Home Blog
Previous | Next

Moving Mode Radar Problems

Contents -- Patrol Speed Shadowing | Shadowing from Moving Vehicles | Batching or Speed Bumping

Moving mode radar measures the closing or opening traffic speed relative to the radar. Traffic speed is derived from patrol vehicle speed, any error in patrol speed translates directly to a traffic speed error. The radar measures patrol speed by measuring the ground speed Doppler shift directly in front of the patrol vehicle, or directly behind for aft antennas. Off angle ground reflections measure patrol vehicle speed low, patrol speed shadowing by definition. The greater off angle from patrol vehicle center line, the greater the patrol and target speed error.

Sponsored links

Patrol Speed Shadowing

Large reflective objects can cause the radar to measure patrol speed off angle. A misaligned antenna will also cause off angle ground reflections. Patrol speed measures low by the cosine of the angle, the cosine effect.

vpm = v cos ß

vpm = Measured Patrol Speed
v = Actual Patrol Speed
ß = Ground Reflection Angle Error
Measured Patrol Vehicle Speed and Error
vp = Patrol Vehicle Speed:
ß = Ground Echo Angle Error: Degrees

Measured Speed
Speed Error

In opposite direction moving mode a low patrol speed reading translates directly to a high target speed reading. The target speed reading is high by the speed the patrol measurement is low.

The error in same-lane mode depends whether the target vehicle is traveling faster or slower than the patrol vehicle. Target vehicles traveling faster than the patrol vehicle have a high speed error for a low patrol speed. Target vehicles traveling slower than patrol vehicle have a low speed error for a low patrol speed.

Traffic Speed Error when Patrol Speed Measures Low
Direction Traffic
Same Direction Traffic
Slower than Patrol Faster than Patrol
Traffic Speed High by
Patrol Speed Error
Traffic Speed Low by
Patrol Speed Error
Traffic Speed High by
Patrol Speed Error

Patrol speed shadowing is not uncommon and can be caused by numerous objects and situations. Police radars have a Patrol Speed Blanking switch to break and reestablish ground lock if the radar gets stuck on an off angle ground echo. Some radars connect to the patrol vehicle speedometer, if speedometer and radar measured speeds do not match speed readings are disabled.

Objects that can cause
Extended Speed Shadowing
Objects that can cause
Momentary Speed Shadowing
  • Sound / Construction concrete barriers
  • Metal Guardrails
  • Crash Barriers - metal cables / post
  • Bridge Trusses
  • Hill-cuts / Depressions / Tunnels
  • Highway Barrels - metal / water filled plastic
  • Ditch Water
  • Moving Vehicles
  • Multiple Parked Vehicles
  • Plowed Snow Piles
  • Pillars
  • Overhead Signs
  • Road Signs
  • Parked Vehicles
  • Ice Patches, Snow Piles

Sponsored links

Shadowing from Moving Vehicles

Traffic traveling in the same direction and slower then the radar can be mistakenly used by the radar as the ground reflection. Radar measured patrol speed will be the difference between the patrol and vehicle speeds.

Figure 3.7-1 -- Moving False Ground Echo
Fig 3.7-1

The below equations use relative Doppler speeds, speed is positive for a positive Doppler shift and negative for a negative Doppler shift.

Traffic Front Antenna Rear Antenna
Opposite Direction vpm = vp - vg
vm = vt + vp - (vp - vg )
vm = vt + vg
vpm = -vp + vg
vm = -vt - vp - (-vp + vg )
vm = -vt - vg
Same-lane vpm = vp - vg
vm = vp - vg - (vp - vt )
vm = vt - vg
vpm = -vp + vg
vm = -vp + vg - (vt - vp )
vm = -vt + vg

Patrol and Traffic Speed Error
Patrol Speed
Measured Traffic Speed
Low by Vehicle Speed
used as Ground Echo
High by Patrol
Speed Error
Low by Patrol
Speed Error

Patrol speed shadowing from moving vehicles can occur when the radar is passing relatively slow moving traffic such as large vehicles, tractor-trailers, and a line of vehicles. Same direction vehicles traveling faster than the patrol vehicle will not be mistaken for a ground reflection because the Doppler shift is negative relative to patrol speed.

Patrol Speed Shadow Management
To manage shadowing from stationary or moving objects some radars monitor patrol vehicle speedometer (VSS - Velocity Speed Sensor). If the radar measured patrol speed does not closely match the speedometer target measurements are not displayed until speeds match. Speedometers are not accurate enough for radar to use in speed calculations, but a good check for patrol speed shadowing.

Batching or Speed Bumping

Batching or Speed Bumping occurs when the patrol speed suddenly changes speed causing a false or inaccurate reading. Radar measured traffic speed and patrol vehicle speed are not updated simultaneously, if patrol speed changes suddenly the radar may still be using outdated patrol speed data leading to a momentary measured speed error. Sudden acceleration, hit the gas, braking, turning, curves, or hitting bumps can cause batching or speed bumping.

Police Radar Information Center

Moving Mode Radar Problems
Previous | Top | Next