Police traffic radar using FFT processing to find fastest target

A method of determining and displaying the speed of one or more target vehicles in a police radar comprising the steps of:

• transmitting a radar signal toward one or more of said target vehicles;
• receiving radar return signals reflected from said one or more of said target vehicles and from other objects in the path of said radar signal which may be Doppler shifted in frequency from the frequency of the originally transmitted radar signals;
• transforming said received radar return signals into digital data and performing a Fourier transform thereon to generate a plurality of frequency components of a frequency spectrum which defines the amplitude and Doppler shifted frequency of all said radar return signals from said one or more target vehicles and other objects from which said radar signal was reflected, each frequency component having a magnitude, defined in terms of either amplitude or power, and a frequency;
• processing said frequency components to determine the frequency component with the highest frequency that has a magnitude which is large enough relative to the approximate average noise magnitude in a portion of said spectrum in which said frequency component was found to pass a qualification criteria defining a minimum acceptable signal quality;
• once a frequency component having acceptable signal quality is found, converting the frequency of said frequency component to speed and displaying said speed.

A method for determining the speed of one or more target vehicles using Doppler radar on a patrol platform comprising the steps of:

• converting received radar signals bounced off one or more target vehicles and stationary objects into digital data and performing a first Fourier transform on said digital data to find the amplitudes and frequencies of the Fourier frequency components that define frequency domain representation of the received radar signals, said Fourier components represented by digital data;
• finding a plurality of the highest amplitude Fourier components representing the frequency domain representation of a plurality of the strongest received radar signals and storing data representing these strong signals in a memory;
• filtering out the Fourier component digital data representing the received radar signals bounced off stationary objects which represent the speed of the patrol platform to generate filtered digital data;
• performing a plurality of overlapping Fourier transforms using Hamming window weighting on the filtered digital data to generate new sample data;
• finding the strongest Fourier component in the new sample data the Doppler shifted frequency of which represents the speed of the target vehicle having the strongest received radar signal;
• finding in said new sample data one or more Fourier components having frequencies higher than the Doppler shifted frequency of said strongest received radar signal, said one or more higher frequency Fourier components hereafter referred to as the fastest candidates, said fastest candidates being representatives in the frequency domain of received radar signals bounced off one or more target vehicles that are moving faster than the target vehicle that reflected said strongest received radar signal and storing said one or more Fourier components representing said fastest candidates;
• for each fastest candidate, determining attributes of the fastest candidate including the approximate average noise power in the vicinity of said fastest candidate, the target signal strength of each said fastest candidate and the signal strength of the strongest signal within a predetermined frequency proximity of said fastest candidate and storing said attributes;
• calculating and storing a first noise magnitude divided by signal magnitude ratio comprising approximate average noise magnitude in the vicinity of the fastest candidate divided by target signal magnitude of the fastest candidate for each fastest candidate, and calculating and storing a second noise magnitude divided by signal magnitude ratio comprising local signal maximum signal magnitude for the strongest signal in a predetermined frequency range of said fastest candidate divided by the target signal magnitude of the fastest candidate;
• receiving an input signal indicating a desired level of receiver sensitivity and choosing an appropriate noise/signal magnitude ratio based upon the selected sensitivity level;
• sorting the fastest candidates and their attributes in order of descending frequency;
• starting at the highest frequency fastest candidate, screening each fastest candidate for first and second noise power divided by signal magnitude ratios that are acceptable when measured against the noise divided by signal power ratio selected based upon the selected sensitivity level and selecting the first fastest candidate that is acceptable;
• screening the selected fastest candidate to prevent display of a speed calculated therefrom if said fastest candidate appears to be a harmonic of either the strongest signal or the signal representing the speed of the patrol platform or appears to the result of a double or triple bounce of a transmitted radar signal between said patrol platform and some other object or appears to be an intermodulation product of said strong signals;
• and
• calculating and displaying the speed of the target vehicle which reflected the received radar signal from which the fastest candidate that passed all the screening tests resulted.

A method of determining and displaying the speed of one or more target vehicles in a police radar comprising the steps of:

• transmitting a radar signal toward one or more of said target vehicles;
• receiving radar return signals reflected from said one or more of said target vehicles and from other objects in the path of said radar signal which may be Doppler shifted in frequency from the frequency of the originally transmitted radar signals using a receiver with a variable gain;
• adjusting the gain of said receiver so that the peak amplitude of received signals is maintained between predetermined limits to decrease distortion and converting said received signals into blocks of digital data, gain adjustments occurring only in the interval between completion of receiving one block of digital data and the beginning of receipt of the next block of data, and storing said gain value during the reception of the radar return signals which were the source for each block of digital data;
• performing one or more Fourier transforms on each block of received digital data to generate digital data representing Fourier spectral components in a frequency domain said spectral components having different frequencies and amplitudes which in the frequency domain represent the amplitude and Doppler shifted frequency of said radar return signals from at least said one or more target vehicles in a time domain;
• processing said Fourier spectral components to compare the power of at least some of said spectral components to the average power of a predetermined number of spectral components which are immediately adjacent to the spectral components whose power is determined, and selecting for further processing any spectral component with a power which exceeds the average power of its neighboring spectral components by a predetermined amount;
• computing the true power of each spectral component so selected using the gain at which said receiver was set when the radar return signal which was the basis for each said selected spectral component was received;
• determining and displaying the speed of the Strongest target vehicle with the strongest radar return signal corresponding to the spectral component with the highest true power and determining and displaying the speed of a faster target vehicle with a faster speed than the speed of the strongest target regardless of whether the radar return signal from said faster target is also the strongest radar return.

An apparatus comprising:

• a radar transceiver having a transmitter and antenna cooperating to transmit radar energy toward target vehicles and having a receiver sharing said antenna with said transmitter for receiving reflected radar energy;
• a memory for storing data and instructions;
• a digital-to-analog converter coupled to said receiver including circuits to sample said reflected radar energy and convert said samples into digital sample points and store said sample points in memory;
• a computer coupled to said memory and programmed to retrieve said sample points and perform a Fourier transform calculation thereon to generate a plurality of frequency components representing the Fourier spectrum of the time domain signal defined by said reflected radar energy, each said frequency component characterized by a frequency and a magnitude, said computer also programmed to process said frequency components to find a fastest frequency component having the highest frequency and which also has a magnitude which is large enough relative to the approximate average noise magnitude in a portion of said spectrum in which said fastest frequency component was found to pass a qualification criteria defining a minum acceptable signal quality.