Speedometer is 10 mph off

[hugo151]

Last year I took my Speedo apart cos it was making a weird buzzing noise & jumping around, after oiling the internals( which cured it), I put it back together & found I'd foolishly put the needle back without zeroing it & now it's 13mph fast, I'm gonna leave it cos it's quite a difficult operation ,even thou it's annoying. Maybe you can do the same but do it properly.

[hillsy]

Just to play devils' advocate here - how do you know your GPS is correct?

[posplayr]

Just to play devils' advocate here - how do you know your GPS is correct?

Because gps in much more accurate than a gs speedo.

[hillsy]

Because gps in much more accurate than a gs speedo.

I would agree, however, how do you know?

It's funny how we just accept the GPS must be right?

[posplayr]

I would agree, however, how do you know?

It's funny how we just accept the GPS must be right?

Because a GPS receiver has several "Figures of Merit" that can be used to qualify the receiver performance above and beyond "Built-in Test" capabilities. The overall GPS system includes both Space and Ground segment which operate in a closed-loop fashion to provide the required Satelite pseudo-range data and ephemeris corrections. In a nutshell the system can check itself and if it is down you will not get a display from your GPS or you will be warned of inaccuracy.

If you consider the complexity of the system, your same questions could just as easily be asked of the cell telephone system. How do I know I'm getting an accurate voice from the person I'm calling? In fact, the voices you hear over a cell phone are more than likely to be much more in error (pitch) that a GPS. This is a qualitative comparison, but as I said the first time a GPS is so accurate compared to a GS speedo, that if it is out they will not report.


GPS doppler based velocity is about 0.25 m/sec or 0.56 mph RMS