[mpatch]

anybody ever check GPS speed against a calibrated radar gun (or some other very accurate speed measuring device)?

 

[greywolf]

Towns around my area regularly have radar operated "YOUR SPEED IS" mobile kiosks parked at major entrances to the town to slow people down. They have always matched the speed on my Zumo 550 exactly.

 

[mpatch]

That's good news. Put new guage faces on yesterday and wanted to check them with my GPS which I just got back from doing. Speedo reads about a good as one can expect. Prett hard to hold exactly at a certain indicated speed but I figure I am 1 mph slow of indicated between 35 and 60 MPH according to my GPS.

 

[GLHS837]

I have bounced my droids GPS against both the cars GPS speed and the speedo, and my GPS equipped radar detector, all against the "self serve" speed kiosks, which they put out on base all the time. All agree.

 

[TnTJ]

Usually pretty close. HOWEVER...

As a wielder of said RADAR for the past decade I have found that GPS can vary enough to put one in trouble when relied upon to "toe the line" so to speak.

RADAR is exact. The counting unit can take a speed out to the .000s digit. It never rounds up though. What the operator sees is the true ones digit- So 59.999 MPH will have a readout of 59 MPH.

GPS has too many variables. Just today I used a GPS app on my phone while riding and the best accuracy was 9'. Now I'm not a mathematician but even I know that +/- nine feet really won't have a bearing on overall speed calculations... unless I'm putting around at .5 MPH.

One thing about those RADAR trailers. There is an effect called cosign that reduces read -out speed compared to actual speed. The further the target object is away from the center of the beam-the lower the speed displayed. So this actually favors the driver in nearly all situations as most RADAR is not operated in a true head-on fashion.

So if your GPS jives with a RADAR trailer on multiple occasions approached at a relatively centered angle, then you are probably safe.

:bom_policeman:

 

[10-95]

RADAR is exact. The counting unit can take a speed out to the .000s digit. It never rounds up though. What the operator sees is the true ones digit- So 59.999 MPH will have a readout of 59 MPH.

Things must have changed since my days in motors. All our manuals from the various manufacturers used to say +/- 2mph was the margin for error. That was in the basic operator course too.

 

[tiller4me]

Checked against laser radar.......spot on.

 

[tmcgee]

Every time I've been tagged for a ticket, I've gone back and checked the GPS track log and what the cop wrote on the ticket was spot on.

And yeah, radar accuracy is much improved since I was a radar tech in the Navy in the late 60's.

 

[GLHS837]

Checked against laser radar.......spot on.

Do you mean laser and radar? Or a LIDAR device? Which is laser only.

 

[TnTJ]

Things must have changed since my days in motors. All our manuals from the various manufacturers used to say +/- 2mph was the margin for error. That was in the basic operator course too.

It may have or it could just be the anal agencies I have worked for. Last one had the RADAR tech come out at a minimum every 2x/ yr and do the "super-duper/ can't be beat ever in court/ look how accurate we tech geeks can make this" calibration where they would show the counting unit go out to the x.000s digit. Had to check calibration w/ tuning forks prior/post every shift and if it wasn't exact we couldn't use 'em. But I do recall some manuals having a MOE for those checks.

 

[ockerstrom]

Towns around my area regularly have radar operated "YOUR SPEED IS" mobile kiosks parked at major entrances to the town to slow people down. They have always matched the speed on my Zumo 550 exactly.

Here in Australia the mobile "kiosks" as you call them [ours are electronic LCD/LED sign units mounted on trailers, not sure what type of radar unit they use but they are often wrong- based on the consistency of several of my GPS units and compared to my speedo].

The important thing with comparing your GPS reading to your speedo reading is to make sure you are travelling at a consistent steady speed.

It takes 2-5 seconds for the signal between the GPS and the satellites to bounce back and be calculated and shown on the GPS screen.

I can make my GPS "lag behind" my speedo reading by up to 7kph under hard acceleration on my Wee-Strom.
When this happens the GPS starts to jump numbers on the reading as it tries to catch up to "real time" speeds- eg. with moderate acceleration it will start off 20,21,22,23, but if I gas it hard it then goes 20,27,35,40,43 etc. until my acceleration rate decreases and my speed becomes constant.

I have found by doing checks at different speeds while maintaining that speed steadily, the the speedo on my particular bike is off by near enough 10% throughout- GPS speed 80kph speedo reads 88kph, GPS 90 speedo 99, GPS 100 speedo 110, easy maths to maintain correct road speeds and when approaching a fixed or mobile speed camera I slow down and match the speedo to the posted limit which then actually ensures I'm under the limit.

It is also a fairly consistent known fact the the Strom speedo is, on average, actually off by 8% on the bulk of them.

For the record, I have 5 different GPS units- 2x Garmin, 2x TomTom, 1x Aldi GoCruise.
I have checked both my bikes plus my car using at least three different GPS units on/in the same bike/car, in each case the % variation between the speedo readings and the different GPS units remained consistent.

I'm happy my GPS units are accurate and I know in my head what the different speed reading variations are for each of my bikes and the car.

And if you are polite to a LEO when he stops you and you are only a bit over the speed limit and not endangering anyone, the odds are he will let you off with a warning.

Those :furious::furious: fixed and mobile cameras don't have the ability to assess the risk/danger factor for each infringement and are only there to fill the coffers of the government.

Here we have a tolerance of around 2kph-3kph from what we can gather [the powers that be in each State won't admit where they draw the line on being over].

The public uproar about the fact the cameras only raise revenue and don't actually reduce the road toll [statistics show they have in fact caused accident rates to rise at some camera sites because people "panic stop" to avoid a ticket thus actually causing multiple car rear enders- many of our cameras are combined red light/speed cameras at intersections] has caused at least one State government to start switching cameras off at some locations.

 

[OydnaR]

and the best accuracy was 9'. Now I'm not a mathematician but even I know that +/- nine feet really won't have a bearing on overall speed calculations... unless I'm putting around at .5 MPH.
n:

that 9' error in position has ZERO bearing on the speed

speed is determined by "differential" GPS, something surveyors like myself have been using since the mid 80's when we started launching the GPS constellation

while GPS postion may not be precise, its relationship to other positions recorded IS precise, as a surveyor, we put one reciever on a known location, the other on an unknown location, while the exact position isn't precise, the bearing and distance from the other reciever IS very precise

it works the same for the speedo, your unit is determining the speed by the differential where it thought you were 1 second ago to the position it thinks your at now, the postion may have been off by 9 feet, but it was off by 9 feet in the same direction at both locations, cancelling any error out



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[greywolf]

Randy, do you know what causes the crazy numbers that a GPS may output for the highest speed reached. I have 303mph on one unit right now and it has never been on an airplane.

 

[OydnaR]

Randy, do you know what causes the crazy numbers that a GPS may output for the highest speed reached. I have 303mph on one unit right now and it has never been on an airplane.

the .002% of the time yer GPS speed is off, its usually waay off, usually as your going thru urban canyons, and your reciever uses signals momentarily with a different set of geometry



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[greywolf]

Thanks. That makes sense.