2Hz GPS speed sailing helmet
by Tom Chalko
Twin-GPS (twin Navi GT-11) equipped helmet ready for 2Hz speed
In speed sailing today the speed is measured using GPS units.
Anyone who is serious about recording sailing speed with high
accuracy and reliability uses more than one GPS unit. Using several
Navi GT-11 gps units it is possible to record gps speed sequences
at a rate up to 4Hz, using the firmware 0702a and above.
Higher speed sampling rate eliminates aliasing errors, increases
the accuracy of the average speed and produces data that better
represents acceleration. Speed measurement accuracy approaching
0.01 knot becomes possible.
I decided to implement a 2Hz speed recording system using two
Navi GT-11 GPS units installed inside the helmet.
Graph below illustrates 2Hz Doppler data from a typical speed-sailing
run at Sandy Point.
Reasons for locating speed recording GPS units inside the helmet
1. Satellite visibility is best on helmet
2. Satellite signal strength is best on helmet
3. GPS accuracy&reliability are best due to 1 & 2
4. Very good aerodynamic performance
5. No Aquapack required
6. GPS units are protected and out of the way
7. Helmet is the driest part of the sailing gear
After a thorough helmet search I have chosen ProTeC snowboarding
helmet, because it has a non-absorbing foam padding that is thick
enough to accommodate two Navi GT-11 gps units.
The ProTeC helmet has a detachable lining, so that the foam padding
can be fully exposed when helmet is not being worn.
I removed the foam padding using a long narrow spring-steel spatula
(designed for turning pancakes) to cut a few spots of glue between
the foam and the helmet shell. Then I removed the foam and cut
a rectangular hole in it to snuggly accomodate my 2 Navi GT-11
The helmet with 2 Navis is very comfortable to wear. I can feel
the slight weight of 2 Navis on my head, but this does not diminish
The only drawback of using GPS units inside the helmet is that
I cannot look at their screens when sailing. I may design an audio
ProTeC helmet viewed from inside with part of the lining lifted
up and foam partly exposed. GPS units are wrapped in a polyethylene
bag, installed in the foam hole and covered with 2mm thick non-absorbing
foam. Note nylon washers
that prevent buttons being pushed. Navis are installed with buttons
facing the shell of the helmet.
Twin GT31 produce synchroneous samples at 1Hz and allow to evaluate
repeatability and reproducibility of GPS speed measurement. When
firmware 1.2 and above is used and GPS antennae are close together,
reproducibility and repeatability approaches 0.01 knot.