Ambiguity resolution
Encyclopedia
Ambiguity resolution is used to find the value of a measurement that requires modulo
sampling.
This is required for pulse-Doppler radar
signal processing.
in the measurement process. This happens with all radar
systems.
Radar aliasing
happens when:
Pulse Doppler sonar uses similar principles to measure position and velocity involving liquids.
(PRF) below about 3 kHz pulse rate produce true range, but produce ambiguous target speed. Radar systems operating at a PRF above 30 kHz produce true target speed, but produce ambiguous target range.
Medium PRF systems produce both ambiguous range measurement and ambiguous radial speed measurement using PRF from 3 kHz to 30 kHz.
Ambiguity resolution finds true range and true speed by using ambiguous range measurements and ambiguous speed measureds using multiple PRF.
For example, a strobe light can be used as a tachometer to measure rotational velocity for rotating machinery. Strobe light measurements can be inaccurate because the light may be flashing 2 or 3 times faster than shaft rotation speed. The user can only produce an accurate measurement by increasing the pulse rate starting near zero until pulses are fast enough to make the rotating object appear stationary.
Radar and sonar systems use the same phenomenon to detect target speed.
The unambiguous zone is in the lower left corner. All of the other blocks have ambiguous range or ambiguous radial velocity.
Pulse Doppler radar relies on medium pulse repetition frequency (PRF) from about 3 kHz to 30 kHz. Each transmit pulse is separated by between 5 km and 50 km of distance.
Each range sample is converted from time domain I/Q samples into frequency domain. Older systems use individual filters for frequency filtering. Newer systems use digital sampling and a Fast Fourier transform
or Discrete Fourier transform
instead of physical filters. Each filter converts time samples into a frequency spectrum. Each spectrum frequency corresponds with a different speed.
A blind velocity occurs when Doppler frequency falls close to the PRF. This folds the return signal into the same filter as stationary clutter reflections. Rapidly alternating different PRF while scanning eliminates blind frequencies.
Modulo
In the mathematical community, the word modulo is often used informally. Generally, to say "A is the same as B modulo C" means, more-or-less, "A and B are the same except for differences accounted for or explained by C"....
sampling.
This is required for pulse-Doppler radar
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...
signal processing.
Measurements
Some types of measurements introduce an unavoidable modulo operationModulo operation
In computing, the modulo operation finds the remainder of division of one number by another.Given two positive numbers, and , a modulo n can be thought of as the remainder, on division of a by n...
in the measurement process. This happens with all radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
systems.
Radar aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
happens when:
- PRF is too low to sample Doppler frequency directly
- PRF is too high to sample range directly
Pulse Doppler sonar uses similar principles to measure position and velocity involving liquids.
Radar Systems
Radar systems operating at a pulse repetition frequencyPulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
(PRF) below about 3 kHz pulse rate produce true range, but produce ambiguous target speed. Radar systems operating at a PRF above 30 kHz produce true target speed, but produce ambiguous target range.
Medium PRF systems produce both ambiguous range measurement and ambiguous radial speed measurement using PRF from 3 kHz to 30 kHz.
Ambiguity resolution finds true range and true speed by using ambiguous range measurements and ambiguous speed measureds using multiple PRF.
Doppler Measurements
Doppler systems involve velocity measurements similar to the kind of measurements made using a strobe light.For example, a strobe light can be used as a tachometer to measure rotational velocity for rotating machinery. Strobe light measurements can be inaccurate because the light may be flashing 2 or 3 times faster than shaft rotation speed. The user can only produce an accurate measurement by increasing the pulse rate starting near zero until pulses are fast enough to make the rotating object appear stationary.
Radar and sonar systems use the same phenomenon to detect target speed.
Operation
The ambiguity region is shown graphically in this image. The x axis is range (left-right). The y axis is radial speed. The z axis is amplitude (up-down). The shape of the rectangles changes when the PRF changes.The unambiguous zone is in the lower left corner. All of the other blocks have ambiguous range or ambiguous radial velocity.
Pulse Doppler radar relies on medium pulse repetition frequency (PRF) from about 3 kHz to 30 kHz. Each transmit pulse is separated by between 5 km and 50 km of distance.
Range Ambiguity Resolution
The received signals from multiple PRF are compared using the range ambiguity resolution process.- Range ambiguity resolution process explanationRange ambiguity resolutionRange ambiguity resolution is a technique used with medium Pulse repetition frequency radar to obtain range information for distances that exceed the distance between transmit pulses.This signal processing technique is required with pulse-Doppler radar....
Each range sample is converted from time domain I/Q samples into frequency domain. Older systems use individual filters for frequency filtering. Newer systems use digital sampling and a Fast Fourier transform
Fast Fourier transform
A fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...
or Discrete Fourier transform
Discrete Fourier transform
In mathematics, the discrete Fourier transform is a specific kind of discrete transform, used in Fourier analysis. It transforms one function into another, which is called the frequency domain representation, or simply the DFT, of the original function...
instead of physical filters. Each filter converts time samples into a frequency spectrum. Each spectrum frequency corresponds with a different speed.
Frequency Ambiguity Resolution
The received signals are also compared using the frequency ambiguity resolution process.- Frequency ambiguity resolution process explanationFrequency ambiguity resolutionFrequency ambiguity resolution is used to find true target velocity for medium pulse repetition frequency radar systems.This is used with pulse-Doppler radar.-Definition:...
A blind velocity occurs when Doppler frequency falls close to the PRF. This folds the return signal into the same filter as stationary clutter reflections. Rapidly alternating different PRF while scanning eliminates blind frequencies.