How do po­lice de­ter­mine if you’re speed­ing?

The Central Voice - - News -

LI­DAR - or Light De­tec­tion and Rang­ing - is a very re­li­able form of speed en­force­ment.

LI­DAR works by trans­mit­ting a very weak, but very con­cen­trated, beam of ul­tra­vi­o­let light. Pulses of light - hun­dreds per se­cond - are sent out from the LI­DAR unit at known in­ter­val times and at a uni­ver­sal con­stant speed of ap­prox­i­mately 300,000 kilo­me­tres/se­cond.

Th­ese pulses re­flect off mov­ing ve­hi­cles and back to the LI­DAR unit, which mea­sures the time be­tween pulses. The change in time be­tween pulses in­di­cates a change in dis­tance over time be­tween the unit and the ap­proach­ing or re­ceed­ing ve­hi­cle, reg­is­ter­ing a speed on the LI­DAR unit.

There are a num­ber of qual­ity con­trol mea­sures built into each unit to en­sure that speeds shown are ac­cu­rate. Most LI­DAR units are ac­cu­rate within plus-or-mi­nus two km/h at dis­tances some­times ex­ceed­ing two kilo­me­tres away.

The main dif­fer­ence be­tween RADAR and LI­DAR is that RADAR uses changes in ra­dio fre­quen­cies - known as the Dop­pler ef­fect be­tween out­go­ing and in­com­ing beams to cal­cu­late speeds, while LI­DAR uses in­frared light to mea­sure time over dis­tance.

Ra­dio waves are sent out from a RADAR unit in a fairly wide beam, and it is nec­es­sary for the op­er­a­tor to de­ter­mine which ve­hi­cle’s speed is be­ing dis­played on the unit.

In the case of LI­DAR, the out­go­ing beam is very fo­cused, al­low­ing po­lice to tar­get spe­cific ve­hi­cles.

One of the main lim­i­ta­tions of LI­DAR is that it must be op­er­ated from a sta­tion­ary po­si­tion, as op­posed to RADAR, which can be op­er­ated from a mov­ing ve­hi­cle.

Be­cause LI­DAR uses light, there also must be a di­rect line of sight be­tween the user and the tar­get ve­hi­cle, so small ob­struc­tions such as signs or branches can af­fect the user’s abil­ity to ob­tain a speed, as can cer­tain weather, such as fog, rain, or snow.

The main ad­van­tage of LI­DAR is the abil­ity to se­lect tar­gets with pin­point ac­cu­racy, even at great dis­tances.

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