Accuracy of GPS devices - Wear them on your head!

I feel this information is so important that it needs a new topic.

In the development of the electronics of the GX-52 GPS, Tom Chalko did a huge amount of technical accuracy testing. The biggest thing that his testing showed up is that the most important determinant of accuracy is the way the GPS is oriented and therefore worn.
Even with a lower gain antenna, accuracy was exceptional with direct line of sight exposure to the sky. This means the antenna is facing the sky directly. He tells me he found that any other orientation degraded the accuracy severly.
With higher gain antenna as in the new GX-52, ideal orientation can give accuracy results that are so good there is almost no significant error. He is talking 0.02 Knots!!!.

Currently, most records the WGPSSRC have ratified have had error deductions of around 0.25 knots. Some have been greater. Very few have been less than 0.2 knots.

By far the best way to get the very best accuracy is to mount the GPS's in or on your helmet!

There are a number of ways to do that, but unfortunately, most helmets don't have enough thickness inside the shell to accommodate GPS's. The Pro-Tek helmets Tom, Mal Wright and I use are exceptions, but ASFAIK, they are no longer available.

There are a number of big advantages to running your GPS in/on your helmet:

1. Orientation. Your head will be almost always oriented towards the sky and more or less level to provide the GPS with the best possible orientation for best reception and signal strength of the most satellites.

2. Best view of the sky. On the top of your helmet the GPS will have the very best, unobstructed view of the sky. Other places, like your upper arm are compromised by you head and body partially blocking their view and resulting is less satellites locked in, poorer reception and more chance of multipath errors.

3. The 'Bio-dampening'. Your head moves and shakes around the very least of any other GPS mounting location virtually eliminating this source of error.

4. You can clearly hear the Genie audio feedback. This allows you to instantly know if your average speed is still increasing, invaluable on a speed run. At the end of the run you can hear the top speed and x-average (10 second) speed beeps so you don't even have to look at your GPS to know the results of that run.

See the original pictures and reasoning here:

web.archive.org/web/20140709180759/http://mtbest.net/speed_sailing_helmet.html

My Practical Experience:

I have had an almost identical setup to that pictured in the link above for quite a few years now. I always have two GT-31's in the helmet when any serious speed sailing is being done, and I also usually wear one GPS on my arm.

The two GPS in the helmet always have almost identical results, and often differ from the GPS on my arm. The tracks are cleaner. The errors much lower and in maneuvers like the Alphas, the data is far more accurate with none of the spikes and errors usually seen in arm mounted GPS in Alphas.

I can always hear the Genie feedback when speedsailing (If I have remembered to turn it on - and only on one GPS!), and I can hear the speed results for that run before I get a chance to look at the GPS on my arm.

I have never even looked like having the GPS in my helmet fail due to water. I put them inside a new zip lock bag and they have never got damp, even after an epic half hour swim across the inlet into 2 foot waves and a howling wind. In that episode, the one on my arm in the 'waterproof bag' drowned and died.

I can happily sail unencumbered by arm bags if I choose to, which is an advantage sometimes in wild conditions.

It is very hard to lose your GPS when it is in/on your helmet. I have lost at least two over the years when the arm bag was ripped off in crashes.


Here is a challenge. I would like to see some practical ideas and designs for mounting your GPS in or on your helmet. Maybe it can be as simple and gluing some sort of Box to the top of the helmet? Maybe you can go wild with space age, high tech styling and design?

Let us all see you ideas and constructions in this thread.

I was thinking of getting a GW-31 and strapping it to helmet through the vent slots at the top. Even had some fantasy about a system of lenses /mirrors/prisms to make a sort of HUD in the upper corner of my vision...then I woke up :-)

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TheSailingMoose said..
I find having the gps on my arm uncomfortable so instead i attach it onto my lifejacket shoulder strap. Would this affect the accuracy at all? Better or worse than on your arm?

About the same as on your arm, possibly slightly better in that it probably won't move around as much. But, your head will block signals from that side, probably more so than on your arm.

Edit, Just saw Mats post above. What he said.....

Black waterproof bag ($4) enclosing clear plastic box with custom silicon seal ($7) housing a GT31 cushioned with pink sponge ($1) all held on top of blue Gath Gedi with black elastic ($8). Bag will not slip out due to being held with tight elastic above the rolled up seal end. A bit messy but looks like it will work well (not had the chance to sail with it yet).

This may not be ideal as its not directly on top of the head, but could be better then the arm or chest???
I'll have to test it out to see how it feels on the water, but I doesn't feel to noticeable when I tried it on.

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choco said...

That would make duck gybing very difficult!

Wouldn't your head also make a good aerial for the GPS. I know you can extend the range of your car remote key a lot by holding it to your head.... :)

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Docssummersky said..
Hello all, well great idea of wearing on your head. Well I wear a beany hat that has a strap so can not be ripped off in wipeouts. Sorted, my Canmore Gps devise is so tiny I simply use the folds in the edge of hat and works for me. Maybe I can sow in a small velcro strap that holds it in. The Canmore has to go in a waterproof bag. I will also try the Garmin 800 edge in the hat to on next windsurfing outing :)

If your beany absorbs water, then no... that would be bad.

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gustfront said..
I wear mine on my wrist. It never even occurred to me that may produce inaccuracy. What tool do you use to determine the accuracy of your data?

The GT-31 has "error data" in the data file - it tells you the calculated error-per-point.

Also, if you use one of the analysis programs (GPSResults, RealSpeed), the speed-graph or track-overlay will show your run -> if there is an error, the graph or overlay looks weird.

Don't put any GPS inside your wetsuit. It's a great way to get spikes and very bad data in your tracks. A wet wetsuit makes it even worse. Why risk a great run with bad positioning .

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byronmc said..
Wouldn't wearing the gps inside the helmet have the same loss due to going thru insulation

Different material and not the same issue. Do the tests yourselves if you don't believe our results.

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byronmc said..
Wouldn't wearing the gps inside the helmet have the same loss due to going thru insulation

Fiberglass and plastic have almost not effect on the signal strength.
With my helmet, I cut the Styrene liner out to make the cavity it sits in and the GPS has only a thin layer of plastic between it and the sky.

I question in another thread about the GW-52 prompted me to go to the Locosys website again and found it has been updated with a lot of GW-52 stuff.

www.locosystech.com/en/product/gps-handheld-data-logger-gw-52.html

So I thought it was a good time to remind those who have heard this and tell those who have not:

Here is an interesting note in the FAQ written, I assume, by Dr. Tom Chalko (who did all the testing and accuracy validation of the device) about how to get best accuracy from the device:

Q:Q1: How to improve the accuracy of GW52 speed measurement?

A:Our main suggestion is to use GW52 in direct view of all satellites above horizon. This is a basic requirement for accuracy. GW52 is a GPS-Doppler instrument optimized to measure SPEED (the magnitude of velocity vector). The Doppler frequency shift df of each satellite carrier frequency (which GW52 measures and determines speed from) is df=velocity*cos(alpha)/c, where c is speed of light. As you can see, the measurement of velocity relies on alpha: the angle between measured velocity vector and the direction to the satellite. When satellite is in direct view, everything is fine: all alpha angles are well defined and the system has good accuracy. When direct view of some of the satellites is blocked by gps-frequency attenuating objects, such as human body for example, the signal reaching GW52 antenna from these blocked satellites is a result of reflections, refractions, diffraction and all the interferences between them. What is the alpha angle that the signal comes from? Who knows? This ambiguity in direction of GPS-Doppler signal is one of the main sources of speed errors. Direct view of all gps satellites above horizon is required due to the physics of Doppler Effect. The actual accuracy of GW52 depends on satellite distribution above the horizon in relation to measured velocity vector, but 1.3 cm/s accuracy (3sigma) in measuring 10s-average speeds above 100km/h have been observed. To achieve comparable speed measuring precision in sport, GW52 must be worn on top of the helmet. All other locations on the body simply ruin the speed measurement accuracy.

In Practice, it does make a significant difference to wear the GPS on the top of your head.

The first and most important reason is clearly given above.

The second important reason is that you head is very stable while you sail compared with other possible body locations. This dramatically reduces high speed frequency movements (vibrations) that can also badly affect accuracy.

Mount you GPS on the top of your helmet!

Many ways to do this easily, but you can also put it inside the foam lining with the antenna facing the sky. The fibreglass/plastic of the helmet shell does not block the radio waves.

This is how I do it: web.archive.org/web/20111210232653/http://mtbest.net/speed_sailing_helmet.html





While at a recent ski trade show, I checked out all the skiing and snowboard helmets and found that most of them were quite suitable to use in this way. Don't worry about the cloth linings in water use, they are fine.

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mathew said..

TheSailingMoose said..
I find having the gps on my arm uncomfortable so instead i attach it onto my lifejacket shoulder strap. Would this affect the accuracy at all? Better or worse than on your arm?

GPS's dont work through water... even just a little bit will affect it quite severely. Your body is a walking-blob-of-water.

- Wearing on your arm will result in your head getting in the way sometimes, for low elevations... you may obscure a single satellite, possibly two at most (due to constellation).
- If you are wearing it on the top of your shoulder, you will probably get about the same obstruction
- If you are wearing it on the front of your lifejacket, then you obscuring the sky probably by close to 50%.

That's what happened to me in 2009 on Big Sunday at The Pit with my old GT-11 on my arm. I knew it was a fast run but no track points. I was fully lit in 50 knots wind and comfy. What I would do to be there again with all the experience I now have.

What if we develop a speed helmet directly with Gath and integrate the 52 unit completely on the top?

That may be true depending on the helmet design and how you go about locating the GPS. But for our use I don't think it is an issue.

It is interesting to look at the different types of helmets.

The Gath helmets appear to be designed with a very thin lining. This suggests to me that they are not primarily designed to absorb a large impact like a bicycle. Motorcycle or Skiing/Snowboard helmet. They appear to be designed to prevent cuts and injuries from surfboards and windsurfing gear, and glancing impacts with sharp rocks on the sea floor.

There is no room in the Gath type helmets to make a GPS cavity.

Since most of us are happy with the lower level of impact absorption protection that a Gath type helmet provides for windsurfing, removing some of the absorption foam inside a ski/snowboard helmet does not seem like an issue for me. It would still most likely outperform a Gath type in that respect.

Attaching GPS to the outside of the helmet can easily be done in the same ways as we do with Go-Pro and similar cameras. It is conceivable that a circumstance could arise where this could affect the protection afforded by the helmet, but it is very unlikely. Mostly, the camera/GPS will part company with the helmet in any serious impact. I would not advise using screws or bolts to attatch anything to you helmet though. Sticky pads and/or velcro seem the best solution, possibly with a chord tether for loss prevention in case it gets dislodged.

I had an interesting conversation at a recent snow trade show with a guy who has started a company to produce helmets with gps, camera, video, radio communication, WiFi, etc all built in and controlled by a powerful mini computer.

His original idea was to service first response emergency workers so they could instantly communicate a scene to their controllers and co-workers. He he's decided to target the adventure sports market now as well.

Of course, the first question I wanted and answer to was, "what is the type and brand of GPS chip in the helmet?" (he had already told me it was 10hz). It is some Russian chip and it was the one thing he didn't know the technical details of. He is getting back to me on that. I told him all it needed was a HUD and he was nearly there!

The downside is that he anticipated a price of over $800, and they were still working on the waterproofing level.

forcitehelmets.com/