Speedometer assisted patrol speed search for DSP traffic radar

A digital signal processor (DSP) traffic radar utilizing pulses from the patrol vehicle's electronic speedometer to steer the DSP's search of Doppler return information for the patrol vehicle's radar return signal, to improve target identification and minimize inaccuracies. In moving mode, when the patrol vehicle comes to a stop, no pulses are received by the DSP and therefore the patrol speed is set to zero, eliminating false association with other moving targets.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in traffic radar devices for law 
enforcement and related applications in which the speed of a target 
vehicle is detected from a stationary or moving transmitter location and, 
in particular, to a method and apparatus for processing Doppler return 
information by analyzing a limited area of the frequency spectrum 
corresponding to the speedometer output of the patrol vehicle, in 
conjunction with analysis of the entire frequency spectrum in order to 
improve target identification and minimize interference and unwanted 
harmonics. 
In previous traffic radars utilizing digital signal processing (DSP), the 
patrol speed is found by using frequency and magnitude criteria, signal 
patterns, and patrol vehicle tracking history. Such a DSP radar is 
disclosed, for example, in U.S. Pat. No. 5,528,246 to Henderson et al., 
owned by the assignee herein. Anomalies can occur using these methods such 
as when the transmitting antenna is released from standby mode, when 
strong returns from other vehicles are received that are caused by a 
shadowing effect when a vehicle is traveling in the same direction as the 
patrol vehicle but at a different speed or when the patrol vehicle signal 
is summed with another vehicle moving in the opposite direction. Another 
problem can occur when the radar is in moving mode and the patrol vehicle 
comes to a stop at a traffic light. A return from moving traffic can be 
confused with the patrol vehicle and a wrong patrol vehicle speed is 
displayed resulting in an inaccurate calculation of a target speed. 
SUMMARY OF THE INVENTION 
It is therefore an important object of the present invention to provide a 
method and apparatus for determining the patrol vehicle speed through a 
combination of speedometer input and radar return. 
Another important object of the present invention is to provide a method 
and apparatus for determining the patrol vehicle speed when the patrol 
vehicle comes to a stop while the radar unit is in moving mode. 
It is also an important object of the present invention to provide a method 
and apparatus for determining patrol vehicle speed within a DSP search 
window by synchronizing the speedometer speed with the radar measured 
speed of the patrol vehicle. 
A further important object of the present invention is to provide a method 
and apparatus for determining patrol vehicle speed that is more reliable. 
More particularly, the signal from an electronic speedometer is input to a 
DSP radar. The signal processing system of the radar unit converts the 
pulses from the speedometer and calculates the speedometer speed. Using 
this speed as the center of a search window, the system searches for the 
patrol vehicle radar return signal that falls within the search window. 
Once the patrol speed is found, other targets can be found and their speed 
accurately determined. If the radar is in moving mode and the patrol 
vehicle comes to a stop, there will be no pulses from the electronic 
speedometer and the patrol speed will be calculated to be zero eliminating 
the need to search for a non-existent patrol vehicle radar return signal. 
If no patrol vehicle radar return signal is found within the search 
window, a search for the patrol vehicle radar return is conducted using 
methods known in the prior art. Likewise, if speedometer pulses are not 
detected at start-up, the search for the patrol vehicle return signal may 
employ prior art methods.

DETAILED DESCRIPTION 
Turning more particularly to the drawings, FIG. 1 illustrates a DSP radar 
unit 10 with an antenna 12 for sending and receiving radar signals. Radar 
return signals received by antenna 12 are converted from analog to digital 
by A to D converter 14, and input to signal processing system 16 
containing a digital signal processor (DSP). It should be understood that 
a DSP traffic radar unit transmits a single frequency microwave signal 
toward moving and stationary targets. A portion of the microwave signal is 
reflected by the targets and is received, mixed, and amplified by the 
receiver section of antenna 12. The signal received is a composite of all 
signals reflected by the moving and stationary targets. Conversion of the 
signal from the time domain to the frequency domain by the DSP separates 
the targets by frequency, which is directly proportional to the speed of 
the target. 
The operator interface, including mode selection and data display, is 
included in logic control system 18. Logic control system 18 and signal 
processing system 16 work in tandem to control calculation, configuration, 
and display of targets and patrol speed. Output 22 from a patrol car 
electronic speedometer 20 is connected to the DSP of signal processing 
system 16. As in a conventional traffic radar, a display window (not 
shown) is provided to display the patrol vehicle speed. 
The electronic speedometer 20 outputs a certain number of pulses for each 
mile the patrol vehicle travels. A common value of speedometer pulses is 
8,000 pulses per mile. Therefore, when the patrol vehicle is traveling at 
a speed of one mile per hour, the speedometer will output (8,000 pulses 
per mile * 1 mile per hour) pulses per hour. The number of pulses per 
second is found by dividing the number of pulses per mile by the number of 
seconds per hour. Thus, the number of pulses that occur in one second (per 
mile per hour) is 8,000 pulses per hour divided by 3,600 seconds per hour, 
which equals 2.222 pulses per second (per mile per hour). The sync time is 
the reciprocal of this value or 1/2.222, which equals 0.450 seconds or 450 
milliseconds. The sync time is the amount of time needed to accumulate 
pulses, which is equal to the speed of the vehicle in miles per hour. The 
sync time is scaled by 15 to determine the synchronization number 
(450/15=30). The synchronization number is stored and used to steer the 
DSP patrol search. Since the DSP does not directly use the speedometer 
pulses as the patrol speed, small inaccuracies of the speedometer 
synchronization are not important. The electronic speedometer output 
pulses are interfaced to the DSP in signal processing system 16 by way of 
a level converter and a Schmitt trigger logic circuit known in the art, to 
an edge sensitive interrupt pin on the DSP chip (such as Analog Device 
ADI2185). The DSP calls an interrupt service routine that counts pulses 
each time a speedometer pulse is received. 
Configuration and control of a traffic radar unit is typically accomplished 
using buttons on the front panel of the radar unit and on the remote 
control unit (as illustrated for example in the U.S. Pat. No. 5,528,246). 
Although specific buttons on the remote control unit illustrated in the 
'246 patent will be used to describe the operation of the present 
invention, their use is for example only and not a limitation of the 
present invention. Other buttons or controls can be used to perform the 
same functions. 
To synchronize the speedometer input to the radar return, the operator 
depresses the "patrol blank" button on the remote control unit twice while 
driving the patrol vehicle at a constant speed. Referring to FIGS. 2 and 
3, the operator requests to synchronize the speedometer input to the 
patrol vehicle radar return 28, which is transformed from the time domain 
to the frequency domain by the DSP of Signal Processing System 16. 
Synchronization is typically done upon initial installation of the radar 
unit in the patrol vehicle or when the radar unit is moved to another 
vehicle. From the main loop 30, the patrol speed is displayed from the 
radar return 32. The entire frequency spectrum is shown in FIG. 2 after 
translation from the time domain with only a patrol vehicle signal 28 
present. If the vehicle speedometer does not match the radar displayed 
speed 34, the operator can cancel the synchronization by pressing any 
button other than the "lock" button on the remote control unit, the 
routine returns to the main loop 54 and the patrol speed is found using 
standard searching parameters. 
If the patrol speed matches the vehicle speed 34, the operator depresses 
the "lock" button on the remote control unit and the cancel timer starts 
and the system enters the sync state 36. If the patrol speed is zero 38, 
any previous sync number and sync valid flag will be cleared 46 and the 
unit will display "0" as the synchronization number 50. This gives the 
operator a way to clear previously stored synchronization data. The sync 
number and cleared sync valid flag are stored 52. The routine then returns 
to the main loop 54 and the patrol speed is found using standard searching 
parameters. 
If the patrol speed is not zero 38, a loop is entered where speedometer 
pulses are accumulated 40 until either the patrol speed equals the 
speedometer pulses 42 or the cancel timer has expired. If the patrol speed 
does not equal the speedometer pulses 42 and cancel timer has not expired 
44, speedometer pulses continue to accumulate 40. If the cancel timer has 
expired 44, the sync number and sync valid are cleared 46 and the unit 
will display "0" as the synchronization number 50 and store the sync 
number and cleared sync valid flag 52. The routine then returns to the 
main loop 54 and the patrol speed is found using standard searching 
parameters. 
If the patrol speed equals the speedometer pulses 42 before the cancel 
timer has expired 44, the sync number is calculated from the elapsed 
cancel timer and the sync valid flag is set 48. The sync number is 
displayed 50 and the sync number and sync valid flag are stored in 
non-volatile memory 52. The unit then returns to the main loop 54. 
Each time radar unit 10 is turned on or a new sync valid has been set by 
the synchronization routine (FIG. 3), the sync number and the sync valid 
flag are sent to the DSP from nonvolatile memory. The DSP synchronization 
software is only executed upon operator command when it is necessary to 
synchronize the radar unit with the speedometer, or to clear previously 
stored parameters. 
As illustrated in FIGS. 4 and 5, from the main loop 72 which is executing 
the conventional DSP software, if the speedometer input routine is called 
before the counting interval has completed 74, the system searches for the 
patrol vehicle 86 using standard searching parameters of the prior art. 
FIG. 5 illustrates the frequency spectrum with a patrol vehicle signal 28 
and target vehicle signal 96. In this example, the patrol vehicle is 
moving at a higher speed and in the same direction as the target vehicle. 
If the counting interval has completed 74 and there were some speedometer 
pulses counted during the counting interval 76, and the number of pulses 
are within the patrol range 78, the pulse count is converted from the time 
domain to the equivalent bin number in the frequency domain of the Doppler 
spectra 80. This conversion will depend on the band of the microwave 
antenna 12 and whether the unit is displaying miles per hour or kilometers 
per hour. The patrol speed search window 81 is set to plus or minus five 
MPH (or KPH) of the speedometer bin 82 and the counting interval timer is 
reset 84. The patrol vehicle return is searched for within the search 
interval 86. The routine returns to the main loop 94 to search for other 
targets 96. 
If no pulses are counted during the counting interval 76, and pulses have 
been counted since power up 88, the patrol speed is zero indicating that 
the patrol vehicle has stopped and since the search range is set to zero, 
the patrol vehicle speed is set to zero and the patrol vehicle search 
range is set to zero 92. The counting interval time is reset 84, and the 
patrol vehicle return is searched for within the search range 86. Since 
the search range is set to zero, the patrol vehicle speed is set to zero 
and the routine returns to the main loop 82 to search for other targets. 
If no pulses are counted during the counting interval 76, and no pulses 
have been counted since power up 88 or if pulses have been counted during 
the counting interval 76 but the number of pulses counted are outside the 
patrol range limits 78, either of which could indicate that the 
speedometer is malfunctioning or a cable is broken, the default search 
range is restored 90. The counting interval is reset 84 and the routine 
searches for the patrol vehicle 86 using standard searching parameters. 
The routine then returns to the main routine 94 to search for other 
targets. 
Accordingly, it may be appreciated that by limiting the search for the 
patrol vehicle return signal in the frequency spectrum to a window around 
the speedometer speed, many false returns can be eliminated. Additionally, 
when the patrol vehicle comes to a stop, there is no need to search for 
the patrol vehicle return, therefore the present invention unambiguously 
determines the patrol vehicle speed eliminating erroneous target speed 
calculation and display.