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Satellite Navigation 101:

From GPS to Galileo

bridges vol. 29, April 2011 / OpEds & Commentaries

By Norbert Frischauf

gps_satellite_usairforce_frischauf_alh_041311_scale.jpgA GPS satellite of the second generation.

Satellite Navigation may not provide glossy pictures, as Earth Observation does, nor can it (yet) produce the commercial profits seen nowadays with satellite telecommunications, but it has one thing the two other applications do not: a name. GPS, Galileo, GLONASS, Compass - all these names represent Global Navigation Satellite Systems (GNSS) and I cannot imagine anyone in the western world whose brain doesn't immediately trigger an association upon hearing the ominous words: "You have reached your destination. The destination is on the left/right side," while stranded in the middle of nowhere with the anticipated destination (obviously) miles away. This however, is another story and has more connection to earth observation data, global information systems, address data, and the fusion of all these different data sources into a clever search algorithm.

GPS, GLONASS and Galileo: "Stealthy" Global Navigation Satellite Systems

ship-compass_scale.jpgGPS-based navigation devices have become part of everyday life.

Despite occasional setbacks, when one navigates a rural area in a car, GNSS has become a commodity you would not want to be without once you've experienced it. Bearing in mind that today GPS serves more than 800 million users(!), the chances are quite good that you - the reader of this article - use a GPS-based navigation device, enjoying the benefits of this technology1. And even if you do not own a personal navigation device, I assure you that you are bound to use GPS in your daily life, because GPS & Co. are all "stealth utilities," enabling many more services than most of us realize. Beside its obvious uses for positioning and navigation, GPS, GLONASS and in the future Galileo, also provide precise timing signals. These timing signals have become key enablers of our society, as they facilitate electronic banking, the handing over of data streams in telecommunication networks, and the switching of power systems. Without this time dimension, the world that we know would cease to exist. From that perspective, GPS et al. have become real infrastructure assets and therefore indispensable for us all.

From today's perspective, one might think of the worldwide usage of the navigation and timing function of GPS - fantastic as it is - as nothing but a logical consequence of the original design. But the truth is that neither the civilian navigation application nor the "stealth" services were envisaged in 1973 when GPS was invented. Conceived as a purely military navigation system, GPS was designed to enable readily available navigation in all places and at all times; everything else emerged "beneath the radar" - to use the military jargon. However, it seems that life beneath the radar can involve some interesting opportunities as, 37 years later, the world depends on navigation signals broadcast by a system of satellites. One has to wonder what ingredients were required to launch this unforeseen success story.

Satellite Navigation: Sailing on rough seas for 2000 years

pin-on-old-navigation-map_scale.jpgThe craving for precise navigation reflects humankind’s longing for orientation and guidance.

The strategic value of precise navigational data for every possible place on earth was clear from the early beginnings of human society and was already acknowledged by the Roman Plutarch (46?-120 AD), who delivered the classical Roman proverb: "Navigare necesse est, vivere non est necesse!"2 Nevertheless, humanity took 2000 years to devise a truly global navigation system - simply because it requires a sophisticated space element.

The start of the first satellite-based navigation system was directly influenced by Sputnik 1, the first satellite. Its famous "beep, beep" transmission, was not only a political signal to the West, but it provided a great stimulus for some clever brains to forecast the future

bus-on-city-street_scale.jpgSatellite-based navigation systems are based on measuring the Doppler shift of a transmitted signal.

orbit of the satellite by measuring the Doppler shift of this transmitted signal. Although this sounds like pure rocket science, it is nothing but a fairly simple extension of a daily observation we all make when we move along a street with passing cars. When one listens carefully to the sound of the engine, one realizes that the frequency changes: When a car comes towards you the sound is higher pitched; it is lower pitched when the car moves away again. The physics behind this phenomenon was discovered in 1842 by Christian Doppler, an Austrian mathematician and physicist. Given the unpopularity of these two subjects among today's students, I assume he would be delighted to hear children intoning the sound of a Formula 1 car as it whizzes by.

Obviously a sound shift will not work in space, but the Doppler effect holds true for any kind of waves and therefore for electromagnetic waves like radio waves as well. By measuring the Doppler shift of Sputnik's beacon, the velocity of the spacecraft along its flight path could be measured. Combining this with orbital dynamics, it was possible to forecast the position of the satellite at the next pass so that those spectators in October 1957 would know where to find the little moving light in a starry sky. Now if you turn the whole system upside down, you have established the foundation of GNSS - assuming that the satellite's position is known and predictable, the Doppler shift of an electromagnetic wave transmitted by the spacecraft can be measured to locate a receiver on earth.

transit_satellite_scale.jpgTRANSIT Satellite

The first to use such a system - conveniently called TRANSIT - was the US and, in particular, the naval forces. Using the system as described above, US submarines were able to acquire a lock on their position with an accuracy between 15 and 500 m. This was sufficiently accurate to permit the launch of a Submarine-Launched Ballistic Missile (SLBM). On the downside, the limited number of satellites (a maximum of 10) and their low altitude of 1100 km restricted the availability of the satellite signals, only allowing for a position fix every few hours. In addition, the time required to fix was not at all comparable to today's GPS, but required 10-16 minutes for the locking procedure. Nonetheless, TRANSIT/NavSat proved to be useful - even the Soviets used NavSat receivers on some of their warships(!) - so the Navy's motivation in designing a better system was naturally not a profound one (to be politically correct). Two thousand years after the risky endeavours of the Roman navy, it seemed that a navigation system was finally in place. But now it was the US counterpart that slowed down the further development into a truly global system with meter-scale performance.

GPS: Head starter against all (military) odds

Luckily, not only the US Navy was in need of accurate navigation systems. The US Army and, in particular, the US Air Force were also interested in such systems. The US Air Force, however, had its own ideas about how such a Global Navigation Satellite System should look. While the navigation issue was of less importance for the Intercontinental Ballistic Missiles (ICBM), the strategic bombers demanded an accurate and available navigation service. A fix every few hours, as offered by the Navy's TRANSIT system, was considered far from satisfactory, simply because the updates were too slow for the high speeds at which the Air Force operated. Consequently, the US Air Force issued its own study on the subject in 1963. This would eventually become "Project 621B," which saw development of a concept with many of the attributes that we see today in GPS.

Still, the US Air Force and the Navy followed separate paths, and were it not for the infamous "Lonely Halls Meeting,"3 which took place in the Pentagon over the 1973 Labor Day weekend, GPS, as we know it, would have never been realized. But over a period of three days, left abandoned in a place usually bursting with activity, 12 military officers discussed creation of the Defense Navigation Satellite System (DNSS), thereby conceiving a system later known as Navstar or  the Global Positioning System - GPS.

The working principle of the system is fairly easy and makes use of only two physical factors: the constant speed of light and the fact that we can measure time differences relatively precisely. The combination of these two things leads to a satellite-based system in which a satellite emits a time-stamped signal. The personal receiver combines this signal with its own time reference, and measures its distance to the satellite by calculations that use the time difference and the signal's speed (the speed of light). By combining the signals of four satellites, the receiver can calculate latitude, longitude, and elevation as well.

working_principle_frischauf_gps1_scale.jpgClick image to enlarge. working_principle_frischauf_gps2_scale.jpgClick image to enlarge.
The working principle of systems like GPS and Galileo builds upon measuring the time difference between emission and reception of a signal sent by a satellite (left image). Dependent on any error of this time measurement, the final position is to a certain degree inaccurate (right image).

Of course, the details of such things are not that easy. In aiming to achieve a better performance than TRANSIT & Co. could offer, it quickly became clear that GPS would need to rely on a bigger space segment with more satellites and certain revolutionary technologies, such as space-qualified atomic clocks. Most of all, GPS would call for significant amounts of money - not millions, but billions of US dollars. Back then, such amounts of money could only be spent by governmental budgets.  The request to spend billions of dollars to allow for the necessary research, development, deployment, and operation of a complex constellation of navigation satellites could be only justified by the need to mitigate a risk of such gravity that it would endanger the very existence of the US  - such as the Cold War arms race.

In the end, it was the nuclear threat to the US that convinced the US Congress to invest in GPS. In the period from 1973 to 2002, US$6.3 billion - excluding military equipment and launch costs - were spent on Navstar-GPS4. The operational costs amounted to approximately US$750 million per year. For this amount of money, the US obtained a system that acted as force multiplier to its nuclear deterrent - and the world got its first, and so far its only, truly operational Global Navigation Satellite System.

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