Speedo Way Off
#11
03-28-2006 | 02:53 PM
Joined: Mar 2006
Posts: 12
Likes: 0
From:
RE: Speedo Way Off
ORIGINAL: abadfish
At best, GPS is accurate to something more like 0.25 mph...and that's only under certain conditions. On average, the accuracy is probably more like 1 - 3 mph.
Recreational GPS receivers are only accurate for velocity if you are travelling in a straight line at a constant velocity for, say, several seconds. This is because all recreational GPS receivers don't actually measure velocity directly. They compute velocity by differencing position from second to second. This method has several deficiencies:
- Because its differencing positions to determine velocity, your velocity errors are very dependent on your position errors. Most recreational GPS receivers have a position error on the order of 15 meters or so. You can get this down to <5 meters with a real-time differential GPS unit or a WAAS-enabled unit.
- The sampling rate is usually at 1 Hz (once per second). A motorcycle's speed can change drastically within that 1 second.
Couple these two together and you will have a large error in velocity. You can see this when you ride. Notice how on a twisty road or a quick change in acceleration (either throttling or braking), there is a bit of latency (up to a few seconds) in the GPS and it takes some time for it to catch up to your speedo. During that time it can be off by as much as 5 - 10 mph.
The only way to get more accurate velocity data from a GPS receiver is to use a GPS reciever that measures carrier phase and/or delta ranges (doppler beat). I don't know of any recreational GPS receivers that do this. It would also help if the receiver has a higher sampling rate like, say, 10Hz or 20Hz as compared to 1 Hz.
Radar is much more accurate for velocity under those dynamic conditions. This is because it uses much more accurate phase ranging techniques (doppler beat). The physics is in its favor. Under any dynamics of the vehicle being targeted, it will be to about 1 mph.
[/geek]
anyway, my point is that GPS is not as accurate for velocity as you think it may be.
ORIGINAL: chainstretcher
Most GPS units are accurate to within .1 mph.
Most GPS units are accurate to within .1 mph.
Recreational GPS receivers are only accurate for velocity if you are travelling in a straight line at a constant velocity for, say, several seconds. This is because all recreational GPS receivers don't actually measure velocity directly. They compute velocity by differencing position from second to second. This method has several deficiencies:
- Because its differencing positions to determine velocity, your velocity errors are very dependent on your position errors. Most recreational GPS receivers have a position error on the order of 15 meters or so. You can get this down to <5 meters with a real-time differential GPS unit or a WAAS-enabled unit.
- The sampling rate is usually at 1 Hz (once per second). A motorcycle's speed can change drastically within that 1 second.
Couple these two together and you will have a large error in velocity. You can see this when you ride. Notice how on a twisty road or a quick change in acceleration (either throttling or braking), there is a bit of latency (up to a few seconds) in the GPS and it takes some time for it to catch up to your speedo. During that time it can be off by as much as 5 - 10 mph.
The only way to get more accurate velocity data from a GPS receiver is to use a GPS reciever that measures carrier phase and/or delta ranges (doppler beat). I don't know of any recreational GPS receivers that do this. It would also help if the receiver has a higher sampling rate like, say, 10Hz or 20Hz as compared to 1 Hz.
Radar is much more accurate for velocity under those dynamic conditions. This is because it uses much more accurate phase ranging techniques (doppler beat). The physics is in its favor. Under any dynamics of the vehicle being targeted, it will be to about 1 mph.
[/geek]
anyway, my point is that GPS is not as accurate for velocity as you think it may be.
I have used my handheld when I fly and it always jives with the $20,000 Garmin 1000 on the panel. Maybe it sucks too since both are Garmin, but for checking speedo error and top speed a GPS is more than accurate enough, however as you pointed out they tend to lag quite a bit.
#12
03-28-2006 | 06:03 PM
Joined: Nov 2005
Posts: 94
Likes: 0
From:
RE: Speedo Way Off
Lower end models do lag a bit,
But they are very accurate with a cross referencing system of the airspeed indicators in our jets... Maybe we need to add a pitot-static system to our bikes. No trigger indication until 40kts, but ya'll can calibrate that to ground speed and you really don't care until ya pass the so-called speed limit. The weight would be a horrendous addition - computers and all...
Now that GPS MAN did the homework - I'll be glad that I'm actually going slower when "THE MAN" catches up to issue that yellow sticky!
"There is nooo waaay, I was going that fast MR. Officer..." You may have read 166, but I'm sure the speedo said 66 dictating 59.4 or thereabouts...
But they are very accurate with a cross referencing system of the airspeed indicators in our jets... Maybe we need to add a pitot-static system to our bikes. No trigger indication until 40kts, but ya'll can calibrate that to ground speed and you really don't care until ya pass the so-called speed limit. The weight would be a horrendous addition - computers and all...
Now that GPS MAN did the homework - I'll be glad that I'm actually going slower when "THE MAN" catches up to issue that yellow sticky!
"There is nooo waaay, I was going that fast MR. Officer..." You may have read 166, but I'm sure the speedo said 66 dictating 59.4 or thereabouts...
#13
03-28-2006 | 10:57 PM
Joined: Mar 2006
Posts: 173
Likes: 0
From:
#14
03-29-2006 | 01:33 AM
Joined: Apr 2005
Posts: 2,072
Likes: 0
From: Spring , Tx
#15
03-29-2006 | 05:12 AM
Joined: Mar 2006
Posts: 117
Likes: 0
From: Johannesburg, South Africa
RE: Speedo Way Off
Ok, so I've gone the whole hogg with sprockets and it got so bad that when my speedo read 260 km/h the GPS said 232 km/h. I then tried the SpeedoHealer. Nice feature is that with the push of a button it shows your top speed achieved. Now I also have a Garmin 2610 Streetpilot hardwired into my bike (WAAS enabled). So, I reset both top speed records to zero and took a leisurely trip to the Vaal River (my favourite strip not closely monitored by the local 'boys in blue'). Top speed recorded on Garmin 282 km/h and SpeedoHealer 281 km/h. I wish I had found the SpeedoHealer earlier as my odo probably reads 15% more than it should.
#17
03-29-2006 | 11:13 PM
Joined: Nov 2005
Posts: 265
Likes: 0
From: Thomasville, NC
RE: Speedo Way Off
ORIGINAL: hobbs51
NOT BEING A SMART YOU NO WHAT AND I NO NOTHING ABOUT NEITHER ONE. BUT IN THE HENDRICK MOTORSPORTS PLANE CRASH IT WAS TOLD THEY WERE USING A HAND HELD GPS AND WAS OFF 300 FT. THE AMOUNT THEY OVER FLEW THE AIRPORT AND HIT THE MOUNTAIN.
NOT BEING A SMART YOU NO WHAT AND I NO NOTHING ABOUT NEITHER ONE. BUT IN THE HENDRICK MOTORSPORTS PLANE CRASH IT WAS TOLD THEY WERE USING A HAND HELD GPS AND WAS OFF 300 FT. THE AMOUNT THEY OVER FLEW THE AIRPORT AND HIT THE MOUNTAIN.
Sidenote-----> Last year I designed the 320-ft Clearspan Hendrix Motorsport Airplane hangar that was to house the airplane that crashed along with 8-9 more located in Concord, NC.....It is a VP Building's structure...I work out of their Kernersville Office..
#18
05-15-2006 | 09:57 PM
Joined: May 2006
Posts: 3
Likes: 0
#20
05-16-2006 | 10:53 AM
Joined: May 2006
Posts: 91
Likes: 0
From:
RE: Speedo Way Off
Just to add to Police Radar, SpeedO, and GPS. the GPS is far better than a stock SpeedO but the police radar and laser is better than the recreation GPS. Police radar will give a speed reading when passed across a wall and that is because the realitive distance has changed. Radar measures realitive distance. Radar will pick up a bigger target further away then a smaller object closer. All these issues are taught and explained and the operator should understand. Moving the radar across the wall isnt gonna change the speed of your vehilce. That would cause a speed of about 5mph and I dont think anyone would be mad about being clocked a 5mph. as far as the larger target furhter away and the smaller closer well here is the explaination.
Radar works like this. It targets the Closest, largest, most reflective , fastest moving target in that order. (Whats that mean). Closest= same vehicle type one 200meters in front of the other, closest one will be displayed(if they are the same vehicle) Largest meaning= pickup truck and motorcycle. (if side by side the truck speed would be displayed). Most reflective= (not the color) Corvette vs. 1977 Cadilac, this is basicly areodynamics the Cadilac speed would be displayed if side by side. Fastest= this is the last and least importance of the group, If both vehicles were the same and one was moving considerably faster and both were in close realation the faster one would be displayed.
I will continue on the next post
Radar works like this. It targets the Closest, largest, most reflective , fastest moving target in that order. (Whats that mean). Closest= same vehicle type one 200meters in front of the other, closest one will be displayed(if they are the same vehicle) Largest meaning= pickup truck and motorcycle. (if side by side the truck speed would be displayed). Most reflective= (not the color) Corvette vs. 1977 Cadilac, this is basicly areodynamics the Cadilac speed would be displayed if side by side. Fastest= this is the last and least importance of the group, If both vehicles were the same and one was moving considerably faster and both were in close realation the faster one would be displayed.
I will continue on the next post