User's Manual

MESSAGE FORMATS

EXPLANATORY NOTES FOR RADAR EVENT & QUALITY MESSAGES

TheEventStart(ES)messagecontainsbothinitialtargetspeedandrangeinformation.Theradarwillhave

tracked the vehicle for a short distance before this message is sent. As only a relatively small amount of target

information is available to the radar at this stage, the range and speed of the target are not fully evaluated by

the radar and are provided for the system to make some initial decisions about whether to be interested in the

target. The initial speed and range readings in the ES message will have been qualiﬁed by the radar against a

series of checks to ensure that the event information is of sufﬁcient quality to proceed with a target track. The

physical position on the road of the target corresponding to the sending of the ES message can be moved by

alteringtheEventStartDistance(*ESD).AlongerESDwillimprovethequalityoftheESspeedanddistance

measurement and make the ES message occur later. Reducing the ESD will have the opposite effect. Making

the ESD too small may lead to premature ES messages.

WhenthevehiclecannolongerbetrackedbytheradaranEventEnd(EE)messageisgenerated.Thismessage

contains the speed of the vehicle which should be used for the Event as all possible speeds would have been

processed during the event. The range of the target in the EE message will generally be different from that of

the ES message because the target will have moved along the carriageway during the event. Generally, the ES

range will be less than the EE range for receding target and vice versa. The distance the target travels after the

trackislosttowhentheEEmessageissentissetbytheEventEndDistance(*EED)message.MakingtheEED

too small may result in multiple events being generated for a single target.

IntheEventEndmessage,thedistancetravelledinthebeambythetarget(EventLength)isreportedinmetres.

This measurement is directly proportional to the length of the target in the event. Depending on the range this

measurement will be approximately 8m for a saloon car and approximately 20m for a truck. Generally, a larger

event length can result in a larger difference in the range measurements reported in the ES and EE messages.

TheQualityMessage(QM)alwaysimmediatelyfollowstheEEmessage.ThereportedPeakPowerSpeedin

the QM is the same as that used in EE speed reading. The radar processes 195 readings per second and the

standard deviation of all those readings for the associated event are reported as a standard deviation in the QM.

The peak power reading is the maximum signal strength seen for the event. The peak power for a given target

will be reduced at increasing range. Generally, for a given range, cars produce a lower peak power reading than

for trucks.

Whilst the radar processes 195 range and speed readings per second not all these readings pass the radars

qualitycheckforagivenreading.Say,100speedreadingsarecollectedforanevent(that’saneventwhichlasts

approx.513mS)buttheradarrejects12ofthereadings,theQMeldPercentageSpeedReadingswillreport

088. That is, 88 good readings from the possible 100 taken is reported as 88%.

Firstly, the speed reading is assessed and then the corresponding range reading for that speed is then assessed.

If a speed reading is rejected, the corresponding range reading is automatically rejected. Continuing with the

example, there will be 88 ranges corresponding to the accepted speeds. Each range is then quality checked to a

given tolerance and rejected or passed. If they all pass the range quality check the Percentage Range Readings

will report 088. The Percentage Range Readings ﬁeld is always less than or equal to the Percentage Speed

Readings. Finally, if the radar had rejected 14 of the available range measurements the reported Percentage

Range Reading would be 074.