'Initialize the sensor.'

def __init__(self, hass, name, latitude, longitude, radius):

    self._session = requests.Session()
    self._latitude = latitude
    self._longitude = longitude
    self._radius = util_distance.convert(radius, LENGTH_KILOMETERS, LENGTH_METERS)
    self._state = 0
    self._hass = hass
    self._name = name
    self._previously_tracked = None
