A method to operate an ultrasonic sensor includes the step of sending an ultrasonic burst as a series of ultrasonic pulses that have a pulse length and a pulse spacing. The sum of the pulse length and pulse spacing represents the pulse period length. The ultrasonic burst starts at a first time and ends at a second time. The current pulse frequency corresponds to an inverse of the current pulse length. The current pulse frequency, during a first time period passes through a first frequency range, in a following middle time period a middle frequency range and in a following second time period a second frequency range. The length of time of the middle time period is equal to or longer than the sum of the first time period and the second time period.