Get device moving speed

I'm trying to get the device's moving speed. I noticed there's a property named speed in Ext.util.Geolocation.

Couple of questions about that property...

i. What's the right way to access it?

ii. Will that get me the information I'm trying to obtain?

iii. If it's value is "NaN", is it because I'm not moving? Or should it return 0?

iv. How come there's no speed property in Ext.device.Geolocation?

Thanks in advance to everyone and excuse my noobiness in this subject.