Recently purchased and fitted an Origin B2 GPS detector which has a nifty digi speedo.

The digi speedo is reading well under the cars speedo eg 92 (ar speedo) - 87 (GPS) ...KmH of course.

Can anyone vouch for the accuracy of the GPS speedo. I don't want to rely on it too much as I am now travelling quicker since I bought it.

 

don,t rely on it.... i use a Navman icn 520 and there is usually about 7mph difference from my speedo

mick

 

This was discussed here:

http://bbs.scoobynet.co.uk/showthread.php?t=466988

The GPS should be very accurate and if it isn't it is for reasons of money saving on the part of the manufacturer rather than because of a problem with the system.

 

The speedo is usually a few percent fast, anything up to 10% high is normal.

The GPS unit is pretty accurate at a steady speed on a level road.

If you rely on the speedo you'll have a 'cushion' between the indicated speed and the actual speed, so if you think you are doing 75 (speedo) you are probably really doing 70.

If you rely on the GPS you'll have no cushion - what you see if pretty much what you get. All very well but may need to pay closer attention since there is less leeway.

 

this is exactly what I'm saying, now that I've got an extra 5mph to play with......

I think I will turn off the digi speedo, it's the safest option

 

I always stick to the GPS speedo, as said, the car speedo is usually a little on the safe side, VW is reading 80, when GPS is 70, my daily workhorse Transit van is reading 83 when the GPS is 70!!!! I use the GPS through speed cameras and not been flashed so far........(touches a piece of wood)

 

I've found my GPS system to be accurate and as said, it reads approx 5/8% slower than the car speedo. As the law requires the car speedo to read faster than your actual speed.

Its error percentage fast or slow can be determined by what size tyres and rims you have, if different from original and errors from different manufacture's.

You can check the tyre/rim size percentage error at this website :-

http://www.tyresave.co.uk/tyresize.html

 

In answer to your question regarding speed acuracy, very is the answer......

 

quote:

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on.




It also depends on how many satelites are being used, like my Road Angle is using 4 out of five for its accuracy.

 

Most GPS systems that i'm awair of will only lock with a minimum of 3 satalite's. Road Angle is no more accurate than any other good quality GPS based system, inc a PDA with TomTom installed......

 

You can do it with 2 sats, called Transit mode, "Transit" was the first name for GPS back in the early 60s before it changed to NavStar as it is now known and Transit worked off 5 sats in polar orbit but you only used 2 at any one time. Some modern GPS units still have this function, but you wont get speed calcs off 2 sats.

 

I don't know very much about GPS's but I do remember seeing somewhere that you have to be careful with the speeds showing on them when you're going up or down a steep incline as you'll be doing more distance on the road than the satelite can 'see' as it essentially sees a flat surface.

I could be wrong though.

 

Always be aware that the GPS speed display has a short delay, so when you accelerate the reading has to "catch up"
As an aside to the above, I recently did the exchange deal with Origin for my old blue i unit (which after less than 4 yrs of occsional use, was literally dying - bad quality!) I have found that if I accelerate fairly quickly I get a warning from the unit! I've e-mailed Origin about this, but so far no reply (no surprise there!) Anyone else found this? Also - beware of the "Demo mode" I tried it and the damn thing just stayed in it! I followed the book instructions to exit but no chance, it was stuck in that mode! I had to phone Origin for an alternative way of escape, which still took 4 attempts - you have to be VERY quick on the buttons!
JohnD

 

Good point! That sounds perfectly logical? You need to know the angle of the incline and a bit of trigonometry!
JohnD

 

That is very true, all calcs done by the unit assume the earth is a perfect sphere.

 

my ipaq lost the signal last night and i was apparently doing 326mph!!!!!!

thankfully i looked down and was only doing 70....

dont rely on it

dazza

 

The GPS will be more accurate than your speedo, most speedo's are inacurate and as they MUST not under-read because of manufacturing regulation so the inaccuracy is taken care of by over-reading and this gets worse the faster you go..

Dont be fooled though as silvilian GPS is not perfect, its a damn sight better than your speedo in most conditions but still not 100% accurate.

This is not true, most GPS units rely on a given elipsoide to carry out their calculations, this accounts for the im-perfect spherical shape of the earth..

Now if you were to drive up a very steep incline you would see an error in speed calculations, it would have to be BLOODY steep though.

I just so happen to work in the industry

 

the ellipsoid correction might account for the non roundness of the earth, but not hills.

if you are going up a 1 in 10 hill, i would expect the gps to be 10% out.
true??

 

Its not that simple..

Depends on the angle of the receiver, if its angled then it will "see" different satelites for reference.

Dont forget the receiver knows its position on 3 axis not just 2.

For example: If your 50,000 ft in the air and you move 10,000 meters around the earth you will have actually travelled a shorter distances than you would if you were 50ft in the air.. The GPS reciever knows its altitude and can account for this.

As for travelling up a steep incline it depends on what equations are being carried out, if changes in altitude and position are taken into account then you will have an accurate recording.

 

According to the Smithsonian GPS works like this. It is calculating a 3D location, so this stuff about spheres etc. is a load of ***** Irespective of the slope of the ground the satellite calculation will give true ground speed so long as the time intervals are sufficiently short, e.g. if you circulate a roundabout at the same interval as the satellite calculation then it will return a speed of 0mph, while your actual ground speed could be much higher. I don't know what the time intervals are but I suspect that they are sufficiently frequent to eliminate this problem.

How do they do this? They must know the slope of the road, then carry out a complex calculation to convert a 3D (true) distance to a 2D projected distance and hence a speed which is inaccurate. Why bother? Just report the 3D speed which is true ground speed irrespective of hills etc.

 

Heres some good reading for you:

http://www.warnercnr.colostate.edu/c...ds_geoids.html

There are 23+ different ellipsoid models that are used depending on where you are located on the plannet.

 

That's relevant to mapping and distance calculations to a destination, obviously there will be discrepancies between a 2D map and travelling over a hill, still don't see why its necessary for speed calculation?

 

They are used to convert the position data into something that makes sense to the surface of the earth i.e. Lat/Long's..

Imagine if you were in a cube (X,Y & Z axis) and there was a deformed footbal floating in the middle and loads of little ball bearings circling the football, you could work out where you were relation to 5 of the ball bearings that were in a line of sight but you still then need to work out what part of the ball you are sat on to get an accurate altitude and position.

Its not 100% neccessary but it makes things more accurate as Latitude & Longitude gives an exact position on the globe regardless of the varying deformations of the earth so you can make a much better calculation of speed.

 

Agree with your second paragraph.

Third paragraph is wrong. Knowing two positions in space and calculating the speed from the time taken to travel between them (where the time is very short) is precise. Calculating long / lat, spheroids / ellipsoids, hills, etc. can only make things less accurate.

 

It uses doppler shift, not a time distance equation. Read my previous posts.

 

Your refering to straight line distance.. which is very inaccurate.

Straight line distances on 2D and 3D models where the journey may involve devation is not accurate.

 

This will give you an idea of how much detail is included in the output from the GPS receiver.

http://home.pacific.net.au/~gnb/gps/nmea.html

Its down to the connected device to use this information how it see's fit, I am only commenting on the more accurate methods of distance/time calculations.

 

It doesn't use distance time calcs, at least not on any commonly used GPS units. I am aware that the GPS nav unit in a Hercules aircraft uses distance time but I don't know of any others that do. Even the Garmin link you sent expains this as it calcs 3D velocity:

http://home.pacific.net.au/~gnb/gps/nmea.html#pgrmv

Once it has 3D velocity it can produce a value for absolute speed if you are going left, right, straight up or round a roundabout, the DSP doesn't care.

If it used distance/time then the this situation would become complex (It isn't so complex in an aircraft if you sit and think about it) and in the days of SA the result would have been utter rubbish as your position appeared to jump about randomly at very high speed. Even today your position isn't totally static and yet your GPS will show your speed as 0mph if you stand still. This is precisely because it is using doppler rather than distance/time to establish your 3D velocity and from this the DSP in the GPS is producing a figure for your speed which is a scalar value.

 

I think I'm going off topic, i'm more relating to projects that I have worked on that have involved very accurate distance/time calculations regardless of alitude and land mass.

 

Hi m8, I had the very same problem.

I am going to assume that you have not 'hard wired' the unit and have it powered from the cigarette lighter?

I spoke to Origin on Monday and apparently it is common fault with scoobs, the problem is that the power feed is 'dirty' ie it has a lot of interference. When you stamp on the gas hard it sends a power surge to the unit which registers as a false laser alarm.

You have 2 options:

1.) Either hard wire the unit professionally.

or

2.) Phone Origin tech support and they will send you a 'gadget' which is basically a small tube magnet which you wrap the power cable around.

This worked for me and it was free, I'm glad I sorted it as the unit almost went through the window the other day on a 100 mile drive.