Problem: The listener returns erroneous distances

If the reported distances are wrong by more than a few centimeters, then something is amiss.

• If the results are wrong for every beacon, the most likely problem is that the batteries on the listener are weak.

• If the wrong distance comes from a beacon that was previously correct, then that beacon's batteries are probably weak.

• Check the setting of the units in which the distance reports are being made. You can use ``G UN'' or ``P UN ... ...'' to set these units. See Section 2.2.20.

• If the listener reports erroneous distance at certain locations but not others, then you may have sources of interference on the path from beacon to listener. Some objects block ultrasonic signals and some others reflect them; some even generate ultrasonic signals at the same frequency as Cricket. All these objects cause the Crickets to report wrong distances.

Obstructions close to the beacon's ultrasonic transmitter or listener's ultrasonic receiver cause the biggest problems. These obstructions could be walls or doors (through which ultrasound does not pass), or could be people. Large metallic plates or cabinets on the path from beacon to listener can disrupt distance estimation by affecting both ultrasound and RF propagation. We have also found that some fluorescent lamps generate 40 kHZ ultrasonic waves that can interfere with the Crickets.

Don't place beacons too close to large objects; if a large object is within 10 or 15 centimeters of a beacon, that beacon's transmissions may be blocked. Dealing with interfering fluorescent lamps could be harder; however, we have found that the intererference usually comes from lamps that are close to dying, so you might think of this intereference as an early-warning system to replace the lamp!