Originally derided as scientifically erroneous, the Doppler effect is now a vital tool in all aspects of technology, ranging from microscopic to galactic.
Doppler, back in the day – and now
The explanation of the Doppler effect is almost intuitive to us, with our understanding of wave phenomena along with available frequency sources and measurement instrumentation. This situation was not the case nearly 200 years ago when Doppler described the now-eponymous phenomenon in the early 1840s as his explanation for how the color of starlight changed with the star’s movement.
The story of Doppler’s trials and tribulations is told in a detailed, readable article in a recent issue of Physics Today. At the time, there was no way to properly and clearly test his assertion. In fact, he was ridiculed by many other prominent physicists and even expelled from a leading scientific association due to his “heretical” and nonsensical ideas. It took several decades, along with the inability of researchers to resolve inconsistencies of data recorded from various experiments that had not been corrected for his eponymous effect, to win over his detractors.
Interestingly, an unrelated advance helped his case: the railroad development in that era. That allowed repeated tests to be run using sound in linear motion along a track at a fixed speed sound (a band was even used on one train!), and which helped confirm his assertions. Once again, a development or advance in one area has influence and bearing on apparently unrelated progress, which was the point of the excellent TV series Connections by James Burke (from the late 1970s, but still quite relevant, most episodes are posted on YouTube).
Doppler’s fall and rise provide a lesson worth remembering and repeating: ideas that are ridiculed at first may eventually be accepted as “correct” (think of Galileo and his solar-centric view of our system). This is the point made in the 1962 book “The Structure of Scientific Revolutions” (personal comment: while the book makes and substantiates many important points, the writing is academic, dense, and turgid, IMO; if you are interested in its points, I suggest you read one of the many available summaries instead!)
Christian Doppler was ultimately vindicated, as his name is now the standard designation for his initially rejected analysis and conclusions. You can find detailed explanations and equations related to the Doppler effect on many websites and at many levels, from descriptive to intensely mathematical.
Note that the Doppler effect is a very useful phenomenon when used intentionally, but it is also a source of engineering challenges and unintended consequences; some are useful and others are disruptive.
How so? While it can be used to measure velocity and acceleration (the rate of change of velocity), it also affects frequency stability over a link. For example, the observed carrier frequency of Earth-orbit satellites (such as GPS) and deeper-space vehicles (such as the recent Mars lander) shifts due to the Doppler effect. Therefore, basic links and transmit/receive paths must compensate and accommodate these frequency shifts and re-tune their carriers accordingly to be able to tune and receive the signal. These carrier shifts can be substantial due to the velocities of these vehicles and will not be constant but will be changing given an accelerating separation, as is usually the situation.
For satellites with the objective and role of providing clocks and timing signals rather than reporting data, the Doppler shifts and relativistic shifts affect their output. So the received algorithm must implement various correction factors. This is feasible since the velocity of the satellites – whether orbital or traveling away with respect to the Earth-based observer – is known, and so timing shifts can be calculated and adjusted.
Doppler gets smaller
As the Doppler effect is so useful, many circuits and systems make use of it as a primary or secondary function. Its versatility means that vendors are innovating to make it easier to embed it via smaller packages and lower power requirements, to expand its use.
For example, most cars now have a Doppler radar to sense vehicles ahead and sense the closing speed between the vehicles, often called the range and range rate. These two parameters tell the car and driver if the vehicle in front poses a possible collision hazard, as just distance (range) alone is adequate to make that assessment.
One of the many recent examples of how the needed components that provide Doppler-effect sensing migrate from high-end applications to more mundane ones are seen in Infineon Technologies’ DEMOBGT60LTR11AIPTOBO1 Development Kit (Figure 1).
Among its uses, this Doppler-based unit is designed to replace the widely used passive infrared (PIR) motion detector, a low-cost technique that works reasonably well as a sensor of occupancy detection for burglar alarms (for example). However, PIR has limitations with respect to sensitivity, precision, repeatability, and the coarse granularity of information it provides.
The Infineon technology offers better performance, response, and user programmability using 60-GHz Doppler-effect technology. Based on the BGT60LTR11AIP fully integrated 60-GHz monolithic microwave integrated circuit (MMIC) (Figure 2), this small kit (3.3 × 6.7 × 0.56 mm) provides a Doppler-based motion sensor. It includes the needed antennas in the package (AIPs) with an 80-degree field of view and integrated detectors for motion and direction of motion.
Its adjustable performance parameters include detection sensitivity, hold time, and frequency of operation. Unlike many 60-GHz devices, it uses standard, low-cost FR4 circuit-board material – yielding a major cost-saving and which is easier to obtain and work with for the OEM or board-production facility than more-exotic GHz circuit-board substrates,
The development kit includes the BGT60LTR11AIP “shield,” as well as the Infineon Radar Baseboard MCU7. The 20 × 6.25 mm shield demonstrates the features of the BGT60LTR11AIP MMIC and gives the user a “plug and play” radar solution. It’s optimized for fast prototyping of designs and system integration as well as an initial evaluation of features and functions.
There’s no doubt that this high level of Doppler performance and functionality is far more than that of a basic PIR-based sensor (Figure 3). Whether the additional cost and complexity of the Doppler approach and technology as represented by this device are worth it, the features and benefits will be determined by users and their applications. Of course, as is often the case, innovators may adapt it for new applications beyond solely replacing existing PIR-based detectors.
Doppler-effect sensing is a cornerstone of many modern systems. It quantifies the use of electromagnetic and acoustic wave energy as noncontact, almost instantaneous way to determine details of motion at a distance. It is used in a wide range of settings and spans ranging from microscopic to astronomical. Modern component and development kits simplify incorporating functions based on the Doppler effect and shift in both designs where there is no viable alternative or an improved replacement for existing approaches.
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The Doppler Effect
- Physics Today, “The fall and rise of the Doppler effect”
- NASA, “Doppler Shift”
- Georgia State University, “Doppler Effect”
- University of Connecticut, “Doppler Effect”
- University of Virginia, “Doppler Effect”
Related or cited topics
- Wikipedia, “Coanda Effect”
- Wikipedia, “Skin Effect”
- Albert Einstein, “On the Electrodynamics of Moving Bodies” (translation from German into English)
- James Burke, “Connections” (TV series)
- Electronic Wings, “PIR Sensor”