A Guide for Online Information About:

Radar Ground Speed Sensors

by Rick Prescott

There are a couple ways to measure true ground speed and traveled distance accurately. One is to use a GPS system that cost thousands of dollars—the other is to use a radar sensor. The radar sensor emits radio beams that bounce off the ground and computes ground speed based on the speed at which objects are passing in front of the sensor.

The advantages of radar over conventional wheel rpm measuring systems is that the radar sensor gives accurate ground speed regardless of wheel slip. Installation is easy, and after the unit is calibrated, it can be forgotten. There are no magnets or wheel sensors to worry about being torn off. Mph updates are extremely quick, and radar sensors usually emit a pulse for every 0.3 inches of travel. Radar sensors will work in all temperature ranges, and dust does not affect the accuracy of the radar sensor. (more)

How They Work (GMH Engineering Products)

The Doppler Radar Speed Sensor is useful in vehicle ground speed and distance measurements. The sensor measures relative motion of the vehicle over the ground and, therefore, is not affected by wheel slip. The sensor has been tested successfully in all types of weather and on several different road surfaces.

The output of the sensor is a digital pulse. The frequency of the pulse can be used to determine vehicle speed, or the pulses can be summed to determine distance traveled. The sensor is compatible with many types of digital speedometers, tachometers, and data logging systems.

Here are some downloadabe application notes for further details:

Fundamentals of Non-Contact Speed Measurement Rev 1.0 (13K)
Using Non-Contact Speed Sensors to Measure Vehicle Ground Speed Rev 1.0 (16K)
Distance Applications Using the Non-Contact Speed Sensor (14K)
Traffic Monitoring Using Doppler Radar Speed Sensors Rev 1.0 (18K)
Wiring Diagram (14K)

Microwave Motion Sensors for Off-Road Vehicle Velocity Data and Collision Avoidance

The Doppler Effect and How it is Used (How Stuff Works)

Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off of a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. The length of time between the moment you shout and the moment that you hear the echo is determined the distance between you and the far surface that creates the echo.

Echo and Doppler Shift

When you shout into a well, the sound of your shout travels down the well and is reflected (echoes) off the surface of the water at the bottom of the well. If you measure the time it takes for the echo to return and if you know the speed of sound, you can calculate the depth of the well fairly accurately.

Doppler Shift is also common and you also experience it daily (often without realizing it). Doppler shift occurs when sound is generated by, or reflected off of, a moving object. Doppler shift in the extreme creates sonic booms. Here's how to understand doppler shift (you may also want to try this experiment in an empty parking lot). Let's say there is a car coming toward you at 60 mph and its horn is blaring. You will hear the horn playing one "note" as the car approaches, but when the car passes you the sound of the horn will suddenly shift to a lower note. It's the same horn making the same sound the whole time. The change you hear is caused by doppler shift.

Here's what is happening. The speed of sound through the air in the parking lot is fixed. Let's say it's 600 mph (the exact speed is determined by the air's pressure, temperature, and humidity). Imagine that the car is standing still, it is exactly 1 mile away from you, and it toots its horn for exactly 1 min. The sound waves from the horn will propagate from the car toward you at a rate of 600 mph. What you will hear is a 6-s delay (while the sound travels one mile at 600 mph) followed by exactly 1 min. worth of sound.

Doppler Shift

The person behind the car hears a lower tone than the driver because the car is moving away. The person in front of the car hears a higher tone than the driver because the car is approaching.

Now let's say that the car is instead moving toward you at 60 mph. It starts from a mile away and toots it's horn for exactly 1 min. You will still hear the 6-s delay, however, the sound will only play for 54 s. That's because the car will be right next to you after 1 min., and the sound at the end of the minute reaches you instantaneously. The car (from the driver's perspective) is still blaring its horn for 1 min. Because the car is moving, however, the minute's worth of sound gets packed into 54 s from your perspective. The same number of sound waves are packed into less time. Therefore, their frequency is increased, and the horn's tone sounds "higher" to you. As the car passes you and moves away, the process is reversed and the sound expands to fill more time. Therefore, the tone is lower.

You can combine echo and doppler shift in the following way. Say you send out a loud sound toward a car moving toward you. Some of the sound waves will bounce off the car (an echo). Because the car is moving toward you, however, the sound waves will be compressed. Therefore, the sound of the echo will have a higher pitch than the original sound you sent. If you measure the pitch of the echo, you can determine how fast the car is going. (more)

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