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:
$GPRMC,180846.600,A,4659.8765,N,07058.4321,W,12.00,270.00,050210,,,D*77
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.
