Can GPS Test Gravity’s Speed of Propagation?

It was good to see Neil Ashby’s article about general relativity and the global positioning system. Let me add a few historical details.

In 1965, I landed a position with Aerospace Corp in El Segundo, California; I had completed a PhD in general relativity some years earlier. Aerospace Corp had become involved in developing what was eventually to become the GPS, and W. Begley of the tracking and radar department asked me to do a study of possible relativistic effects on clocks carried by satellites.

The project was classified, so all I was told was that the military had become very interested in setting up an ultraprecise navigation system. I was happy to help by writing a research report; a brief, unclassified version of it was later submitted for publication. ¹

Many years later, pocket-sized GPS receivers hit the civilian market, and I began to realize the full implications of my research. I could finally tell my wife what I had been up to 30 years before!

It is remarkable that the GPS is presently the only practical application of Einstein’s gravitation theory. I urge that the general public be made more aware of this very useful result of a very abstract physical theory.