An apparatus comprising a plurality of beacons, the plurality of beacons comprising:
- a first beacon to:
- transmit, during a first occurrence of a first time slot, a first wireless electromagnetic signal comprising a first identifier that identifies the first beacon as the source of the first wireless electromagnetic signal,
- transmit, during the first occurrence of the first time slot, a first ultrasound signal that lacks an identification of the source of the first ultrasound signal,
- transmit, during a second occurrence of the first time slot, a second wireless electromagnetic signal comprising the first identifier, and
- transmit, during the second occurrence of the first time slot, a second ultrasound signal that lacks an identification of the source of the second ultrasound signal;
- and
- a second beacon to:
- transmit, during the first occurrence of the first time slot, a third wireless electromagnetic signal comprising a second identifier that identifies the second beacon as the source of the third wireless electromagnetic signal,
- transmit, during the first occurrence of the first time slot, a third ultrasound signal that lacks an identification of the source of the third ultrasound signal,
- transmit, during the second occurrence of the first time slot, a fourth wireless electromagnetic signal comprising the second identifier, and
- transmit, during the second occurrence of the first time slot, a fourth ultrasound signal that lacks an identification of the source of the fourth ultrasound signal;
- wherein the first wireless electromagnetic signal and the first ultrasound signal are received by a mobile device during the first occurrence of the first time slot, the first ultrasound signal received after the first wireless electromagnetic signal;
- and
- wherein the fourth wireless electromagnetic signal and the fourth ultrasound signal are received by the mobile device during the second occurrence of the first time slot, the fourth ultrasound signal received after the fourth wireless electromagnetic signal;
- and
- wherein the first ultrasound signal and the third ultrasound signal have non-overlapping areas of coverage;
- and
- wherein the second ultrasound signal and the fourth ultrasound signal have non-overlapping areas of coverage;
- and
- wherein the first wireless electromagnetic signal and the first ultrasound signal are used to estimate a first location of the mobile device, the first location corresponding to a location of the mobile device during the first occurrence of the first time slot;
- and
- wherein the fourth wireless electromagnetic signal and the fourth ultrasound signal are used to estimate a second location of the mobile device, the second location corresponding to a location of the mobile device during the second occurrence of the first time slot.