Satellite Navigation

P. F. LammertsmaInstitute of Information and Computing Sciences

Utrecht University

February 2, 2005

Abstract

Satellites already play a significant role in our daily lives, aiding communication, explorationand research. In the future, we will undoubtedly see their influence grow. GPS is perhapsone of satellite’s most successful applications, and for consumers, receivers are becoming evermore affordable and reliable. The recent signing of a cooperative agreement between the UnitedStates and the European Union will expand this system, laying the foundation for a compatibleand interoperable Global Navigation Satellite System, the GNSS. With this relatively youngtechnology, improved accuracy, better reception and altogether new applications lie in wait forus in the near future.

CONTENTS 1

Contents

1 Introduction 2

2 History 2

3 TRANSIT 3

3.1 Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.3 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3.4 Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.5 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 GPS 5

4.1 Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.3 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.4 Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.4.1 Ideal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.4.2 Inaccuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.5 Broadcast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.6 Reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.7 Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.7.1 Header . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.7.2 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.8 Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Galileo 12

5.1 Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.3 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

6 Conclusion 15

2 HISTORY 2

1 Introduction

“I’m astounded by people who wantto ‘know’ the universe when it’s hardenough to find your way around Chi-natown.”

- Woody Allen

Using today’s state of the art global navi-gation satellite systems (GNSS), you can pin-point your location anywhere on earth with anaccuracy of less than fifteen meters. Currently,the only system available to the general pub-lic is the American Global Positioning System,which has been fully functional since mid-1994.

The upcoming European competitor,Galileo, promises to improve accuracy. Thiswould in fact make it possible to find yourway through a city, solely based on the datagathered by your receiver from space. Perhapssomeday GNSS will find extremely usefulapplications, such as replacing seeing-eye dogsand guiding motor vehicles.

This article discusses the historical signifi-cance of the TRANSIT and GPS systems, in-cluding their technical specifications and work-ings. Then it will briefly cover Galileo, a newsatellite positioning system. The article willconclude by explaining the advantages of thissystem and why we need yet another satellitenavigation system.

2 History

The history of satellite navigation goes back tothe era of the ‘space race’. With the launch ofSputnik I in 1957, the Russians had to keepthe Doppler effect in mind [5]:

To maintain radio contact with amoving object, you have to keepchanging your frequency.

The Monitor Station would search over a cer-tain frequency range until it could acquire alock on Sputnik ’s signal. By calculating thefrequency shift, they could determine its veloc-ity relative to the station. Consequently, theycould determine its position in orbit.

In fact, they expressly chose a frequencywhich was audible on a normal transistor ra-dio. Listening to it, you would clearly hearthe Doppler effect, unmistakable proof thatthe Russians had launched the first man-madesatellite into orbit.

This led to the development of the TRAN-SIT system, which was immediately followedby several other projects such as Nova andTimation. These were primarily experimentsthat eventually led to the research of GPS in1969.

Russia has also played its part in satellitenavigation, but constantly was one step be-hind. They followed the American’s TRAN-SIT with the Russian Tsikada, and answeredGPS with the exclusively military GLONASSsystem.1

1Russia no longer keeps GLONASS under mainte-nance, and now only 8 of the 10 satellites are still func-tional.

3 TRANSIT 3

3 TRANSIT

In 1959, the American Navy discovered thebenefits of inverting the Doppler shift con-cept, thereby starting the development of theTRANSIT system, initially known as the NavyNavigation Satellite System [3].

If you know the exact position of thesatellite, you can determine your rel-ative position to it.

3.1 Configuration

The TRANSIT system is configured in sucha way that the six satellites in orbit providemaximum coverage. To do so, scientists putthe satellites in uniform orbital precession, insix separate polar orbits.

• Six satellites

• Six polar orbitsAltitude: 960 kmPeriod2: 106 minutesInclination3: 90◦

• Three ground-based Monitor Stations

3.2 Requirements

For a successful positioning measurement, theTRANSIT system specifications state thatonly one satellite is required. A position can becalculated as soon as the satellite passes over-head.

TRANSIT could guarantee a successful mea-surement within 110 minutes at the equator, aslong as a satellite was in range of the receiver.4

The greater the latitude, the more satellites

2The period of a satellite is the time it takes to com-plete one orbit.

3The inclination of an orbit is relative to the equator.4This is directly related to the orbital period.

Figure 1: TRANSIT configuration

become visible. For example, at 80◦ latitude,the average fix time was only 30 minutes.

Presently, as the system is no longer main-tained, the satellites have lost their uniformorbital precession, this guarantee no longer ap-plies.

3.3 Concept

The theory of the relationship between thesatellite and receiver is described in the fol-lowing five steps.

1. A satellite sends its exact position andtime over frequency f0.

2. A receiver, expecting to receive signalsfrom the satellite on frequency f0, searchesfor the signal over a certain frequencyrange above f0.

3. If the signal can be found on a certainfrequency f , the receiver will continue totrack this frequency as it continues todrop.

4. When f0 = f , then the satellite is some-where overhead.

3 TRANSIT 4

5. At this point, a calculation can be per-formed, and the receiver can stop listen-ing.

3.4 Positioning

Figure 2: TRANSIT positioning

As mentioned, we know that the satelliteis neither approaching nor departing (i.e. thesatellite’s relative velocity is zero) when f0 =f . This can be proven with the Doppler fre-quency expression in equation 1, where f is theobserved frequency, f0 the source frequency, vs

the source velocity, and c the speed of light.[14]

f =f0

√1 − v2

sc2

1 − vsc

(1)

When vs = 0, the frequencies must evidentlybe equal.

f = f0

With the information from the signal carriedover the frequency, the receiver knows the ex-act position and time of the satellite at the mo-ment that it passed overhead (figure 1). There-fore, the receiver must be located somewhereon a line perpendicular to the orbit of the satel-lite.

Although it may seem that we don’t knowthe distance from the orbit, we can calculatethis using a Doppler shift expression. How-ever, this equation is rather complex and goesbeyond the scope of this article.

What we don’t know, is to which side of theorbit the receiver is located, because on the lineperpendicular to the satellite’s orbit, we onlyknow the distance to the intersection. Thismeans that still two possible locations remain,shown in figure 2.

3.5 Disadvantages

Not only do the calculations only narrow thereceiver’s position down to two possible loca-tions, they’re based on it being at sea level. Foranything but maritime expeditions, this wouldrender the system useless unless the altitude isknown. Other disadvantages include bad cov-erage, poor accuracy and the requirement thatthe receiver physically has to wait until a satel-lite passes overhead.

The TRANSIT system was abandoned in1996, due to the great success of GPS.

4 GPS 5

4 GPS

Before the first TRANSIT satellite even wentinto orbit, the US Department of Defense al-ready had something much bigger in mind:the Navigation Satellite Timing and RangingGlobal Positioning System (NAVSTAR GPS),known to most as GPS. Development, however,only started in 1973.

Just like TRANSIT, GPS satellites send thetime and position in a signal carried over agiven frequency. However, the similarity stopsthere. GPS receivers now only use the fre-quency as means of obtaining the signal that’scarried over it. The Doppler shift is no longerrelevant to the positioning, and the receiversimply transposes the observed frequency tothe source frequency. This is done to ensurethat the carried signal is decoded at the cor-rect bitrate.5

Since 1994, GPS is fully functional with all24 satellites in orbit. The United States hadplanned on reaching this stage by the late ’80s,but due to several delays – amongst the Chal-lenger Space Shuttle disaster in 1986, the dead-line could not be met.

4.1 Configuration

Scientists developed a configuration for theGPS system that provided global coverage us-ing at least 21 satellites in a medium earth or-bit (MEO).

• 21 active satellites3 spare satellites

• Six orbital planesAltitude: 20,200 kmPeriod: 11 hours 58 minutesInclination: 53◦

• Four satellites per plane

5Explained in section 4.5 on page 8.

• Five Monitor Stations

Figure 3: GPS configuration [9]

Initially, researchers contemplated a geosta-tionary earth orbit (GEO) configuration at36,000 km. This was discarded, however, be-cause the satellites would require a strongertransmitter and a more powerful launch vehi-cle. More importantly, a GEO would providepoor coverage of polar regions.

Figure 4: GPS Monitor Stations [7]

Instead, the preliminary test configuration –Block I – specified that the planes be inclinedat 63◦. Being the first generation of GPS satel-lites, the ten satellites successfully launched

4 GPS 6

Figure 5: Coverage diagram of GPS satellites

into orbit from 1978 to 1985 are designated asthe GPS/Block I satellites.

The 24 current GPS satellites are in theBlock II configuration, and were launched be-tween 1989 and 1994. This configuration spec-ifies that the six planes are inclined at about55◦. Evenly spaced at 60◦ longitude, this incli-nation provides best global coverage, includingthe polar regions, as shown in figure 3.

These satellites are in fact divided into fourgenerations: II, IIA, IIR and IIF. The primarydifferences have to do with accuracy and max-imum number of days without contact frommonitoring and control stations.

The Monitor Stations upload new, correcteddata to each satellite every four hours. Thisdata includes a corrections on the exact timeand position of that and other GPS satellitesin orbit. An update of the satellite’s positioncan be determined by performing a GPS mea-surement to a ground antenna of which theexact location is known. The Monitor Sta-tions are located near the equator to reducethe ionospheric effects (figure 4).

4.2 Requirements

The GPS system specifies that a measure-ment requires data from four different satel-lites. This means that at any point in time,at least four satellites must be in range of thereceiver (figure 5).

If all four of these separate signals can be re-ceived, GPS guarantees a successful measure-ment within 36 seconds.6

Each GPS satellite must know the exacttime, with an accuracy of at least 10 nanosec-onds. Each satellite is therefore equipped withtwo rubidium and two cesium atomic clocks.

4.3 Concept

The basic relationship between the satelliteand receiver is described in the following fivesteps, which will be explained in greater detailin sections 4.4 through 4.7.

1. A receiver receives a signal from a GPSsatellite.

2. It determines the difference between thecurrent time and the time submitted overthe frequency.

6Explained in section 4.7.1 on page 10.

4 GPS 7

3. It calculates the distance of the satellitefrom the receiver, knowing that the signalwas sent at the speed of light.

4. The receiver receives a signal from anothertwo satellites, and again calculates the dis-tance from them.

5. Knowing its distance from three known lo-cations, the receiver triangulates its posi-tion.

4.4 Positioning

4.4.1 Ideal

In figure 6a you can see that by calculating thedistance d0 to satellite A, the receiver can placeitself on a sphere with radius d0 from A. Wecan then continue to determine the radius d1

from satellite B. These two spheres must touchor intersect if the measurement was successful.

If the spheres merely touch, which is highlyunlikely, we can already determine our posi-tion. However, if they intersect, we must besomewhere on a circle where any point of thecircle is d0 from A and d1 from B, as shown infigure 6b.

Finally, figure 6c shows us how a third mea-surement will put the receiver d2 from satel-lite C. This narrows our position on the circledown to one position (if the circle and spheretouch) or two (if they intersect).

One of these positions can be disqualified,because the location or velocity is virtually im-possible. For example, it could put us at analtitude above the satellites.

4.4.2 Inaccuracy

Since a GPS completely relies on correct tim-ing to make a successful measurement, bothreceiver and satellite must know the time veryprecisely. While the satellites are equippedwith four atomic clocks that are updated every

Figure 6: Ideal triangulation by GPS

four hours, the receiver only has a simple clock,no better than a cheap digital watch.

To explain this, we’ll simplify things to twodimensions. An ideal triangulation would thenonly need two satellites (assuming we can stilldisqualify one location). However, becausewe’re using an inaccurate clock, we need anextra measurement. The ideal situation withthree measurements is shown in figure 7.

In reality, these three circles don’t align per-fectly at all. The receiver cannot keep the ex-act time as precisely as the satellites, so we’llhave to make the circles align by hand.

We can do this, because we know that thecircles are supposed to align. If the circles aretoo large, we adjust our clock by moving it for-ward in time until the circles are small enoughto intersect in one point (figure 8). If the circlesare too small, we move our clock backwards.

In essence, the receiver’s clock doesn’t haveto know the exact time; it only has to deter-mine the travel time of each satellite’s signalrelative to each other. Since it only takes a sig-

4 GPS 8

Figure 7: Ideal 2D positioning

nal 63 to 70 milliseconds to reach the receiver,it’s still possible that the clock loses accuracyduring the measurements, which results in thatthe four spheres still don’t align properly.

This means that there’s not a single exactposition where the receiver can be. Instead, itis a certain area, called the pseudo range.

In some cases, it may be that only one spheremisaligns. Receivers are equipped with an al-gorithm that makes an educated guess withinthe pseudo range.

4.5 Broadcast

Now let’s take a look at the actual broadcast.We already know that the actual frequency isnot important; it is only used for carrying thesignal. This signal contains all the vital infor-mation to the proper functioning of the GPSsystem.

There are actually two frequencies that arebroadcasted. Both are in the microwave range(i.e. above 1,000 MHz), and are so identifiedby the prefix L.

The developers decided to use the rubidiumatomic clock to set the frequencies. The clock

Figure 8: Pseudo-range reduction in 2D

has a nominal frequency f0 of 10.23 MHz [6],which is used internally by the satellite as thefundamental frequency, and can simply be mul-tiplied to reach the microwave range.

The first frequency, L1, is 1575.42 MHz,which is derived from the fundamental fre-quency f0. It carries a signal for both civiland military receivers.

154 · f0 = 1575.42 MHz

The second frequency, L2, is 1227.6 MHz,and is used only by the military.

120 · f0 = 1227.6 MHz

Within the satellite, a data signal is createdthat contains the Navigation Message. Thismessage contains all the information requiredby the receiver.7 The signal is made at 50 bitsper second (50 Hz).

The satellite must also be able to identifyitself over the frequency. To do so, a PseudoRandom Noise code, or PRN-code is created.This is a signal that seems to be pure noise,

7Explained in section 4.7 on page 10

4 GPS 9

but in fact is a sequence of n bits that repeatsafter the nth bit. Each satellite is assigned oneof the 32 unique PRN-codes.

There are two different kinds of PRN-codes:one called the coarse acquisition code, or C/A-code, used for civil receivers, and anothercalled the precise code, or P-code for militaryreceivers.

The C/A-code is 10n − 1 bits long, where nis the number of digital shifting elements thedevice contains. In GPS satellites, this is 10,so the code is 1023 bits long. Subsequently, itis sent at a rate of 1.023 megabits per second,which is a tenth of the fundamental frequency:

f0

10= 1.023 MHz

At this frequency, it takes precisely 1 ms tosend the 1023 bit C/A code.

The P-code is sent exactly at the fundamen-tal frequency f0. Because the frequency is 10times as high, the data is 10 times as accu-rate. The P-codes are not publicly known, andtherefore cannot be used by the general public.

For security reasons, the P-code is not asshort as the C/A code. It repeats preciselyonce every seven days, therefore making itpractically impossible to find, unless you knowwhat you’re looking for.

Finally, the PRN-code is combined with theNavigation Message by a modulo-2 adder, thenmixed with the frequency, which will carry itto the receiver. This process is illustrated infigure 9.

4.6 Reception

For the receiver to revert the frequency to theNavigation Message, it replicates both the fre-quency on the L-band as the 32 PRN-codes forevery possible satellite. Subtracting the purefrequency from the received signal, the PRN-code combined with the Navigation Messageremains.

Figure 9: The PRN-code and Navigation Mes-sage are carried over the L-band [10]

Figure 10: Acquiring full PRN correlation [10]

Once again, exact timing is essential to re-ceive the message correctly. As the receivergenerates the matching PRN-code, it doesn’talign properly. The receiver tries to make thesignals line up, acquiring full PRN correlation8, correcting its clock to do so (figure 10). Oncealigned, it will remove the PRN- code from thesignal to recover the Navigation Message.

The chosen unique PRN-code used to decodethe Navigation Message identifies which satel-lite sent the broadcast. The Navigation Mes-

8In this context, correlation is the so-called ‘code-phase lock’ on a signal. Only when two identical signalsare aligned is the signal strength high enough to bedetected (called the correlation peak).

4 GPS 10

sage, which contains the time, ephemeris9 andother data, can now be analyzed.

4.7 Signal

The Navigation Message is a continuously re-peating frame of 1500 bits, split up in 5 sub-frames. Each sub-frame, being 300 bits andsent at 50 Hz, takes precisely 6 seconds to send(see figure 11), and contains a 60 bit headerand a 240 bit data block.

Figure 11: Removing correlation to line up thesignals [13]

4.7.1 Header

The header is subsequently split up in twowords10.

The first is the Telemetry Word (TLM). Ledby an 8 bit preamble, this is the first stage ofrecognition for the receiver. Followed by 16bits of reserved data, which matches the ending6 bit checksum, or parity.

A receiver would search for the preamble,which is defined as 10001011. This marks thebeginning of a new sub-frame. To confirm this,

9An ephemeris is a table giving the coordinates ofthe satellite at a number of specific times during a givenperiod. In the Navigation Message, only the currentcoordinates are sent.

10In GPS, words are 30 bits long.

the receiver gathers the reserved data, createsa parity, and checks to see if this correspondswith the last 6 bits of the TLM. If this doesn’tmatch, the receiver backtracks to look for thenext preamble.

The second part of the header is the Han-dover Word (HOW). Spanning the first 17 bitsof the HOW is the time of week (TOW). Thisis the first bit of significant data the receivercan use.

Although we have already synchronized ourclock correctly, we have not discussed the dif-ference between the way time is kept by satel-lites and how it is kept here on earth. TheGPS systems do not take leap seconds into ac-count, while this is a requirement. Therefore,the Navigation Message tells the receiver howto update its clock to ‘correct’ CoordinatedUniversal Time, UTC.

The next 7 bits contain general sub-framedata. This consists of the sub-frame ID (thenumber of the sub-frame, 1 through 5), a re-served alert flag and the Anti-Spoofing flag11.The alert flag notifies the receiver that thesatellite may be giving an inaccurate measure-ment. 12

Before the receiver can store any data, it hasto double-check that it is actually reading theheader, and not some series of bits that so hap-pens to resemble it. It does this by makinganother parity, and checking it with the last6 bits of the HOW. If this does not match, itbacktracks to look for the next preamble, fol-lowed by the rest of the TLM, etc.

Each sub-frame has a separate header for theprimary reason that a receiver can step into themiddle of a broadcast, and not have to wait up

11Anti-Spoofing is a special mode the GPS systemcan be put in, in which the P-code is replaced by ahighly encrypted Y-code. This only affects the P-codemilitary receivers, which then require a special crypto-graphic key to function at all.

12This can happen, for example, when the satellite’sorbit or rotation is being adjusted.

4 GPS 11

Figure 12: Deviation of GPS measurements with and without Selective Availability [2]

to 30 seconds for the next cycle. This way, afull data frame can be downloaded in less than36 seconds, as long as the signal persists.

For example, if a receiver starts listening di-rectly after the header of sub-frame 3 was sent,he will have to wait until sub-frame 4’s headeris sent to start downloading, which could takeup to six seconds. It will take an additionalthirty seconds to download the full frame, to-taling 36 seconds in the worst-case scenario.

4.7.2 Data

While the two header word types are identicalin each of the five sub-frames, the enclosed datais entirely different.

• Sub-frame 1 contains the exact time gath-ered from the four on-board atomic clocks.

• Sub-frame 2 and 3 are grouped, and con-tain satellite ephemeris data.

• Sub-frame 4 and 5 are grouped, and splitup in 25 separate pages, which can begathered over 12.5 minutes. This data is

pretty much only of interest to the Mon-itoring Stations, and isn’t covered in thisarticle.13

Using the data from sub-frames 1 through3, a receiver can pinpoint his relative positionto a particular satellite, because he knows (1)the time elapsed to send the signal and (2) theposition of the satellite at the time the broad-cast was sent. As explained in section 4.4.2,repeating this process for three more satelliteswill put the receiver in a pseudo range.

4.8 Disadvantages

In reality, GPS coverage is relatively poor.This is especially the case in cities, in denseforests and other places where there are manylarge obstructions in the receiver’s horizon.This is because the microwave frequencies thatGPS broadcasts on is very sensitive. Signalsmay bounce, or be blocked entirely; both ofthese effects obviously have negative effects toa successful measurement.

13Some more expensive receivers make use of thisdata to update their ephemeris almanac of all satellites,as this data is enclosed in sub-frame 4 and 5.

5 GALILEO 12

When a signal bounces (off a building, forexample) and is read by a receiver, a miscalcu-lation occurs known as a multipath error. Sincethe signal didn’t travel in a straight line fromthe satellite, the receiver will expect to be fur-ther away from it than it actually is.

From a civil point of view, another disad-vantage of GPS is Selective Availability (SA),used by the military to reduce accuracy overa certain region. When enabled, SA increasesthe C/A code’s inaccuracy by adding randomnoise.

As GPS becomes integrated in more andmore systems, perhaps the most important dis-advantage is reliability. Because GPS is runby the U.S. Department of Defense and is theonly open service satellite navigation system,there is no guarantee that the service will al-ways be available. The United States govern-ment could, for example, decide to block civilGPS altogether.

5 Galileo

A European global navigation satellite systemwas proposed in the early 1990s, an initia-tive to boost the economic advantages alreadyproven by the GPS system. Disadvantages ofGPS had come in to play, such as the long-lasting reduced accuracy over southern Italyresulting from Selective Availability during theBosnia crisis.

Until the final agreement to start the Galileoproject, the EU was not in a favorable positionwith respect to the current GNSS systems, be-ing a consumer in the background instead of anactive partner. From a civil point of view, ac-curacy was very limited and the whole systemwas in military hands. Therefore, civil avia-tion is still restricted to land-based navigationbeacons.

In 2004, the first stage of development wascompleted and deployment started in 2005.Satellite contracts have been awarded amongstthe Galileo Industries, a consortium of severalEuropean aerospace companies, therefore man-ufacturing the satellites will progress rapidly.The relatively lightweight 625 kg satellites willbe able to be deployed in clusters of two toeight satellites per launch vehicle, thereby re-ducing expenses and deployment time.

5 GALILEO 13

Figure 13: Galileo satellite configuration [12]

If Galileo continues to keep on schedule, thesystem should be fully functional by 2008.

5.1 Configuration

Galileo’s configuration is slightly different thanGPS, the leading difference being that it usesonly three planes instead of six.[4]

• 27 active satellites3 spare satellites

• Three orbital planesAltitude: 23,616 kmPeriod: 14 hours 4 minutesInclination: 56◦

• 10 satellites per plane

This configuration describes three planes inmedium earth orbit. The satellites in MEOare at such a steep inclination to improve cov-erage at higher latitudes, unfortunately reduc-ing coverage at the equator. The next stepof development would allow the installation ofeight geostationary navigation satellites. Al-though this doesn’t have the highest priorityat the moment, GEO satellites will certainly

be included later, to compensate for this re-duced accuracy. (See figure 13.)

Totalling 38 new satellites, this more thandoubles the current 32 GNSS satellites in orbit.

5.2 Requirements

While GPS only provides two services – onepublic and one private – Galileo will providefive:

1. Open ServiceFree of user charge, this service is avail-able to anybody with a GNSS receiver. Itprovides simple positioning and timing.Accuracy: 5 to 10 meters14

2. Safety of Life ServiceThis addition to the Open Service pro-vides authentication of the signal, im-proved integrity, and timely warnings ofany problems.Accuracy: 5 to 10 meters

3. Commercial ServiceSubscribed users can decode two addi-

14If your receiver only supports single-frequencies,the accuracy is 15 to 20 meters.

5 GALILEO 14

tional signal to improve accuracy.Accuracy: less than 1 meter

4. Public Regulated ServiceFor subscribed users requiring high con-tinuity, an additional two signals can bedecoded.15

Accuracy: 4 to 6 meters

5. Search and Rescue ServiceThe satellite listens for beacons broadcast-ing a distress signal, and globally broad-casts its position and message.

Galileo satellites broadcast over a widerspectrum of frequencies than GPS satellites,not only to improve reception, but also to in-corporate the numerous services listed above.The system will send over the L1 and L2 dis-cussed in section 4.5 on 8 and on another fre-quency, L5 (1176.45 MHz).

The Open Service is free of charge, so thereis no liability on the part of the system op-erator when the service is disrupted. How-ever, concession-holders will charge for otherservices that offer a guarantee of continuity ofservice. As stated in the anticipated modusoperandi, Galileo will be liable for damages inthe case of failure.[8] This legal jargon would,for example, finally allow civil aircraft to makefull use of satellite navigation. This, in its turn,will lead to a new era in civil air transporta-tion.

Equally important is the economical aspect.The estimated cost being between 3 and 3.5 bil-lion euros, Galileo will cost no more than lay-ing 150 kilometers of highway. Subscriptionsalone will repay this in a matter of years, but

15This service will also be integrated into the 112-network. For example, a user in distress calling 112with his GNSS-receiver will submit his current location,and emergency response teams equipped with receiverswith the Public Regulated Service will be able to trackhim in ‘real-time’.

the primary reward is the growth of the GrossNational Product.

According to the CBA EC, Europe can ex-pect a benefit of 62 billion euros in total fromeconomic benefits and 12 billion euros in totalfrom social benefits. Including approximately2.5 billion euros in operation costs, the 74 bil-lion euro profit significantly outweighs the 6billion euro expenses within twenty years.[11]

Additionally, Galileo will be interoperableand compatible with GPS and GLONASS. Be-cause more than twice as many satellites willbe available from which to take a position, evenin places that would normally obscure signalsfrom GPS satellites low on the horizon will re-ceive coverage.[1]

Although Galileo can be used in combinationwith the existing GNSS satellites, it will beself-sufficient and independent. This makes itnot only a back-up to GPS, but also a complete,alternative system.

5.3 Concept

The relationship between receiver and satelliteis identical to GPS, described in section 4.3 onpage 6.

The European Space Agency put a lot of ef-fort in designing a state of the art satellite foruse in the Galileo system. Their design, whichthis year will go into production, has severaladvantages:

• Increased power output with Lithium-Ionbatteries

• Lightweight and compact

• Laser retro-reflector allows ‘pinging’ fromearth by laser

• Upgradeability with extra payloads

• Communication amongst satellites byInter-Satellite Link (ISL)

6 CONCLUSION 15

• Can be injected directly into the correctorbit by the launcher

• Launchers can accommodate two to eightsatellite vehicles

5.4 Applications

The scientists and politicians moving Galileoforward are very enthusiastic about the newEuropean civil GNSS. In 2008, we can ex-pect our cell phones to have built-in GNSS re-ceivers, allowing us to use them to find ourexact position not only worldwide, but eveninside a building. When making a 112 distresscall, our telephones will automatically divulgeour exact position to help emergency servicesfind us.

Someday GNSS might be steering our carsor guiding the blind. Until then, we’ll cer-tainly see more reliable receivers in aircraft,shipping and road transportation for naviga-tion and search and rescue.

6 Conclusion

GPS is perhaps one of satellite’s most success-ful applications so far, and for consumers, re-ceivers are becoming ever more affordable andreliable. Now over 10 years old, the systemwas revolutionary in its day and its popularityis still growing.

However, being under military control, thereis no guarantee that the service will alwaysbe available. In 2008, the ESA’s civil Galileosystem will be fully functional and will grantconsumers accuracy equal to that of the mil-itary. Anybody who owns a GNSS receiverwill be able to make free use of Galileos openservice. Commercial users will be allowed topurchase subscriptions to two additional ser-vices, which provide extra accuracy and conti-nuity. Aircraft will be granted a special servicethat provides authentication of the signal, im-proved integrity and timely warnings of prob-lems. Finally, a search and rescue service willhelp broadcast distress signals, wherever theymay originate from.

In the 45 years of satellite navigation, GNSShas proven its economic, technological andpractical value. Certainly new innovations willcontinue to show the benefits of this continu-ously expanding field.

REFERENCES 16

References

[1] European Space Agency. Why Europe needs Galileo.http://www.esa.int/esaNA/GGG0H750NDC index 0.html.

[2] Stuart at Random Useless Info. GPS Stuff.http://www.randomuseless.info/gps/.

[3] Robert J. Danchik and L. Lee Pryor. The legacy of transit. Johns Hopkins APL TechnicalDigest, 11(1,2), 1990http://www.globalsecurity.org/space/systems/transit.htm.

[4] European Commission Directorate-General Energy and Transport. Galileo: EuropeanSatellite Navigation System.http://europa.eu.int/comm/dgs/energy transport/galileo/index en.htm.

[5] William H. Guier and George C. Weiffenbach. The Early Days of Sputnik.http://sd-www.jhuapl.edu/Transit/sputnik.html.

[6] PerkinElmer Inc. Rubidium Frequency Standard Model RFS-IIF.http://optoelectronics.perkinelmer.com/content/Datasheets/rfs2f.pdf.

[7] US Army Space Institute. Army Space Reference Text.http://fas.org/spp/military/docops/army/ref text/.

[8] Laurence Nardon. Galileo and GPS: Cooperation or Competition?http://www.brookings.edu/fp/cusf/analysis/nardon.pdf.

[9] University of California Berkeley. Earth Sciences & Map Library.http://www.lib.berkeley.edu/EART/.

[10] Delft University of Technology. Eurofix: PRN codes.http://www.eurofix.tudelft.nl/prncode.htm.

[11] Hans Herman of the European Space Agency. Galileo: The European Initiative in SatelliteNavigation.http://tcmc.tugraz.at/PDF/tcmc2001/pdf/1 1/Fromm.pdf.

[12] Mobilecomms Technology. Galileo Satellite Radio Navigation System.http://www.mobilecomms-technology.com/projects/galileo/.

[13] Department of Geography University of Colorado Boulder. Global Positioning SystemOverview.http://www.colorado.edu/geography/gcraft/notes/gps/gps.html.

[14] Steve M. Yionoulis. The transit satellite geodesy program. Johns Hopkins APL TechnicalDigest, 19(1):36–38, 1998http://techdigest.jhuapl.edu/td1901/yionoulis.pdf.