The Doppler effect is the name given to the apparent change in the frequency and wavelength of a wave, due to motion of either the source or observer.

Consider the following two examples:

1. Moving source: If an ambulance with its sirens on passes a stationary observer, the pitch (frequency) of the siren will change to the observer's ears as it passes him. The siren will increase in pitch as it approaches, and decrease in pitch as it passes. However, this change is merely "apparent change" due to the fact that the source and the observer are not in the same frame of reference. If the siren and the observer were traveling together at the same velocity, then the siren would sound constant to the observer. Hence, the pitch of the siren does not change for the EMTs who are riding in the ambulance.

2. Moving observer: A man is standing on the beach, watching the tide. The waves are washing into the shore and over his feet with a constant frequency and wavelength. However, if he begins walking out into the ocean, the waves will begin hitting him more frequently, leading him to perceive that the wavelength of the waves has decreased. Again, this phenomenon is due to the fact that the source and the observer are not the in the same frame of reference. Although the wavelength appears to have decreased to the man, the wavelength would appear constant to a jellyfish floating along with the tide.

Radar instruments use the Doppler effect to calculate velocity of a target object. In a radar instrument setup, neither the source of the radar beam nor the observer is moving. Instead, the velocity of the target object creates a Doppler shift. These instruments emit a radar beam at the target, which reflects the beam back to a receiver (often the same device as the transmitter). If the object is moving away from the sensor, each subsequent wave must travel farther than the previous wave before reaching the target and being reflected. Thus, the amount of time between each wave (ie: wavelength) increases. Conversely, if the object is moving towards the sensor, each subsequent wave must travel a shorter distance before reaching the target and being reflected. Therefore, the wavelength decreases. Given the original wavelength and the magnitude of this wavelength shift, the velocity of the target can be calculated.

Several oceanographic instruments have been designed to record flow velocities based on the Doppler shift principle:

1. High-Frequency (HF) Radar systems: These systems have either a central transmitting / receiving unit or separate transmitting and receiving units which emit radar beams over a body of water, such as the ocean. Surface waves act as radar targets and reflect the radar beams back to the receiver. Based on the wavelength shift, the surface velocity of the body of water can be calculated. Read more about our work with HF Radar systems in an estuarine setting here. Read more about our work with HF Radar systems in a coastal setting here.

2. Acoustic Doppler Velocimeter (ADV): These submersible instruments emit an acoustic beam into a body of water, which is reflected back by naturally or artificially seeded sediment that is traveling with the flow. Based on the wavelength shift of the returning acoustic beam, the velocity of the flow can be calculated. Read more about the ADVs that we have at the BLASST Lab here.

Page author: Megan Schuler