The basic principles of active noncontact range finding are similar for electromagnetic (radar, laser, etc.) and active acoustic sensing. A signal is radiated toward an object or target of interest and the reflected or scattered signal is detected by a receiver and used to determine the range. As shown in Figure 5.1, a source of radiation is modulated and fed to a transmit antenna, or aperture, which is usually matched to the impedance of the transmission medium to maximize power transfer. This can take the form of a horn for acoustic or radar sensors, or an appropriately coated lens for a laser. The antenna also operates to concentrate the radiated power into a narrow beam so as to maximize the operational range and to minimize the angular ambiguity of the measurement. When the transmitted beam strikes the target, a portion of the signal is reflected or scattered because the target has a different impedance, or refractive index, than the medium through which the signal is propagating. A small percentage of the reflected power travels back to the receiver (which is often collocated with the transmitter), where it is captured by the receiver antenna and converted to an electrical signal that can be filtered to remove extraneous noise before being amplified and detected. Distance measurement methods can be classified into three categories: interferometry, time of flight, and triangulation. The method used by a particular sensor usually depends on the maximum range and the measurement accuracy required. For example, interferometric methods can be extremely accurate, but are prone to range ambiguity, while time-of-flight methods operate at longer ranges with poorer accuracy.

Chapter Contents:

• 5.1 Overview
• 5.2 Triangulation
• 5.3 Pulsed Time-of-Flight Operation
• 5.3.1 Sensor Requirements
• 5.3.2 Speed of Propagation
• 5.3.3 The Antenna
• 5.3.4 The Transmitter
• 5.3.4.1 Radar Transmitters
• 5.3.4.2 Underwater Sonar Transmitters
• 5.3.4.3 Ultrasonic Transmitters
• 5.3.4.4 Laser Transmitters
• 5.3.5 The Receiver
• 5.4 Pulsed Range Measurement
• 5.4.1 Timing Discriminators
• 5.4.2 Pulse Integration
• 5.4.3 Time Transformation
• 5.5 Other Methods to Measure Range
• 5.5.1 Ranging using an Unmodulated Carrier
• 5.5.2 Ranging using a Modulated Carrier
• 5.5.3 Tellurometer Example
• 5.6 The Radar Range Equation
• 5.6.1 Derivation
• 5.6.2 The dB Form
• 5.6.3 Worked Example: Radar Detection Calculation
• 5.6.4 Receiver Noise
• 5.6.5 Determining the Required Signal Level
• 5.6.6 Pulse integration and the probability of detection
• 5.7 The Acoustic Range Equation
• 5.7.1 Example of Using the Acoustic Range Equation
• 5.8 TOF Measurement Considerations
• 5.9 Range Measurement Radar for a Cruise
• 5.10 References

Inspec keywords: distance measurement; sensors; signal denoising; receiving antennas; interferometry; transmitting antennas

Other keywords: time-of-flight methods; range ambiguity; radar sensors; radar; interferometric methods; distance measurement methods; transmit antenna; electromagnetic sensing; scattered signal; receiver antenna; triangulation; angular ambiguity minimization; laser; transmission medium; power transfer maximization; active noncontact range finding; reflected signal; refractive index; maximum range; active acoustic sensing; electrical signal; active ranging sensors; acoustic sensors; measurement accuracy

Subjects: Sensing devices and transducers; Single antennas; Spatial variables measurement; Signal processing and detection