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Satellite Navigation 101:

From GPS to Galileo

bridges vol. 29, April 2011 / OpEds & Commentaries

By Norbert Frischauf

A 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

GPS-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

The 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

Satellite-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

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.jpg working_principle_frischauf_gps2_scale.jpg
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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ual Use: From the nuclear triad to the potato chip market

At the peak of the Cold War, "nuclear triad" was the buzzword of the hour. In short, it refers to a nuclear arsenal that consists of three components that are independent of each other. Of all the nuclear powers, only the US and Russia/USSR have maintained a nuclear triad for most of the nuclear age. As such they operate(d) both strategic bombers, land-based ICBMs, and submarine-launched ballistic missiles. In this way, each country would significantly reduce the risk that all its nuclear forces could possibly be destroyed in a first-strike attack - thereby ensuring a credible threat of being able to launch a counterstrike and ultimately increasing their nuclear deterrence.

I assume it is apparent that the costs of a full-fledged nuclear triad are extremely high. Although it offers the best level of deterrence from attack, only the US and the USSR wanted and were initially able to afford such a system. China eventually became the third nation, and by now Israel may have full nuclear triad capabilities as well. India is expected to join the "club" in 2012, when its Arihant-class submarine is likely to be commissioned.

A major cost driver for the nuclear triad is the extent to which a deterrent is supposed to harm the enemy. This is particularly true for submarine-based missiles. Although they increase the chance of survival from a first strike and are therefore the weapon of choice for the second strike, their limited range requires that the submarine move closer to the target, thereby increasing risk of early detection. In aiming to optimize the flight path of an SLBM, and hence extending its range to the maximum possible, it is necessary to obtain an accurate value of the SLBM's launch position. Thus, being able to rely on an operational GPS-like system is a clear force multiplier. Consequently, GPS is a GNSS with a clear military focus and, according to the classical military doctrines, it therefore needs to be counterweighted by a country's own system. GLONASS is the Russian response, while COMPASS is seen as the Chinese answer - in military terms.

To repeat once more: GPS, GLONASS, and COMPASS are all military systems, primarily serving the interests of their country's armed forces. Were it not for the US, the story would most likely end here. Instead, the civilian society of the US enabled the development of civilian applications; although the use of the signal by private entities was not explicitly encouraged, at least it wasn't prohibited. And so clever minds created applications that transformed GPS from purely military into a dual-use system. This transformation was so successful that the GPS market has become the second largest space market (after satellite communications). GPS serves millions of civilian users, and to this end more than 1.4 million handheld and vehicle-mounted GPS receivers have been produced each year since 1997. The rapidly growing GPS market, including equipment and applications, reached US$6.2 billion in 2000 and is forecast to surpass the US$50 billion threshold this year5. To put this in perspective: The GPS market is twice the size of the worldwide potato chip market!6.

GPS is great - but the world needs Galileo

Let's face it: GPS is an excellent GNSS that has set THE worldwide standard. But there is one major flaw: It is - and will remain - under military control. Being operated by the US Department of Defense, there is always the risk that in times of crisis, GPS will be degraded and/or switched off over specific regions if deemed necessary. This is, of course, the sole right of the US government, as it has invested billions of dollars in the design and development and devotes an annual budget of approximately US$750 million to keep the system operational. The money is spent to maintain and control a fleet of 24 satellites, which circle the earth at an inclination of 55° once every 12 hours at an altitude of 20,200 km. This constellation allows users to estimate their position with an accuracy of 10 m in the horizontal and 35 m in the vertical plane. The "selective availability" - an artificial degradation of the navigation signal, leading to an accuracy limitation of approximately 100 m - was switched off in May 2000 by a directive of President Bill Clinton.

Still, the word "availability" hangs like a sword of Damocles over GPS. Besides the fact that usage of GPS can be temporarily denied (some sort of "political availability"), there is also a physical limit to availability of the GPS signals. Twenty-four GPS satellites sounds like a lot, but when one takes into account that the accuracy increases with the number of satellites, that a typical airport runway is "only" 45 m wide, and that this system needs to cover the whole surface of the earth, one can see that physical availability has its limits as well. What the world needs is a civilian system with 24/7 availability and with performance as good as, or preferably better than, GPS.  All this is Galileo!

Galileo: European quality has a price

Bearing in mind that systems like GPS are becoming more and more important for our economy and our daily life, the European decision to build Galileo as an independent, better-performing system under exclusive civilian control was quite logical. In 2003 the final decision was made, when ESA and the European Commission agreed on building Galileo with European industry. Until 2007, €1.5 billion had been invested, while an additional €3.4 billion have been secured until 2013 for finalizing the project. With the recent delays, which now see Galileo delivering its first signal in 2014 and reaching Full Operational Capability (FOC) in 2016, these budget numbers are now being questioned, as is the assessment of the operational costs in the utilization phase: These costs had originally been estimated at €220 million per year. Even if Galileo is to cost €6 billion, which is close to the costs of its US counterpart - don't forget that the US$6.3 billion does not include launch costs, which are included in the costs for Galileo - this is not a shocking cost. Although €4.9 billion or possibly €6 billion seems high at first glance, the Galileo's costs are not that extraordinary when compared with an infrastructure project such as building a highway. Here the costs can reach values like €100 million per km, so a 150 km-long highway between two cities - say Brussels and Rotterdam - can easily require €15 billion.7  If one looks at Germany with its dense autobahn system, one can estimate how much money has been and is continuously being invested in infrastructure projects in Europe.

Once completed, Galileo will comprise 30 satellites (27 operational and 3 spares), circling the earth in three distinct orbital planes with 9+1 satellites each, at an inclination of 56° and an altitude of 23,616 km. Because of the larger number of satellites and more advanced technology (better atomic clocks, dual frequencies, etc.), Galileo aims to provide better availability and accuracy than GPS and will even offer 24/7 service under all but the most extreme circumstances. One search-and-rescue and four navigation services are part of the "Galileo package."

 Advantages of the "Galileo package"
  • Open Service (OS): Combines open signals, free of charge, interoperable with GPS. Position and timing performances will be competitive with other GNSS systems (especially GPS III) - an accuracy of 1 m is envisaged.
  • Safety of Life Service (SoL): Improves OS performance by providing timely warnings to the user when system integrity is hampered. A service guarantee is anticipated. This is supposed to become the key service for aviation navigation.
  • Commercial Service (CS): Provides access to two additional signals to enable a higher data rate throughput and to enable users to improve accuracy to the centimeter level. Signals will be encrypted and a service guarantee is expected.
  • Public Regulated Service (PRS): Provides position and timing to specific users (government agencies, military, etc.) requiring a high continuity of service (also during times of crisis), with controlled access. Two PRS navigation signals with encrypted ranging codes and data will be available.
  • Search and Rescue Service (SAR): Will broadcast globally the alerting messages received from distress-emitting beacons. When an incoming emergency message is received, it will provide feedback to the sender, confirming that help is on the way. The service will contribute to enhancing the performance of the international COSPAS-SARSAT SAR system.

This bundle of four navigation services and one service to support Search and Rescue operations has been designed to cover the widest range of users' needs, including professional users, scientists, mass-market users, safety of life, and public-regulated domains. The major question that now remains is when Galileo will be ready.

Rome wasn't built in a day ...

... and Galileo won't be either. It is a complex program, with numerous parties involved and is designed to provide a plethora of signals to serve all sorts of customers. The current planning foresees Galileo being able to provide the Open Service (OS), the Public Regulated Service (PRS), and th

Galileo … a journey worth following.

e Search and Rescue Service (SAR) as of 2014; and the complete set of signals once Full Operational Capability (FOC) is achieved in 2016. There is, however, still some uncertainty about these dates, mostly due to political and financial issues, which is nothing new for a program of these dimensions. Looking at its US counterpart, one could argue that Galileo's current state is comparable to where GPS was in the mid-1970s: The technical concept was finalized but the constellation was not yet in full operation. A few years later, GPS was a reality and it remains so today, with unrivalled success8.

  Galileo is here to challenge the success of GPS - to give us a truly global navigation satellite system that we can rely on completely. A system that we can use to guide airplanes to the runway, a satellite system that will help ships and cars to navigate, a navigation satellite system to better direct the traffic on the roads, organize the transport of goods, and reduce carbon dioxide emissions, a global navigation satellite system that will be used in Europe, the US, Japan and elsewhere in the world.

Galileo will come with a price tag - that's for sure. But the price tag is comparable to that of other infrastructure projects, except that Galileo is a force multiplier in infrastructure, supporting numerous applications from positioning and navigation, to the synchronization of power, telecom, and financial networks. These networks are too indispensable in today's world to leave them at the mercy of global navigation satellite systems serving primarily military interests. The world is not the same as it was in the ‘70s, when GPS was conceived - and it will not be the same when Galileo becomes fully operational in 2016.

But even if it takes until 2018, Galileo is a GNSS (brand) name to become well acquainted with today, as it will be an integral part of our life in years to come - just as DVB, UMTS, and the World Wide Web are already.

The above article is based on a book chapter written by the author, which will be published in September 2011. The book is called Outer Space: An Ever Growing Issue in Society and Politics (Studies in Space Policy) and can be pre-ordered online via amazon.com.

Norbert Frischauf is a high energy physicist by education and is currently with the Joint Research Center Institute for Energy (JRC-IE) of the European Commission in Petten, Netherlands, where he works as scientific officer at the Gas Hydrogen Storage Test Facility.


1. Prof. Bradford Parkinson, Highlight Lecture: Origins, Surprises and Future of GPS, IAC 2010, 9/28/2010.

2. "Sailing is necessary, while survival is not!" According to Plutarch, Pompeius exclaimed these words when boarding a sailing ship, whose crew was reluctant to leave the harbor because of a strong gale lingering outside on the open sea.

3. Prof. Bradford Parkinson, Highlight Lecture: Origins, Surprises and Future of GPS, IAC 2010, 9/28/2010.

4. As stated on the NavStar Global Positioning System Joint Program Office Web site.

5. http://www.astronautix.com/project/navstar.htm  (accessed October 2010).
6. http://www.potatopro.com/Pr/E-shot/
Savory%20Snacks%20Global%20Industry%20Guide.aspx  (accessed October 2010).

7. http://www.skyscrapercity.com/showthread.php?t=495808  (accessed October 2010).

8. It should be noted at this point that GPS was declared fully operational in April 1995, 18 years after the launch of the first GPS satellite! With slight irony, one can therefore say that Galileo is currently on track to beat that schedule.


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