1. A method of passive radio frequency energy harvesting, the method comprising: harvesting radio frequency energy using a crystal receiver; powering a processing unit only, from the radio frequency energy being harvested when the radio frequency energy being harvested or an associated signal is above a threshold level; generating an interrupt signal to switch the processing unit to a low power mode when the radio frequency energy being harvested falls below the threshold level; and alternatively powering the processing unit only from an on-board power source when the radio frequency energy being harvested or the associated signal is below the threshold level.