Interferance

[RichForrest]

Looking at the track log for the 7 across ride yesterday and came across this
Would the BFO mast on the nearby hill cause these problems or is it just as it was sat under the garage canopy?

[delthebike]

Stationary, doesn't need to be under anything. I used to get it when I left my GPS running during stops in cafes, I put it down to satellite and planet wobble.

[Zoidburg]

The GPS switching from one set of satellite triangulations to the other as the earth revolves?

The grids arent going to be perfectly meshed.

[RichForrest]

I usually leave it on at cafes/controls also just so it records the whole ride, never had that much movement before though.
Mind you it was on top of a big hill so may have picked up more satellites.
 

[rogerzilla]

I rode past Membury with the CTC yesterday!

[FatBloke]

I rode past Membury with the CTC yesterday!

Not all of the CTC!   

[delthebike]

I rode past Membury with the CTC yesterday!

Not all of the CTC!   

After May and the AGM it will be! 

[andygates]

Wait, you were under the canopy and need to ask? 

[delthebike]

Wait, you were under the canopy and need to ask? 

Too near to the services food (in some loose definition of food type stuff) hall! 

[RichForrest]

Wait, you were under the canopy and need to ask? 

Yes, but I'm always sat under the canopies on audax rides, it's part of the experience 
It's never happened before so just seemed odd.

[Jaded]

My 705 is good when moving - very accurate indeed, you can even see which way you went round a roundabout. However when it is not moving, it does what you have shown - jumps about. I think it must be to do with the algorithm that processes the GPS signals. It doesn't cope with stationary.

[Greenbank]

GPS "accuracy" is a load of shite. I've had it report accuracy to "4m" and been 50m off a known position. Elevation differences are even worse.

Stick the GPS on a windowsill for an afternoon and then look at the resulting GPX file, both in terms of distance traveled and "climbing".

GPSes are quite good to give you a pretty good idea of where you are, especially if you are moving. If you're static then it'll wander. If you're under a canopy of a petrol station then expect what you've seen.

[frankly frankie]

Canopy.  The satellite transmitters are only about 50W, and even the closest ones are over 20,000km away.

If it's an 'H' type GPS it will pick up and use reflected signals, which will generate false positions (but still better than nothing, the theory goes).  If a signal takes an extra 50 nanoseconds to arrive, the GPS will think its about 15m further away from that particular satellite, than it really is.  50ns is about the limit of accuracy of the leisure GPS system.  For comparison, the Chilean earthquake changed the length of a day by about 1260ns.

The shape of the constellation changes surprisingly rapidly so the relative strengths of the reflected signals may also change, if a different set of 3 or 4 is prioritised the resulting calculation (which involves bogglingly large and ridiculously small numbers) will give a different result.