GPS systems

I for got to mention the the VY'dore is stock standard as it left the factory in 2003 im at a loss as to how the speedo can be so far out.

cheers

Mick
Back Expand Un-Read 0 Moderator

Reply By: Robin Miller - Sunday, Jan 11, 2009 at 08:46

Sunday, Jan 11, 2009 at 08:46
Hi Mick

Is the odometer out as well?

Its illegal to make a car with the speedo error you reported.

The speedo has to not read slow and can legally be up to 6% fast.

I had one Patrol once that was like this , its speedo said 107 when doing 100 on gps and this is actually pretty common.

I actually took mine out of the car and got a new one from the dealer which was also wrong by the same amount.
On mine there was a calibration adjustment (adjustment screw) and I re-calibrated it.







Robin Miller

Member
My Profile  Send Message

AnswerID: 343663

Follow Up By: Mick_J - Sunday, Jan 11, 2009 at 16:12

Sunday, Jan 11, 2009 at 16:12
Hi Robin i will mention it to the dealer next service and see what they say.
When you say you re-calibrated it are you talking about the speedo?

Cheers Mick
0
FollowupID: 611532

Reply By: Road Runner - Sunday, Jan 11, 2009 at 09:31

Sunday, Jan 11, 2009 at 09:31
You probably need to ensure that your vehicle is tracking level and straight when comparing a GPS speed measurement against your speedo. I suspect that the mathematics in the GPS solution measure all acceleration and you are provided with a cumulative effect. I have noticed a difference in GPS reading against speedo (an increase) when following a curved path. This may also apply if climbing or descending at a constant speedo reading. The speedo is simply measuring wheel rotation and converting it to speed regardless of change in direction.

Another factor affecting GPS is the rounding due to inaccuracies of constant position updates. Even when stationary a GPS will show movement in various directions and velocities.

The elevation of the satellites above the horizon will affect accuracy of the GPS. If all the satellites are overhead the altitude solution will be more accurate but if the satellites are closer to the horizon the geographic position is more accurate.

On the subject of speedo accuracy I have a Prado that under-reads about two per cent and a Hilux that over-reads twelve per cent! Both vehicles are less than four years old. I raised the issue of the Hilux during a recent service and was advised that the error was not uncommon. Keeps me out of the radar traps anyway!
AnswerID: 343672

Follow Up By: Bonz (Vic) - Sunday, Jan 11, 2009 at 09:41

Sunday, Jan 11, 2009 at 09:41
Road runner, a GPS only calculates the speed from two known locations set relative to the geoid that the GPS references to. WGS84 uses a ellipsoid which approximates +/- 100m to the geoid, which is basically the mean sea level around the globe.

The GPS knows where you where 1 second ago and knows where you are and then calcs the speed and bearing to make that distance. As the distance is rarely more than 50m, and the satellites are almost a constant with respect to that small distance then the speed is very accurate regardless of the satellite location.

The only thing that would affect the GPS accuracy would be if you were climbing or descending, but again as this is always gradual in a vehicle its only a minor error, in the order of 0.5kmh or so.

You are right about a stationary GPS though, all it knows is where it is, and its internals tell it to approximate the last speed and distance it recorded until it gets a fix again by two known points 1 second apart.

.
Time is an illusion produced by the passage of history
.

Lifetime Member
My Profile  My Blog  My Position  Send Message
Moderator

0
FollowupID: 611493

Follow Up By: Richard Kovac - Sunday, Jan 11, 2009 at 15:37

Sunday, Jan 11, 2009 at 15:37
Bonz

Some time my GPS is so good the Troopy can do over 200 k/hr ..LOL
0
FollowupID: 611526

Follow Up By: Mick_J - Sunday, Jan 11, 2009 at 16:10

Sunday, Jan 11, 2009 at 16:10
Thanks Road runner.
I was travelling on the M5 from King Georges Road to the River road Exit when i noticed the difference in readings so may be what you say is correct.
So what i will do is when next service is due i will mention it to the dealer and see what he says even tho out of warranty.
Thanks for all the feed back.

Cheers
Mick
0
FollowupID: 611531

Follow Up By: Road Runner - Sunday, Jan 11, 2009 at 23:47

Sunday, Jan 11, 2009 at 23:47
Hello Bonz,

You may be right but my theory relates to Newtons law. Firstly a vehicle travelling in a straight line at a constant speed is in equilibrium. If the vehicle is turning there is an acceleration called centripetal force acting on the vehicle towards the centre of the turn circle. This is also a velocity. It is the force you experience when your bodyweight tends to move toward the outside of the turn. So now we have two velocities, the 100 kph measured by the wheels and the unbalanced turning force that is accelerating the vehicle during the turn. So as well as measuring longitudinal velocity, the system must also measure lateral velocity and the cumulative effect is displayed on the GPS. Only my theory and I would appreciate an argument to the contrary.


0
FollowupID: 611646

Follow Up By: Bonz (Vic) - Monday, Jan 12, 2009 at 18:53

Monday, Jan 12, 2009 at 18:53
Hey Road Runner, your GPS is rather dumb when it is reading out your speed, all it knows is your speed and direction over that past second. It doesnt know you are going around a corner or up a hill, just relative locations and it simply calculates your speed and direction based on you going from there to here in one second.

So there is no centripetal forces taken into account, no GPS yaw sensors or anything, it just knows that it knows. Same as a radar gun it just knows how fast you are going. Measured as a velocity. There is no cumulative effect shown on the GPS it only calcs your speed and direction over a second.
.
Time is an illusion produced by the passage of history
.

Lifetime Member
My Profile  My Blog  My Position  Send Message
Moderator

0
FollowupID: 611791

Sponsored Links

Popular Products (14)