that are being considered are  :

-   to build a small inventory of waterproof trackers, and develop an application to visualise the boat positions aboard the committee boat

-  to develop the communication link between the committee boat and the clubhouse (GPRS cellular network or more powerful radio)

- from the clubhouse, to broadcast the live data on a web site, allowing display on local TV set and any mobile device.