gps and ground speed
 

I always assumed that gps would give accurate ground speed. Is this the case? I have a new gps in the motor and it shows a -4mph discrepancy with the speedo. New motor (SAAB) so I would hope that the speedo is accurate.
I am interested both from the aviation point of view and the motor. 4 MPH ground speed over 2 hrs flying is not too much of a discrepancy I know but it is interesting.

The speedo is wrong. They are desisgned to overread as this puts you on the safe side. I do not think they are allowed to under read.

A consistent 4mph over read over its range is a bit large. I take your point though.

Max permitted error I believe is 10% (well it used to be anyway) so @ 40mph 4mph would be right on the limit, @ 70mph well within the permitted tolerance.

The permitted tolerance is -0%, +10%. Since the accuracy is affected by rolling radius (tyre wear and air pressure) as well as inherent variations within the tachometer itself, the manufacturers generally aim at the centre of the permitted range, i.e. +5%.

I take it then that the GPS can be taken as accurate. There isn't a variation betweet types. Logic tells me that GPS must ba accurate. Makes me woner how accurate the other prameters are such as MPG as expressed by the on board computer.

Keep in mind also that the speedo knows nothing about hills or turns, which will affect your groundspeed somewhat, especially the former.

Bit of blind faith in GPS on here.
As the GPS position accuracy is at best 35meters and gps speed is relative to a geometric spheroid based on the centre of the earth I would stick with your taco.

You may well find that the GPS will give you some G/S when you are stationary - for "raw" GPS this is normal given the accuracy... the true value is probably somewhere inbetween the GPS and taco values.

Ah that is the thing that was niggling me. It is obvious now I see it.:ugh:

Still the error was consistent.

GPS and ground speed
 

Interesting- I purchased a hand held gps which is stated to be accurate to 0.1 knot,to check ground speed when sailing to take advantage of tides and current flows.Out of interest i thought I would check my nissan's speedo. To my surprise it was reading 5mph over on the car. the car is also fitted with sat -nav but only gives average speed. The average speed calculations for both gps was indentical.I have checked my colleages cars and all vary between 1-5 mph over read which I understand the manufactures adjust to protect liability.

"A consistent 4mph over read over its range" is actually pretty typical.

The EU accuracy spec is as follows:

2.3.7. the speed displayed must never be lower than the actual speed. Between speed V1, read on the speedometer and actual speed V2: there must be the following relationship with the test values specified in item 2.3.5 and between those values:

0 <= (V1 - V2) <= 0,1 · V2 + 4 km/h.

As Sallyann1234 says, manufacturers will always aim for mid range of such a spec, but modern speedometers are remarkably accurate and so cluster quite closely around an overread of about 5 mph.

Really? The Garmin 1000 I use regularly not only accurately records taxiing around the apron, it re-orients as the aircraft turns (track up, so map rotates during turns) while taxiing at walking pace. I'd tech it if it showed map shift while stationary.

GPS over tachometer any day.

There are 2 issues here... The GPS "raw data" and the software processing of that data by the Garmin.
GPS raw data will move around within the error tolerance, caused by inherent errors, receiver quality and signal strength. When stationary it will give +-2-3Kts, and the Track outputs rather random - in short unreliable.
It now comes to the software as to how to handle "smoothing" these errors. "Over smooth" and you get lag and essentially best guess information - as your Garmin is showing. "Under smooth" and you get small shifts, track / GS jumping around. The latter is more accurate if you really want the know what is going on - the former more pleasing if you just like to believe it ;)

GPS will generally give very accurate speed readings, though by definition, it displays an average spped over a short period of time. So it will under-read when increasing speed, and over-read when slowing down. It is also subject to various errors from time to time, which will affect the reading, especially when the signals are weak, or the the geometry of available satellites give rise to significant errors. These errors have become significantly reduced since Selective Availability was switched off in the late 90s.

I have yet to see a car speedometer which does anything but over-read by some 2 - 10%. However car speedometers are usually very consistent at given speeds, over time.

So the best approach could be to calibrate a car speedometer at various speeds when at a steady speed using GPS, then use this knowledge of the error to derive an accurate speed in any situation (much like a deviation card).

Under smoothing & over smoothing
 

Fair enough NigelOnDraft. I bow to your technical knowledge. Perhaps most users don't want to see velocity vectors jumping rapidly in direction and magnitude, it being easier to read time-averaged digital outputs.

True of course, but it's good enough for government work.

Some of you guys need to get out to play more!

I have a Garmin 430 in the aeroplane and a Garmin 296 portable that I use in the aeroplane, the car and the boat.

A few observations about GPS.

1) Steady state GS is very accurate and at 100 kph is spot on with the digital speedo in my car (GMH Commodore SV8) - this has been calibrated against a number of police radars - at some expense !!!!!!!

2) If you have a residual GS while stationary, your GPS is stuffed. I have ever never seen this. When stationary, my GPSs read 0 GS.

3) Although they say GPS is accurate to 35m in my experience it is closer to 1m. I regularly use the 296 to locate underwater structure when fishing. I can navigate to previously marked waypoints to within 1m.

Dr:cool:

My understanding is that GPS errors are (mainly) systematic:

This is why there are augmentation systems (Differential GPS, WAAS) built around the concept of a ground station at a known location providing offset information as to the system error, allowing further calibration of receivers in the area.

This also would explain the ForkTailedDrKiller's observations - the *accuracy* may well be 35m, if comparing the reported position to the actual position. However, the *repeatability* is rather better if all you want to do is hit the same place / waypoint again, rather than finding a spot from a given lat/long.

This would also suggest that the velocity measurement (rate of change of position) is somewhat more accurate than the absolute position.

Interestingly enough, I also used to drive a saab (9-5 with an electronic speedo), and found a pretty consistent 4mph underread accross the range - suggesting that the spedo is pretty accurate, and has a built in factor.

Other cars including older saabs with mechanical speedos tend to show a percentage error.

Being involved in a business that does GPS mapping/survey that guarantees a data accuracy less than 4cm and aims for 2cm whilst moving, I have a little understanding of this issue.

First of not all GPS's are created equal. The gear we use, uses the Russian Glonass as well as U.S. satellite system and is also conected by radio to a base station to give a real-time correction. We consistently achieve 2 cm accuracy. Hand helds, car systems and basic aircraft GPS all vary from 1m to 35m accuracy and worse in some cases.

The point he Mark1234 raises is very valid though. It is more about 'repeatability' rather than variability. Even the 35m accurate systems will probably get you back to the same spot, pending clarity/accuracy of the screen read-out, within a short time period. 2 hours later, next day, change in the weather, satelites in view and position can very quickly see that position change however. What they will not do is reliably take you to a pre-detirmined spot on the earth with any accuracy. Thus the position of a 10m accurate system can vary 10m radius from the given point or up to 20m from a point marked previously.

Given all that I would generally trust the GPS more in a steady state on level ground. Tyre pressure and wear does have an impact on the speedo reading in my car as well. As they wear the difference tends to go from an indicated 2kpk faster than the GPS to 4kph when needing replacement. If you were on a 45degree incline then the GPS would obviously be under-reading drastically.

If you wish, it is easy to use a stopwatch to check your speedo. I once had a car that when I first drove it I seemed to be passing cars all the time even when driving at the limit. A police radar fine a few months later alerted me to the fact it under-read by 12 kph. This was traced back to the wrong drive gear fitted to the speedo gearbox drive. Still had to pay the fine though.

My thought anyway. ;)