Saturday, April 09, 2011

How Nike+SportWatch GPS calibrates its foot sensor automatically using GPS

The Nike+SportWatch has no manual calibration of the foot sensor as the Nike+Sportsband does. As I surmised in my first review here is how they do it.

From official response by  Nike on Nike Forums

"Thanks for asking about the calibration and how it works.

You're correct in noting that there isn't any manual calibration, as there is with other devices, where you actually tell your device, "I want to calibrate this run."

Instead, you set up an outdoor run where you are using both the Nike+ sensor and the GPS option. (In other words, on your run setup screen, both GPS and Footpod are toggled to "on.") Once you've run a certain distance with both good GPS reception and good footstrike data capture from the Nike+ sensor, the sensor will automatically be calibrated using that data, without you having to do anything.

Let's say you run a mile at a steady pace without pauses and have good GPS reception and good footstrike data. Internally, your SportBand might determine (based on sensor data) that you ran .91 miles. It would then adjust that .91 miles to match the GPS data--one mile--internally, and would save that correction factor and use it to adjust your future sensor-based runs (i.e., your treadmill runs).

It's the same principle that's always applied. The difference is that now, instead of you telling your device, "Hey, I ran a mile, not .91 miles!" your GPS is telling your device, "Hey, Upstate ran a mile, not the .91 miles captured by the sensor."

I hope this makes sense! It's actually a pretty cool system, and I hope it yields comparable accuracy to what you've come to expect."

I am still wondering if it works the opposite way when GPS signal is weak the foot sensor picks up the task. I think it may.


PatinChCh said...


I just purchased a sportwatch - I have not yet taken it on a run. I have been trying to get it to recognise the sensor to no avail, and am now thinking maybe I need to be out and in touch with the satellites for it all to connect up. Is this right, or have I got a dud shoe sensor?

Sam Winebaum said...

You do need to be outside for the GPS sensor to be picked up. Preferably with a view of the southern sky and no buildings or trees immediately in the way, For the foot sensor to get acquired you need to walk a bit. It won't pick up if you are standing still. Hope this helps.

PatinChCh said...

Hey - thanks Sam. I did some walking around inside (my weather is so bad) but did not get it sensed. Maybe dud, maybe I'll try tomorrow outside. Appreciate it!

MattD said...

Hi Sam,
Newcomer to your blog tonight. Ran 3 small runs before running a 3:33 at the Flying Pig Marathon in Cincinnati last wknd. My issue, which you mentioned briefly, is that the "instant pace" measurement was wildly variable during the entire 26mi. Always had me at 7:04 or 7:37 when my avg race pace was 8:10ish. Trust me, I looked at it all the time and it never ever once registered an 8:10 pace. Do you know if the July firmware update will address this? I did not run with the foot sensor as I thought the GPS would be accurate. I love the watch, actually wear it as a timepiece, but the instant pace and overall average page are almost deal-breakers for me. Please let me know your thoughts. Thanks so much!!

MattD said...

Sorry - let me clarify - ran 3 small runs with the Nike Watch before the marathon. All 3 runs had wildly inaccurate instant pace readings.

Sam Winebaum said...

MattD-I heard unofficially from Nike at the Boston Marathon Expo that average pace could come by July. I really like the watch a lot but lacking this feature is a big big gap. One way around it is to set your laps to a shorter distance. I am not sure instant pace can ever be really accurate unless they smooth it out over a decent amount of time and thus you are back to in a sense setting it to shorter laps. I now have the heart rate monitor which after some wild fluctuations in the first 6-10 minutes of every run works for me as a substitute for average pace, for now. I am starting to have real doubts about the foot sensor being calibrated by the GPS accurately. When I first ran foot sensor alone vs. GPS alone on a measured course they were pretty close. Now they are way off: yesterday foot sensor alone 4.03 miles, Monday same route 4.77 miles. I think I will reset the foot sensor to factory setting and run that route again to see what happens. All of these issues would seem to be solvable through firmware updates, unless someone like Garmin has a patent on average pace which is hard to work around.

MattD said...

Sam - Thanks for your input. I guess I'll hang in there for the July firmware update. Tonight, I borrowed a Garmin305 from a friend and the "instant pace" is refreshingly dialed in! I don't claim to understand the technicalities of how GPS works, but Garmin seems to have mastered a calculation that Nike hasn't (yet). I can live with average pace, as long as it's accurate. I'll do what you did - comparing the foot sensor against the GPS for distance - and report back. If there are any other tests you'd like me try, just let me know. Happy trails.

Anonymous said...

I was excited to see the new watch and was looking to purchase a GPS enabled watch for my runs. Glad to hear the comments here...I definitely want an accurate piece for that price. I'm trying to find a piece smaller than the Garmins. Any suggestions? I'm just getting tired of driving the car to track my mileage when I decide to take different routes and definitely need the heart rate sensor too. But I don't want a massive watch (I'm 5'1")...I would rather take my iphone on my arm. Thanks in advance for any information.

Sam Winebaum said...

Now that the Nike+ has average pace: see my other post here it is a contender. It seems about as thick as a Garmin 205 for example but is narrower in width. Not as small as pictures may seem to indicate but not a block. See this post for a side by side on my skinny wrist readable in sunlight. Simple to use. The optional heart rate sensor works great. The foot sensor seems to still be a work in progress in terms of calibration, remains to be seen if the update took care of the obvious issues. Not a total stat junky solution but certainly good enough for someone like me who is an active road and trail racer. The beta version of the Nike+ web site is far faster to load than the old version and will display all the important stats: route, laps, elevation, pace, heart rate, etc.. Hope this helps.

Sam Winebaum said...

I did not comment on accuracy in my previous comment. I was 99.05% accurate with the SportsWatch at the Boston Marathon or off about 0.2 miles which is great in my view. I do think that if you run near tall buildings ( as I saw at Boston) or in deep valleys on trails accuracy suffers. I have not done a side by side comparison to my Garmin in these situations but I suspect the Garmin as it uses a different and more power hungry GPS chip may be more accurate in those marginal reception situations. This is where the foot sensor, work in progress to date, is supposed to pick up for GPS when signal is lost. Except for the last 1/4 mile at Boston I have not been in an area where signal might get seriously blocked yet.