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by Steve Hocker

Did I miss something?

January 25, 2018

Same Direction Radar – why does my radar miss targets?

In my years of teaching how to run radar there is one question that I can always count on to come up,” Why did my Same Direction Radar drop a reading?”.

This reality can be frustrating and sound all too familiar to a traffic cop.  Allow me to explain why this happens here. 

The Same Direction radar (sometimes referred to as Same Lane radar) was first developed in 1985.  Since that time, advances have been made, but certain inherent Doppler radar limitations exist for the minimum and maximum speed differences in Same Direction mode. 

Remember that the minimum difference in speed between the target and patrol vehicle must be 3 MPH before any speed will be shown.  The maximum difference in speed between the target and patrol vehicle is 70% of the patrol speed.  Once that maximum difference is exceeded, no speed readings will be shown.

Everything seen by the radar unit is relative to the radar not the roadway.  Same direction radar works on processing the returned signal from the target vehicle traveling in the same direction as the patrol vehicle.  For example, consider a patrol vehicle that is moving at 50 MPH with a target ahead of the patrol vehicle traveling at 90 MPH (relative to the ground) in the same direction.  The radar sees a 50 MPH signal and a 40 MPH signal (relative to the radar).  Due to electronics and physics, the radar unit cannot show any speeds where the difference in speed is greater than 70% of the patrol speed.  70% of patrol speed (50) is 35 MPH.  The maximum speed the radar can display with a 50 MPH patrol speed is 85 MPH (50 + 35).  This is the reality of running any Doppler radar.

As you can see in the graph above, for a patrol car traveling at 50 MPH, the patrol speed cosine reflections are as low as 39-40 MPH.  The maximum speed that a radar can look for same direction targets is 70% of the patrol speed or 70% x 50 MPH = 35 MPH.  So, to prevent patrol speed cosine reflections being picked up as an actual target speed, the radar is not allowed to look above the 70% limit.   The graph shows a true target that is traveling 20 MPH faster than the patrol vehicle.  This 20-MPH signal would be added to the patrol speed (50 MPH) and show a target speed of 70 MPH.  If the vehicle were traveling above 70% of the patrol speed, say 40 MPH (actual speed of 90 MPH), the radar would not be able to display that speed.

Steve Hocker is the Senior Product Manager at Stalker Radar.  Steve joined Stalker Radar in 2006.

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