First Fix: Satellites and spacetime

Matteo Luccio

Sitting comfortably in a thin aluminum tube at 35,000 ft, I can continue to communicate via e-mail — and, soon, via video — and write this editorial, while on my way from Portland, Oregon, where I live, to Cleveland, Ohio, where North Coast Media, this magazine’s publisher, is based.

I can safely assume that the pilot knows our position, heading, and speed with great accuracy and receives excellent weather reports. The computer on my wrist (made by the largest manufacturer of GNSS-based consumer devices) and the much more powerful one in the holster on my belt, can do way more than Dick Tracy’s creator, Chester Gould, could have ever imagined a gadget produced by Diet Smith Industries to do.

One thing that communications, navigation, and weather forecasts currently share is reliance on satellites — be they in geostationary Earth orbit (GEO), at 22,000 mi, which are used mostly for weather data, broadcast television and, increasingly, data communication; medium Earth orbit (MEO), at 3,000 mi to 12,000 mi, including GNSS satellites and those that provide Internet connectivity; or low-Earth orbit (LEO), 300 mi to 745 mi, with thousands of satellites in operation today, primarily addressing science, imaging, and low-bandwidth telecommunications needs — and, coming, a new generation of satellite-based positioning, navigation, and timing (PNT) services.

Another thing these feats of engineering share is their foundation on the purest science and mathematics. To take one example, had the designers of GPS failed to adjust the system by 38 ms per day to account for both Albert Einstein’s 1905 Special Theory of Relativity and his 1915 General Theory of Relativity, positional errors would cumulate at a rate of about 6.2 mi each day, making GPS utterly worthless for navigation in a very short time. That’s because Einstein’s 1905 theory leads to the prediction that the atomic clocks on GPS satellites should fall behind clocks on the ground by about 7 ms per day because of their slower ticking rate due to the time dilation effect of their relative motion — while his 1915 theory leads to the prediction that they would be ticking faster than identical clocks on the ground by 45 ms per day due to the curvature of spacetime.

As with most complex technologies, the scientific principles, technical challenges, and policy debates behind GNSS are unknown and irrelevant to more than 99% of the public, few of whom even know that GPS is not the only global navigation satellite system in existence today. The technology is transparent to them. Most of them say “GPS” to refer to GNSS receivers, digital maps, driving directions and traffic data without understanding the separate, though overlapping, technologies, business models and data sources involved. This routinely results in misunderstandings and misattributed complaints and praises — such as when drivers blame “their GPS” (meaning their GPS receiver) for leading them up a dead end that was due to a mapping company being one step behind new construction or praise it for traffic alerts for which they should thank crowd-sourced data and algorithms.

Matteo Luccio | Editor-in-Chief
mluccio@northcoastmedia.net