AirToob Lightning
What we take for granted...

The Global Positioning System (GPS) seems to be part of our lives now in all kinds of navigation and positioning applications, from the aircraft we fly in (and Tomahawk cruise missiles) to smartphones and domestic Sat Navs.

I first encountered the GPS in my flight simulator, mirroring what was happening at the time in real aviation. One of the joys of the very realistic sim was learning to navigate from charts and radio beacons. GPS and flight computers took some of the fun out of that, while making aviation safer - although the radio beacons are still there and are still essential.

(Not too many years from now, I fear, much the same might be said about self-driving cars... but that's another story.)

Having only recently acquired a modern smartphone, which packs a GPS receiver (and much more) into a mind-bogglingly small space, I became curious to know more about the GPS system.

I hadn't realised, for example, that GPS satellites follow 6 different orbital paths, with several satellites distributed in each path. Each satellite takes about 12 hours to orbit the Earth once, timed so that it passes over nearly the same locations on Earth every day (as you can see if you keep your eye on a particular satellite in the graphic for two revolutions).

The graphic (from Wikipedia) is visual example of a 24 satellite GPS “constellation” in motion with the earth rotating. It shows how the number of satellites in view from a given point on the earth's surface, in this example from Golden, Colorado, changes with time. (As of February 2016 there were 32 satellites, a few of which are not in use, to improve receiver calculations with redundant measurements.)

A GPS receiver has to be able to receive signals from at least 4 satellites in order to function correctly... but why?

And how do the satellites themselves know exactly where they are in space at any given time as they orbit?

The key to the GPS, it turns out, is time - very accurate time taken from atomic clocks. The GPS receiver in my phone doesn't need an atomic clock itself, but it has to know (among other things) how long the signal from each satellite that it can “see” has taken to reach it. Since this signal is moving at the speed of light, a difference of 1 metre is a difference in signal arrival time of a little over 3 billionths of a second (3 nanoseconds).

The GPS satellites know very accurately where they are at all times because they are tracked from the US Air Force's monitoring stations around the world in the GPS's Control Segment, as described here. As part of this process, the Control Segment updates each satellite with knowledge of how it is moving and with fine time corrections - satellites carry atomic clocks that are synchronised with each other and with atomic clocks on the ground. Why and how both of Einstein's theories of relativity (which have opposite effects on time) are taken account of by the GPS is described in this fascinating article - or else try here.

Satellites that are currently having their orbits changed are marked “unhealthy” so as not to be used by GPS receivers.

How a GPS receiver uses the transmissions from 4 satellites to work out its 3‑dimensional position is described here. The reason that it needs 4 satellites instead of 3 has to do with the fact that a GPS receiver's clock is not synchronised to the satellite clocks, for cost and complexity reasons.

And that's not all, folks... (at least, not for me).

I look at my slim smartphone and wonder: how, with its tiny GPS antenna, and certainly without several parabolic dishes, does my phone receive usable signals from satellites that are at least 12,600 miles away (the shortest red lines on the moving graphic above)?

A GPS satellite is powered by solar panels, generating only a few hundred watts, not all of which is available for transmission. Furthermore, the transmitted signal is not, of course, sent straight to my phone... it spreads out over a large area of the Earth, its power diluted enormously, and a tiny, tiny part of that power falls on my little smartphone's GPS antenna. That antenna must be receiving each satellite's signal as the faintest electronic whisper in a sea of electronic noise.

As Arthur C. Clarke famously wrote: “Any sufficiently advanced technology is indistinguishable from magic.”

Is there also a danger that we become too reliant on the GPS? Following extraordinary political decisions in the USA, the GPS was made fully available for civilian use around the world, with the same precision for civilians as for the military. On the other hand, of course, there are safeguards in the event of hostilities, and occasional disruptions from activity in our sun, particularly Coronal Mass Ejections (CMEs). The ability to read maps and navigate for ourselves might be a skill worth preserving...

It's also good to know that real-world pilots still have to be able to navigate using radio beacons, as well as by dead-reckoning (and navigating by the stars when available) over a large ocean.


If you like this...

[What we take for granted... what would a megabyte (gigabyte, terabyte) look like if we could see individual bits?]