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  Setting up Navi GT31


2Hz GPS speed sailing helmet
by Tom Chalko

Twin-GPS (twin Navi GT-11) equipped helmet ready for 2Hz speed recording

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 are important:
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 units.

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 comfort.

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 interface...

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.


Sailing speed sampled at 2Hz using two Navi GT-11 units inside the helmet Mal Wright, who had the helmet on his head and saw the graph said: "I am amazed how well the two Doppler Speed data sets agree. When combined together in your graph they look like one continuous result even though two data sets have come from two separate GPS units. We know that Doppler recordings from the GT11 are very accurate, but this graph confirms how consistent they are, which is a very important consideration for GPS competitions". Mal has also asked me to make the helmet for him. Ian Fox, after putting my helmet on suggested that I should make them for sale. I prefer to describe what I did for people to help themselves...