While hardware makers are racing to develop various wearable devices that can track our activities and perform other functions, most of us are already carrying around a smart phone all the day. So Nokia has developed a software-based solution called SensorCore that lets these handsets use their internal sensors to track activities with very low power, sparing battery life. So if you have a compatible handset—SensorCore is currently limited to a small range of Lumia handsets—you might not even need a separate fitness tracker.

As I noted yesterday in my overview of the new Fitbit app for Windows Phone, I've been using an activity tracker for a few years now. I started off with a Nike Fuel band (and ended up owning two of those) and since moved on to a Fitbit Force (which, go figure, Fitbit has discontinued because of complaints about skin rashes; I've not experienced that).

But with the arrival of a Lumia 635 review unit—and, more recently, the delivery of Windows Phone 8.1 plus Lumia Cyan to the Lumia 1520, I now have some devices that can utilize SensorCore. And I was curious to see how it works.

To be clear, you currently need a Nokia Lumia 1520, Icon, 930, 630 or 635 to use SensorCore, plus of course Windows Phone 8.1 and Cyan. SensorCore will be expanded to other Lumia devices later, Nokia/Microsoft Mobile says, and I would assume that most new Lumia devices will support it. That said, the recently-announced Lumia 530 will not support SensorCore, I'm now told. (I had originally written that the 530 did support this. --Paul)

The trick here is that some combination of sensors needs to be running in the background at all times for your phone to track your movements. And as anyone who's used HERE Maps, HERE Drive + or any other mapping, location, or navigation-type app will tell you, nothing drains your handset's battery faster than tracking your movements. But if Fitbit and other companies can achieve real time movement tracking in tiny wrist-worn devices with days-long battery life, why can't your smart phone do the same thing?

Well, we're not going to get days-long battery life in a smart phone anytime soon. But Nokia did at least figure out a way to run a combination of phone sensors in a special low-power mode that it claims negligibly impacts battery life. Dubbed SensorCore and supported by a set of free APIs that developers can use for their own apps, this technology provides a step counter, an activity monitor, a place monitor, and a tracks monitor.

The first two are probably pretty obvious, but the last two are perhaps less well-understood. The place monitor "can identify known places," Nokia says, "such as the general area of a consumer's home or workplace, which can then be utilized to trigger actions with the Windows Phone 8.1 Geofencing API ... With Geofences, apps can request to be triggered, such as when a user enters or exits their home location." And the tracks monitor (sometimes called a tracks point monitor) "provides points along user’s latest journeys between general areas without resorting to power hungry GPS."

In other words, a step counter and an activity monitor are obvious and necessary for any basic fitness tracker. But those last two bits could lead to some truly interesting apps down the road. (Pardon the pun.)

In case it's not obvious, SensorCore is on all the time, and isn't something that needs to be manually engaged or disengaged. That said, you do need to opt-in before you can use it: Two services need to be running on your phone, Location services, which has been around since the first Windows Phone version, and Motion Data, which is new to Windows Phone 8.1 and Lumia Cyan. You must enable Motion Data in Settings before SensorCore will work.

There are a few other interesting aspects to SensorCore: It logs ten days of data, and that data always stays on the phone—i.e. is not stored or retained in any Microsoft/Nokia online service (unless you set that up via something like HealthVault, of course)—and can be wiped out by the user at any time. This means that you could enable SensorCore on a Monday, walk around and perform your normal routine all week, and then install a compatible app, like Bing Health & Fitness, on that Friday, and it will still be able to access the previous weeks' worth of activity. That's neat, because previous solutions required you to install an app and then move around for a few days before you had anything interesting to see.

To test SensorCore, I've walked each day with a Lumia 635 (listening to Audible, I'm currently on The Martian by Andy Weir, by the way, which is excellent) in my pocket, and I've then compared the results with my Fitbit Force. I'm surprised by how close they measure my steps, given the disparity between the Nike Fuel Band and the Fitbit previously. (They were generally under 50 steps off in walks of 3000+ steps.)

There is one thing I'm kind of curious about. The Fitbit is better than the Nike Fuelband when it comes to registering movement on the elliptical, but these devices don't record certain types of movement—like bike riding—very well. It's possible SensorCore will do a better job with that, though I've not tested that aspect of this technology yet. Obviously, it would be ideal to find a single solution that measured as much physical activity as accurately as is possible.

And you're not limited to Bing Health & Fitness. There are also a number of other SensorCore-compatible apps out there, and many offer additional functionality. One I'm looking at now is Track Runner for Windows Phone 8.1, which, among other things can utilize a separate Bluetooth LE-based heart rate monitor to make sure you're always in the zone. The Jarv heart rate monitor I'm testing works like the one I use on my elliptical trainer, meaning you need to strap it on your skin, and around your chest. But the ability to monitor this information in addition to stride rate/cadence, auto-pause, and step counting, all of which require SensorCore, is very interesting. I plan to use this app—and SensorCore—more in the coming weeks, as we'll be visiting Barcelona and I expect to be out and about in the heat quite a bit.

Developers who are interested in SensorCore should check out Nokia's documentation. But if you have a compatible device (and Windows Phone 8.1 and Lumia Cyan) be sure to check it out too: This technology could literally save you from a separate wearable purchase.