# Positioning through Satellites (GPS)

A GNSS calculates the location of fixed and moving objects anywhere in the world by means of precise timing and geometric triangulation (see Figure 1-1). GNSS is composed of a constellation of satellites that send radio signals.

A combination of personalised radio signals, which are encoded with the precise time they left the satellite, allows a ground receiver to determine its position through geometrical triangulation (Figure 1-1). Satellites are equipped with high-precision atomic clocks enabling them to measure time accurately. Receivers hold information regarding the position of any satellite at any given time. Thus, a precise position can be calculated by timing how long the signals take to reach the receiver from the satellites in view. By reading the incoming signal, the receiver can recognise a particular satellite, determine the time taken by the signal to arrive, and therefore calculate the distance between itself and the orbiting satellite.

Figure 1-1 Satellite triangulation for positioning

A ground receiver should theoretically be able to calculate its three-dimensional position (latitude, longitude, and altitude) by triangulating the data from three satellites simultaneously. However, a fourth satellite is necessary to address a "timing offset" that occurs between the clock in a receiver and those in satellites. The more satellites there are, the greater the accuracy is. As satellites are synchronised with Coordinated Universal Time (UTC), they provide precise time.

Functioning around the clock, GNSS satellites provide accurate three-dimensional positioning to anyone with appropriate radio reception and processing equipment. Although the coverage provided by a GNSS is "global," its availability and precision varies according to local conditions. Signals tend to be weaker over the poles and in low-lying urban areas surrounded by buildings.

## GNSS Systems

Nowadays, there are only two systems that provide global coverage: the U.S. Navstar GPS and the Russian GLONASS. Although the American system is fully operational, the Russian programme is only partially available due to the decaying constellation of its satellites, owing in part to financial constraints stemming from the collapse of the Soviet Union. Both systems began as military applications and continue to be funded and operated by their respective departments of defence. Nevertheless, both systems were made available to the civil population, although they remain under total military control and are less precise than the original systems. The European Galileo is set to become the third GNSS provider, as it is planned to be operational in 2012. Galileo will offer total interoperability with GPS and GLONASS. The spectrum for the three systems is shown in Figure 1-2.

The current GPS system is based on 24 satellites circling the earth every 12 hours, located in six orbital planes at a height of 20200km (see Figure 1-3). Each satellite sends UTC and navigation data using the spread spectrum code-division multiple access (CDMA) technique.

Figure 1-2 GNSS spectrum, GPS, Galileo, and GLONASS

Figure 1-3 Satellite constellation

A receiver can calculate its own position and speed by correlating the signal delays from any four satellites and combining the result with orbit-correction data sent by the satellites. Currently, two services are provided by GPS: a precise positioning service (P-code), which is mainly restricted to military use; and a standard positioning service (C/A-code), which is less precise than the P-code but available to the public. All 24 satellites transmit signal L1, which carries the C/A-code and the P-code, and signal L2, which carries the P-code. The characteristics of the L1 and L2 signals are shown in Table 1-1. Interference between signals of different satellites is avoided by using pseudorandom signals with low cross-correlation for code division multiple access (CDMA) modulation[ARINC06].

TABLE 1-1 GPS signal characteristics

 Signal Modulation Central frequency Bandwidth L1 QPSK 1575.42MHz ~20MHz (C/A Code 2MHz + P-Code 20MHz) L2 BPSK or QPSK 1227.6MHz ~20MHz (P-Code 20MHz or P-Code + C/A Code)
 TABLE 1-2 GLONASS signal characteristics Signal Modulation Frequency L1 BPSK 1602MHz + n0.5625MHz L2 BPSK 1246MHz + n0.4375MHz

The GLONASS system, like the GPS, consists of 24 satellites placed in three orbital planes at 19100km. Each satellite orbits the Earth approximately every 11 hours and 15 minutes. Two services are offered: standard accuracy (SA), designed to be used by civilians worldwide; and high accuracy (HA), used only by authorisation of the Russian Ministry of Defence. Both signals sent by GLONASS, the characteristics of which are summarized in Table 1-2 [GLONASS02], have frequency division multiple access (FDMA) technology in the L-band for both SA L1 and HA L2.

Similarly, the Galileo system will consist of 30 satellites (27 operational, 3 in reserve), positioned in three circular Medium Earth Orbit (MEO) planes at 23616km above the Earth and inclined at 56° to the equator for planet coverage. As in the GPS system, a receiver will be able to calculate its own position and speed by correlating the signal delays from any four Galileo satellites and combine the result with orbit correction data sent by satellites. Four services will be provided by Galileo: Open Service (OS) (available to everyone); Safety of Life (SoL), Commercial (CS) and Public Regulated (PRS). All these services will be provided by a complex signal structure, which includes as many as 10 signal components. The E1 and E5A-B signals are designated for Open Service. Their characteristics are summarised in Table 1-3[Guenter02].

The GNSS architecture typically consists of three subsystems: a satellite constellation (space segment), a ground segment (control and monitoring ground stations), and end-user mobile receivers. These subsystems can be enhanced through space- or ground-based augmentation [HP AN1272].

## Commercial Applications

Over the last few years, the United States and the European Union have been in a race to launch new versions of GNSS, GPS, and Galileo. For the United States, it will be its second generation of GPS, as U.S. Commerce Department secretary announced last January, "the second generation has been born with a commercial focus as it has a second channel for civilian use."

TABLE 1-3 Galileo signal characteristics

 Signal Modulation Central frequency Bandwidth E1 BOC(1,1) 1575.42MHz ~24MHz E5A-B Alt-BOC(15,10) 1191.795MHz ~51MHz

This means an increase in accuracy and reliability. Some companies such as General Motors, IBM, Lucent Technologies, and Trimble Navigation have already shown interest. On the other hand, the EU, as well as its partners such as China and India, among others, is involved in the launching of Galileo. In December 2005, the first Galileo satellite, Giove-A, was put into orbit from the Baikonur Cosmodrome in Kazakhstan. At the same time, another important achievement was made when ESA, Europe, and their partners signed an agreement, pledging €950 million to carry out the second phase of the system. This phase consists of the validation of the project, the addition of four satellites, and the establishment of the Galileo ground network. The third phase, which will see the rest of the Galileo satellites put into orbit, is expected to cost around €3.6 billion.

To understand the interest behind those millionaire investments, we need to take a harder look at the possibilities offered by GNSS.

### GNSS Applications

Apart from military applications, GNSS offers a multitude of commercial opportunities. The growth of the transport sector, the skyrocketing evolution of telecommunications, and the development of services requiring precise positioning capabilities – such as rescue services – reinforce the promise of GNSS as an invaluable multiple-use technology (see Figure 1-4).

Signal transmissions are an integral component of aviation, shipping, telecommunications, and computer networks, to name just a few applications in which they are used. Positioning plays an important role in these fields due to its ability to enhance economic efficiency. For example, in aviation, savings may be obtained through more direct flights (attained through improved traffic management), more efficient ground control, improved use of airspace capacity, and fewer flight delays. GPS is already an important tool for in-flight safety, assisting in such aspects as en route navigation, airport approach, landing, and ground guidance. It is estimated that Galileo’s economic benefits to European aviation and shipping sectors will reach €15 billion in 2020[EU-Galileo].

Many industries will benefit from advantages offered by GNSS, such as defence, aeronautics, and mining. Similar effects on the mass market, motor vehicles, and surveying will be explained in detail in the following section.

### Mass Market

People are starting to discover the realm of recreational possibilities offered by GNSS. Experts predict that more than 40 million potential users in Europe will use GNSS for recreational purposes such as sport fishing, sea navigation, and hiking. As with any mass market, demand elasticity is a key factor, as is price. Nowadays a basic receiver costs around €100, yet consumers will soon demand retail prices of less than half that cost. GNSS manufacturers, therefore, have no choice but to decrease receiver cost and size.

Figure 1-4 Applications for GNSS

On the other hand, mandatory services such as the European E112 and American E911 will force telephone providers to pinpoint the location of their users from any call down to a 100m radius. Thus, mobile phones will have to include a GNSS receiver. Taking into account the 860 million users of mobile telephones in September 2002 and the prediction of over 2 billion users by 2020, sales for GNSS mobile phone receivers alone will be huge. The U.S. market is currently gearing up for this change thanks to companies such as Qualcomm or Motorola, which are offering GPS-equipped mobile phones.

GNSS will also prove to be an invaluable medical and social tool when it comes to locating Alzheimer’s patients and the blind, among others. Moreover, GNSS already plays an important role in emergency services such as search and rescue, disaster relief and environmental monitoring. Current emergency beacons operate within the Cospas-Sarsat satellite system. However, with no real-time service guarantees and inaccurate estimates (provided in kilometres), there is room for improvement.

### Motor Vehicles

The motor vehicle market is continuously expanding. It is estimated that more than 670 million cars, 33 million buses and trucks, and more than 200 million motorbikes and light vehicles will be on the streets by 2010. Moreover, by 2020 at least 450 million vehicles will be fitted with GNSS. Thanks to their low cost, GNSS devices will become standard features even in mid-to-low-priced cars. Furthermore, most of the installed devices will be dual systems that work with GPS and Galileo simultaneously. This will increase receiver accuracy and introduce new features, including collision prevention, emergency service notification of airbag activation, or the location of stolen vehicles. This market is expected to be worth €25 billion by 2016.

On the other hand, according to DGTREN, the social and economic costs of road accidents and fatalities amount to 1.5 to 2.5 percent of the European Gross Domestic Product (GDP). Road congestion adds additional costs equivalent to 2 percent of European GDP. The use of high-precision GNSS devices could lower these social costs by increasing road safety, reducing travel time, and minimising road congestion. More efficient use of fuels may also have positive effects on the environment. Additional road applications presently gaining attention include in-car navigation, fleet management of taxis, and driver assistance.

Surveying A huge increase in surveying-based applications is expected, namely in the trucking and shipping industries, especially if the price for surveying systems drops. This can be achieved by reducing the cost of system electronics.

Sales Estimates Sales estimates for upcoming years have been thoroughly studied by market survey consultants in[DGTREN03]. Figure 1-5 shows the expected profits generated by GNSS hardware over the next few years, and Figure 1-6 shows annual earnings in the navigation and location system market, including hardware. The growth in the next decade of GNSS-related markets can be seen. It presents a great opportunity for those able to offer technological solutions to market needs.

Figure 1-5 GNSS hardware profits [DGTREN03]

Figure 1-6 Navigation and location system market profits [DGTREN03]

Figure 1-7 shows which industries made up the GNSS business market in 2001 and the market forecast for 2015. The consumer and motor vehicle markets will see the highest growth. The former went from being almost nonexistent to very significant. Location and surveying markets will experience more steady growth than the first two.

GNSS systems will undoubtedly play an important role in the world economy, specifically in regard to services offered and products sold (see Figure 1-8), intensifying the interest of Europe and its partners in having their own system.

## System Limitations and Vulnerabilities

Despite military and commercial advantages, GNSS has its limitations. There are three frequently documented weaknesses. First, positioning signals tend to be less precise in urban environments or under foliage, in areas where the number of satellites in sight are low (typically at upper and lower latitudes around the poles) and under certain weather conditions such as thick clouds. Transmission strength also affects GNSS precision. A more powerful and less distorted signal could increase precision significantly. To address this, ground or space-based augmentation such as additional ground stations can be used to improve precision in localised areas.

Figure 1-7 Market share for GNSS applications [DGTREN03]

Figure 1-8 Market share for GNSS applications [DGTREN03]

In addition, GNSS services may suffer from intermittent service coverage. Given the limited lifespan of the space component, the system needs to be replaced and/or reconfigured periodically. For example, during certain upgrading operations, receivers relying on information from ground stations or satellites under maintenance may be affected. Even if service suffers setbacks of only a couple of seconds or minutes, the impact may be significant for many applications, such as air traffic control.

Finally, as a vital component for a growing number of commercial and military applications, global navigation and positioning systems may be vulnerable to hostile parties. For example, a ground station may be physically attacked or taken over, resulting in any number of consequences to service, or parts of the system can be electronically jammed. In the distant future, these threats may also affect the space sector, resulting in potentially severe consequences.

The greater the dependence on the system is, the more serious the economic consequences of system failure or shutdown could be. In addition, any system failure could prove to have direct consequences on sectors (such as aviation) requiring continual and precise signals[ISS02], [EU-Galileo].

To provide enhanced services, receivers will have to be able to obtain specific information through signals sent by satellites of both GPS and Galileo systems.

To meet this need, highly integrated low-cost GPS/Galileo receivers will be required. The interoperability of both systems will offer a number of very important advantages. Moreover, to ensure low price and reliability, developers will design receivers with the lowest possible number of external components, low power consumption, and smaller size, and use low-cost technology to fabricate the devices.

As the majority of satellite navigation applications are currently based on GPS, great technological effort is being spent to integrate satellite-derived information with a number of other techniques in order to obtain better positioning precision with improved reliability.

This scenario will significantly change in the near future since the GNSS infrastructure will double in size with the introduction of Galileo. The availability of two or more constellations, more than doubling the total number of available satellites in the sky, will enhance service quality, increasing the number of potential users and applications.

Galileo-specific characteristics will include significant enhancements. First, for urban areas or indoor applications, the design of Galileo signals will improve service availability by broadcasting dataless ranging channels, in addition to the classical pseudorandom ranging codes. Second, the high-end professional market will also benefit from the characteristics of Galileo signals, which will lead to centimetre-sensitive accuracy over large regions[EU-Galileo].

A comparison between Galileo and the current GPS system is helpful in providing a better understanding of the needs of the European GNSS system. According to the Directorate-General for Energy and Transport within the European Commission (EC), it is crucial for Europe to have an option independent of the current U.S.-GPS monopoly, which is less advanced, less efficient, and less reliable. As stated by the Commission, the specific drawbacks of GPS are identified as:

■ Mediocre and varying position accuracy Depending on the time and place, GPS accuracy is sometimes given within "several dozen metres." From a European perspective, this inaccuracy is blatantly insufficient, particularly within the transportation sector. With its better precision, Galileo is set to fill this gap.

■ Questionable geographic reliability In northern regions that are frequently used as aviation routes, GPS provides limited coverage. This also affects the coverage accuracy in northern Europe, which includes several EU member states. In addition, Galileo would boost overall urban coverage from the current rate of 50 percent (provided by GPS alone) to 95 percent.

■ Questionable signal reliability With GNSS services playing a significant role in society, there is concern about the possibility of service shutdown. If the GPS system became dysfunctional or was turned off (accidentally or not), it has been conservatively estimated that the cost to European economies would be between €130 and €500 million per day.

A GPS/Galileo receiver offers a range of new services not currently available. The interoperability between GPS and Galileo will present new possibilities beyond the realm of imagination. Including Galileo technology in the receiver would not only improve the accuracy down to the centimetre and maintain current services, it would also improve the following:

■ Data integrity This opens a broad range of applications for different products, especially for those where data integrity is critical, such as user authentication. Other examples include, but are not limited to, security applications, the surveillance and transporting of dangerous or sensitive goods, and railway transport security.

■ Emergency management Galileo technology can help avoid sudden accidents or speed up emergency assistance vehicles when required.

■ Rescue services Galileo technology could locate specific boats, planes, and vehicles after an accident, especially in the wake of natural disasters.

■ Data confidentiality Applications depending on data confidentiality will be possible because of information encryption’s compatibility with Galileo.

■ Advanced assistance Galileo is capable of using an auto-pilot feature for remote control of motor vehicles.

Galileo is a civil service, which ensures constant signal availability, while GPS could be stopped at any time for any military emergency, which would result in economic losses. Galileo improves information accuracy and continuity as well as service availability. It is especially useful for locating users in hostile environments such as geographically rugged or highly developed urban locations.

Finally, it is worthwhile to point out that to carry out the aforementioned developments, it will be technologically necessary to introduce new, small, low-cost, highly autonomous dual receivers (GPS/Galileo).

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