GPS Versus Speed Radar — Misuse Of Both Technologies

Accused speeder to cops: My GPS proves your radar gun is wrong

By Ken Fisher | Published: October 26, 2007 - 10:27AM CT

GPS-based tracking devices don’t just help you figure out where you are and where you’re going, they might also help you avoid a speeding ticket. Shaun Malone of California is contesting a speeding ticket in that state by arguing that his GPS-based tracking system shows that the officer’s radar gun was wrong ..   Full GPS vs Radar article here.

Well, let’s just see here, how many mistakes are being made by how many people, and what the heck is the proper answer to this dilemma?

F irst of all, what is speed?  If we don’t know that and accept a definition we can never get on the same page.  There are essentially two common definitions that would make sense here … speed and instantaneous speed.   Aren’t they the same?  Certainly under the laws of physics they are not … under the laws of the various states on their highways they often seem to be enforced as if they are.

In general when we talk about speed we mean the rate of displacement as it moves through space.  We find speed by dividing the total distance covered by the total time taken and we receive and answer in “scalar units”, such as miles per hour or feet per second.  It’s quite clear that when law makers institute laws regarding speed of vehicles this is the sort of “speed” they are thinking of … in the US “Speed Limit” signs even read MPH … in countries which aren’t “hard of thinking” they restrict kilometers per hour, but it all comes out in the wash.

Problem is, we have no commonly accepted way to accurately (as in take legal action “accurately”) to measure this sort of speed.  In our care we have a device called a “speedometer” which counts, indirectly the revolutions of one or more of the vehicles wheels.  The device is actually a “wheel tachometer” … that is it measures the revolutions of the wheel per second and by simple math integrates the individual rotations, over time, into a representation of the vehicle’s speed across the ground.  How accurate is the “speedometer”?  Essentially no accuracy at all … just think of the case of the vehicle sitting on a patch of ice … gun the engine and see 100 mph on the speedometer and note that you aren’t moving at even a single mile per hour.  Extreme example, yes, but don’t forget the vehicle’s wheels are always slipping to some extent or another and the integration of the distance/time = speed equation does not take into account any other variables, such as tire radius changes based on speed, etc., etc.  Speed law enforcement used to depend solely on “pacing” behind the speeder’s car and noticing the officer’s speed on his own speedometer, or “clocking”, measuring the time the suspect vehicle takes to travel a known distance, using some form of stop watch.  For the type of “speed” it appears out law makers were interested in, this is the only truly accurate way to measure speed.  It’s interesting to note that official speed records are set, important research tests of all types of vehicles , labs that measure the speed of bullets from guns … you name it, this is how they measure speed.  they don’t use GPS and they don’t use radar, and they sure don’t use 19th-century technology which calculates the rate at which the wheels on the wagon are turning.

Enter radar.  “Pacing” a suspected speeder takes time and work and interferes with doughnut runs, so police ling ago fell into the stupid trap of using a device commonly called a “speed gun’ or “Doppler radar”.  Christian Doppler (1803-1853) was an Austrian mathematician/astronomer who is credited with introducing the scientific proofs that an object in motion which is emitting or reflecting detectible forms of energy changes the apparent frequency of the energy based on the objects speed.  A police radar gun points a beam of radio frequency energy in the general direction an officer is pointing the device and reflected energy received by the gun is measured.  The device calculates the shift in frequency and derives an “instantaneous speed” based on the observed shift.  Note that “instantaneous speed” is a snapshot of the vehicle’s speed at a specific moment in time and, in fact, is only a cousin of the “speed” law makers wanted measured.  I worked for years repairing and calibrating radar guns for a DoD agency.  The inaccuracies of both these devices and the typically poorly-trained operator would fill 10 web articles.  It’s interesting to note that California was one of a select group of states that for years did not allow the use of radar for speed enforcement … and it is sad that it has fallen into the trap of expediency over accuracy and citizen’s rights.

Enter GPS.  All GPS receivers calculate an instantaneous vehicle speed as part of the GPS navigation equation.  Over a period of time these instantaneous speeds can be integrated into a highly accurate average speed.  But at any given time this speed can be wildly inaccurate.  Depending on the algorithms selected by the GPS receiver designer, the device may present to the user an average speed number over time.  There are currently no standards that dictate how this speed will be calculated, how many instantaneous speeds will be averaged, over what time the calculation will take place and what, if any, biasing or smoothing will occur.  Take just one very common error that nearly all commercial systems suffer with:  The receiver calculates a position … let’s say the position shows the vehicle sitting still in Omaha, Nebraska at 41°18′11″N 95°53′39″W.  For whatever reason the next time the receiver calculates the current position the latitude and longitude are calculated as 00°00′00″N 00°00′00″E … which is the point where the Prime Meridian cross the Equator in the Atlantic, east of Africa.  A certain period of time later the unit calculates its position again and this time it is right where it was before … 41°18′11″N 95°53′39″W. During the time from calculation one to calculation three the devices apparently has moved more than 13,000 statute miles across the earth’s surface.  So what’s the vehicle’s speed? 

Obviously, the vehicle hasn’t moved at all, it is sitting still and thus has no speed.  But what, then, will you report to the user?  A speed of zero?  This example makes the decision look trivial.  Just “throw out” the obviously erroneous fix and get back to business.  What is only one digit went falsely to zero and then popped beck in normally?  How will you detect that instantaneous error and how will you account for it when recording a speed scalar for the vehicle.  What if two ‘bad” calculations come in one after the other?  One hundred?  Where will you “turn off” speed reporting and throw an error code instead?

Speed is a very elusive “beast” at times, and both Radar and GPS have very strong points and very weak points when it comes to the measurement of speed.  I’m glad I’m not the judge in this case, and I’ll tell you what I did a couple years ago when I got a speeding ticket in a car equipped with commercial-grade GPS tracking … I paid the damn ticket.