GPS and relative motion

GPS – where does the US military choose its origins?

The Earth is often taken as a fixed point of view, without always taking the care that one should about making this explicit.

The GPS location system reinforces this, appearing to give an unequivocal location.

In both cases you ought to be careful, occasionally bringing to students' attention the implicit assumptions behind the rather definite sounding data. In some ways one needs to be even more careful when the GPS unit starts providing out speed and velocity data.

Even if you don't move, while sitting in your chair reading this sentence, the orbital velocity of the Earth will ensure that you're not at the same 'place' by the time that you get to the end. (The rotational velocity of the Earth at the equator is 465 metre inverse second, so from a certain point of view you'll have moved about 1000 metre in the 3 second it took to read.) Then there is the orbital velocity, of the Earth around the Sun.

That's all before considering that you might be reading the sentence on a train, or in a plane.

Teacher Tip: What's the point of view of the GPS system?

You may go further by going more into the system, showing that it relies only on timing and so differences in distance, from which everything else is computed.

This links rather well to the idea of making images by timing, in the SPT: Radiations and radiating topic.