title: How is it possible that Google Fit app measures number of steps all the time without draining battery?
body: <p>The <a href="https://play.google.com/store/apps/details?id=com.google.android.apps.fitness">Google Fit</a> app, when installed, measures the duration you are walking or running, and also the number of steps <em>all the time</em>. However, strangely, using it does not seem to drain the battery. Other apps like <a href="https://play.google.com/store/apps/details?id=com.protogeo.moves">Moves</a> which seems to record number of steps pretty accurately declares that it uses a lot of power because of it constantly monitoring the GPS and the accelerometer.</p>
  
  <p>I imagine several possibilities:</p>
  
  <ul>
  <li>Wakes up the phone every minute or so, then analyses the sensors for a few seconds and then sleeps again. However it seems that the records are pretty accurate to the minute, so the waking up must be frequent.</li>
  <li>Actually turns on the accelerometer all the time, and analyzes it only after the accelerometer measurement data buffer is full. However I think the accelerometer has a small buffer to store the latest measurements.</li>
  <li>Use GPS to <em>estimate</em> the number of steps instead of actually counting it. However this should not be the case, since it works even indoors.</li>
  </ul>
  
  <p>The app still feels <em>magical</em>. Counting steps the whole time without perceptible battery drain.</p>