I drove my brother back to Purdue tonight from Chicago and on the way back...the interstate was COMPLETELY empty...so I decided to have a little fun....I waited for a long straight and just got on it from about 80mph.....I topped my car out where 150mph on the speedo would be if it went that high....the needle was actually pegged (when I did my indiglo gauges I noticed that the needle stops completely around when 150mph would be).
I wish I had my GPS along for the ride like last time.....last time I hit 146.4 on the GPS...and it looked like my car was doing 148 on the speedo.....needless to say...I'll bet I hit 150mph tonight

pic of where the needle on the speedo stops is attached

 

I thought(was told by sources) that NORAD throws positioning on GPS systems off up to 100m on pinpoint positioning. The reason why I ask this is because I want to know how realistic the speed on these are since I want to use one to reset my speedometer since i've had it on and off 4 times.

 

SA (Selective Availability) was turned off on May 1, 2000. As a result GPS is more accurate since then. Also the error that GPS systems have is not relevant to a speed reading. Sequential points are quite accurate. The error found in a GPS signal is from satellite drift. Between two sequential points the drift is almost identical. So there is virtually no error.

Stereodude

 

Will all GPS systems do this? Calculate speed

 

Not anymore. The government gave approval opened up a few more satalites for public use to give more accuracy a couiple of years ago.

 

Yes, it does.

 

The speed readings from a GPS are VERY accurate. We use GPS all the time in the planes that I fly...they are priceless as far as navigation and ground speeds are concerned.

 

even though the speedo doesn't move.. you can still are going faster

 

Yes, the GPS signals are being dithered currently ever since military action began in Afganistan. I work with the Air Force and use GPS in my work for them. They sent me an e-mail a few months back informing me that civilian GPS data will have much more error now that the system has been turned on again. Another thing to remember is that civilian GPS units only use 3 satelites and aren't remotely as accurate as the military units used. Currently, I've got about a 60 sguare foot margin of error (vs true survey points) with a store bought GPS unit. Another thing to consider is that a GPS used in a car will have more error simply because of the refraction of the windshield and metal frame. Glass, buildings, trees, clouds, strong magnetic fields (car), etc will throw off a GPS somewhat. Whether you want to believe me on this is up to you, but I think I have two pretty reliable sources feeding me this information. My first source of info is the US Air Force. My second source is Garmin which is only 2 miles from my office. I talk to these guys all the time to verify data, programming, signal strength, etc.

The faster you go, the harder it is for 3 satelites to continue to track your movement accurately.


Dave

 

My max speed of 146.4mph on my GPS was taken last April BEFORE the whole Sept. 11 thing.

Also...My Garmin Street pilot uses up to 11 sats at one time to track my position...there's a display mode that shows the sats.

If GPS was so innaccurate then why does the FAA approve it for use on instrument approaches??? When I fly the Ground speed calculations and time to next waypoint are RIGHT on....

 

Well I would take a stab at saying that even navigating by GPS doesn't have to have PIN point accuracy, you can be 10m or even 100meters off when youa are 5000 ft off the ground. It's not like your landing without seeing your runway lights. Until I got out of Active Duty in July of 2000, we were always told that there was a +/- error of 10m even with Mil. Spec. GPS systems. Thats why I asked.

 

I've also gone by those radar signs that display your speed at well over 70mph...and the GPS always reads EXACTLY what the radar sign says....you can't get much more accurate than that...

 

I'm not arguing with you, I think its awesome if it is that accurate. I'm going to go get one.

 

They are incredible for long trips....they tell you every turn, distance, ETA, Total time enroute, time to next waypoint (turn), average driving speed, total average speed, max speed, total mileage, and so on....it can even tell you were the closest McDonalds is

 

dave b - even though you just listed off a bunch of reasons why it would be inaccurate, would that affect the speed it reads out, as opposed to the currect ground position..?

 

Which model and type, how much?

 

Again, I will take exception to what you say. You do not understand how GPS works apparently. It uses far more than 3 satellites to compute your position. It uses time codes to compute distance, speed, and everything else. SA has not been turned back on. Garming units can tell you how accurate they are, or how certain they are of your position. I've compared reading taken before Sept. 11th and after. They are within meters. SA has not been turned back on.

Stereodude

 

Mine is the Garmin Color Street Pilot.....it's about 3 years old right now....got it for $400 in late 98.

 

Yeah i did that about a month ago and i have a pic



Bruce

 

Nice air bag-

 

Thank you i like not having it very much.

Bruce

 

I'll agree with the pilot(I'm one) about the accuracy of GPS. Plus many sets will tell you how many satellites are being received. Last time I looked at my simple Garmin StreetPilot it was receiving 5. P.S.--- That StreetPilot is a good sub if you don't have the "Nav" panel.

 

I've seen as many as 9 locked satellites on my E-trex at times. It reports accuracy as 17 feet with that many satellites. It can tell how accurate it is.

SA is not effective at making GPS inaccurate. A simple FFT on the data can reduce its effects greatly. When GPS was invented the smallest receivers were the size of a VW beetle. You couldn't get enough processing power to do a FFT on the data. No one ever antipated that the type of processing that can make SA negligable would be so easily done or readily available. Now days SA is fairly easily defeated with the processing power available. That's why it's been turned off. It wasn't doing it's job, so there really wasn't any point in leaving it on.

SA should have be turned off, until a time of war. If there was no error in the signal no one would have spent years developing systems and methods to circumvent it. When switched on suddenly civilian GPS would be inaccurate and there'd be no way to compensate because it would be a surprise. However, they tried the other method and left it on all the time. This gave everyone a long time to figure out how to defeat it.

It should also be noted that the only modern major wartime action that took place post GPS deployment was the Persian Gulf War/Conflict. During this SA was turned off and civilian GPS units were given to the troops because there were not enough military GPS units available. It was shown that civilian units were just as accurate as their military counterparts once SA was turned off.

Stereodude

 

i dont trust my speedo when i blow by the 5-0 at 90+ MPH in a 65 and he doesn't move... do i need to recalibrate?

 

I wonder if the police will use GPS in the near future and have every car always scanned to get their speed rating all day? We'd get speeding tickets everyday.

 

I think that would be unconsitutional. (here comes the flames)
They have no just reason to monitor your speed, and they would need to have such put a tracking beacon onto your car.

 

There's something not quite right here. You mean to suggest that these satelites, which were built to be able to track the shuttle launching from Cape canaveral at 3,000 mph, or find a Boeing 747 at 600 mph, or cruise missle headed for DC at some other un-godly mach speed, may have trouble accurately tracking a Maxima going a measely 150 mph?

DW

 

I'm sure that NASA has dedicated satelites for the shuttles and most planes are tracked by beacons on the plane. Do you have a beacon DW? If so I'm sure that NASA and the FAA would have no problem tracking you down. I think we are losing track of this thread.

 

All I have to say is . . .



DW

 

Yea, this thread was about how cool it was that I got my max up to 150mph....and now you guys are talking about satellite tracking

 

OK, back on track

150 mph?? Man that was fast!

Did you do that NA, or shot a little juice?

DW