Shurshot

GPS works through determining the time taken between two GPS reference points and then does the math to come up with a speed. The shorter the distance being measured while radical changes in speed are taking place, the accuracy of the unit heads south.

Civilian GPS is something like within 10 feet plus or minus with good reception

It is not a like laser or radar that works by recording time from a sender to an object back to the receiver

camarodoctor

So you're saying that it's possible that a PDR reading at 60 mph may actually be 65 mph?

Jet Stream Blue Z06

Doesn't the PDR get the speed from the car just like it knows how much brake or gas you are applying, or what gear you are in?

DRLC5

So is the PDR speed separate from the actual speedometer in the car? I do not own one just yet so I am curious.

CaryBob

I don't think the OP is questioning 0-60 times, but rather the time to cover a distance. GPS has nothing to do with the speedometer.

Does the car use tire revolutions or GPS to measure distance covered?

peterjank

Possible, but doubtful. In my race cars, I have used Aim data acquisition systems that measure speed via both GPS and actual wheel speed since 2008. When cars are going straight and wheels have good traction, GPS speed and wheel speed rarely differ by more than 1 mph.

When you have wheel spin, GPS obviously provides more accurate speed numbers than wheel driven measurements.

GPS speed may be less accurate than wheel speed when the GPS antenna is struggling to get a fix on satellite locations due to heavy cloud cover, rain, surrounding buildings, etc.. The Aim GPS produces its best results when its antenna is able to lock on 9 or more satellites. It normally won't work if it can only spot 5 or fewer satellites. As the number of satellites falls from 7 toward 5 you may start to see some discontinuities in GPS speed measurement.

I don't have any direct experience with Corvette PDR yet, but my guess is that its GPS speed measurements exhibit characteristics similar to what I have described for the Aim systems that I use.

dar02081961

I think if you did more research you would find these statements arent accurate.

Speednet

From an engineering talk given earlier this year, I seem to recall the person saying that the GPS in the PDR is separate from the GPS in the navigation system, and that the PDR's GPS is more accurate and takes more readings per second. It's accurate enough to know what part of the curve you took. Which is pretty accurate.

So I'd say the OP is not correct when he says (with no backup proof) that the GPS is not accurate over short distances for measuring speed. I think the OP does not seem to have experience with high-accuracy GPS systems. These systems can be very accurate for recording speed, distance, and location.

begone

If I can download a $20 app on my phone called "Harrys Lap Timer" , I'm sure GM can figure out a GPS based accurate system for all of these times and speed figures...

PushNut

does the 2 things have to do with each other?

Your phone skills do not in any way relate to GM's ability to allow civilian access to high accuracy GPS satellites...

CHJ In Virginia

Harry's Lap Timer is an APP that uses the IPhone GPS system to collect speed and other data while a car is on track. I used it for two years and found it to be accurate. The poster above is saying that if an independent person can write a program to access and use successfully GPS data, it should be a piece of cake for "Big GM" to do the same.

Steve Snake Driver

Cosworth worked with GM in developing the PDR. They do the data gathering/telemetry for the C7R. I suspect they know a thing or two about how this stuff works.

Shurshot

Maybe so?

I was acting on the info posted on another thread stating the PDR ran off of GPS. If it is getting info from the speedometer it may well be getting info from the odometer as well. If that is the case the point I am making about the limitations of accuracy associated with GPS is moot

However, if it does gain speed, location and distance traveled information from GPS sensors as most plotters do, the shorter the distance with the greatest variance in speed will be more prone to error than a longer distance with more constant speed will be.

Or, in other words, concerning percentage rates of error, a PDR running off of GPS inputs will be much more accurate with the information posted on a one mile run then it will be for measuring 60 foot hole shot times, or a 1/4 mile run versus a 2 mile run

Shurshot

Through my experiences navigating at high speed in low light conditions where time and distance accuracy could mean the difference between living and dying, I can say my statements are very accurate.

Fair enough?

begone

You learn something everyday. No but seriously, check out that app. It's highly respected and accurate. But you'll need what those pesky kids call a smart phone.

Blvdbrawler

The MPH on the PDR is very accurate. Honestly, why have it and give it to customers if its going to give you the wrong information?? I think its very close or right on point as to how the car is actually performing. (Unless the OP or anybody else can prove otherwise?)

FLATOUTVIPER

Without running it at the track and comparing it to the time slips you guys will never know how it compares to the actual track. I don't really see why this is hard to understand.

c6z07

THANK you someone with intelligence lol. How can you claim it's accurate but not have a slip to prove it

Shurshot

This X2 ^^^^

Steve Snake Driver

I believe a big difference is that you didn't have a wheel in contact with the ground adding data to the overall equation (at least if I catch your drift). The PDR has 30 or so acquisition channels. To assume it only relies on GPS is probably wrong.