GNSS principles and comparison Safaa Dawoud Potsdam University Potsdam, Germany Email: [email protected] Abstract—Over the past years, there has been an increase in calculation, but also increases the accuracy of the receivers’ number of Global Navigation Satellite Systems (GNSSs). Some of position calculation. these GNSS are already in service, but some of them still under The past decade has seen a rapid development of several development. As a result of developing these systems by different countries spread through different continents, users will soon be GNSSs. Some of them are already in service (e.g. GPS by overwhelmed by lists of concepts, features, and idioms, which USA). However, some GNSSs still under planning or partial are announced by different competing systems. operational phase (e.g., Compass by China and Galileo by The purpose of this paper is to review main principle of European Union). Moreover, GLONASS struggled for many any GNSS then to compare different GNSSs including GPS, years until being back to full service lately at 2011. In this GLONASS, Galileo, and Compass. However, in this paper, we focus more on GPS system as it is the only GNSS that is presently paper we overview the principles of GNSS depending on GPS at a full operational capability and widely used in the last ten but also compare the other GNSS systems. Section II starts years. with a brief introduction to GNSS, then it is followed in section III by intensive details about GPS. Sections IV and V study I. INTRODUCTION in detail GLONASS and Galileo systems respectively. Section VI shows the revealed information about other GNSSs that are GNSS is used to determine the position of a receiver under development. Section VII is a comparison between GPS, on land, at sea, or in space by means of constellation of GLONASS, Galileo, and Compass systems. Finally, section multiple artificial satellites. Determining receiver position (i.e. VIII concludes this paper and shows how the growth in the latitude, longitude, and height) relies on calculated distance number of GNSSs could be turned into an advantage. to several satellites. Each satellite continuously broadcasts a navigation message. The receiver uses the received message, II. GNSS CONSTELLATION AND SEGMENTS from satellites in the view, to determine the transit time of GNSS systems should have a constellation with sufficient each message and compute the distance to each satellite. number of satellites launched to ensure that at least four Constellation with a sufficient number of satellites must satellites are simultaneously visible at every site. The GNSS be launched for each GNSS to ensure that at least four satellites have atomic clocks, radio transceivers, computers satellites are simultaneously visible from any point on Earth. and supporting equipments used for operating the system. The satellite is able to stay in a stable orbit due to gravity The signals of each satellite allow the user to measure an force, which is sufficient to accelerate the satellite in its orbit. approximate distance from the receiver to the satellite, which Fuel energy is only used to move the satellite to another is called the pseudorange. The pseudorange is calculated from orbit or to maintain the satellite in its correct orbit in case the signals time of travel from a satellite to the receiver. of deviation caused by external object gravity (i.e. Moons and Since radio signals are traveling at the speed of light c, where Suns gravity). Solar panels are used to power all electronics c = 299; 792; 458 m/s. and radio receivers and transmitters on board. Each satellite pseudorange = (time difference) × c covers a range; each range defines a surface of sphere, where GNSS usually consists of three segments: space segment, the satellite is in the center of this sphere. control segment, and user segment [1]. Typically, three satellites are enough to determine longitude, Space segment consists of many satellites that are placed latitude and height from the three sphere equations. The above the Earth in nearly circular orbital planes. There are satellites have atomic clocks which can be synchronized to three different Orbit altitude, low earth orbit (LEO), medium the level of nanosecond, but this is not the case with current earth orbit (MEO), and geostationary earth orbit (GEO) satel- receivers, as the atomic clocks are expensive (i.e., more lites. The relation between orbit altitude and Earth circulation than $100,000 each). Therefore, receivers’ manufactures use period are fixed. LEO satellite are located at an altitude of inexpensive crystal clocks. However, one µs synchronization under 2; 000 km and circulate the Earth in the range of 95 to error leads to a position error of 300 meters. To overcome 120 min. MEO satellites are located at an altitude of 5; 000 the synchronization problem, a fourth satellite signal will to 12; 000 km and take around 6 h to circulate the Earth. be needed to calculate ∆t. Mathematically, to solve four The altitude of GEO satellites is fixed at altitude of 35; 786 unknowns (i.e., x,y,z and ∆t), there must be four equations. km, in which they exactly match the earth rotation speed (i.e., As a matter of fact, using more satellites not only allows ∆t circulate the earth once in 24 hours) and remain exactly at the GPSBasics u-bloxag 2.1.1 Generating GPS signal transit time 28 satellites inclined at 55° to the equator same point from the earth view. Eachorbit satellite the Earth is equipped every 11 hours with and 58 minutes at a height of 20,180 km on 6 devices that are used for navigationdifferentorbitalplanes(Figure3). or other special tasks. The satellite receives, stores, and processesEach onetransmitted of these satellites information has up to four from a ground control center. Toatomic identify clocks each on board. satellite, Atomic the clocks are currently the most precise instruments satellites have various identificationknown, systems losing (i.e., a maximum the launched of one second sequence number, the orbital positionevery30,000to1,000,000years.Inorderto number, and the system specific name). make them even more accurate, they are regularly adjusted or synchronised from Control segment is responsiblevariouscontrolpointsonEarth.Eachsatellite for controlling the whole system including the deploymenttransmitsitsexactpositionanditspreciseon and maintenance of the boardclocktimetoEarthatafrequencyof system, tracking of the satellites in1575.42MHz.Thesesignalsaretransmitted their orbits and the clock parameters, monitoring of auxiliaryat the data, speed and ofupload light (300,000 of the km/s) and data message to the satellites. Thethereforerequireapprox.67.3mstoreacha control segment is also position on the Earth’s surface located responsible for data encryption anddirectly service below protection the satellite. against The signals unauthorized users. Moreover, trackingrequire stations a further located 3.33 us around for each excess kilometer of travel. If youwish to establish the world coordinate the activitiesyourpositiononland(oratseaorintheair), for controlling and moni- toring the system using bidirectionalall you communication require is an accurate between clock. By these stations and GNSS satellites.comparing the arrival time of the satellite signal with the on board clock time the User segment consists of passive receivers able to decode Figure 3: GPS satellites orbit the Earth on 6 orbital planes moment the signal was emitted, it is Fig. 1. GPS satellites received signals from satellites. Usingpossibletodeterminethetransittimeofthat these receivers is not signal(Figure4). associated with any fees. However, civilians are not allowed to access GNSS military signals. Therefore, besides the special B. GPS Signals and Navigation Message receivers designed for military applications, there is a diversity Satellite and Satellite and of GNSS receivers available on the market today.receiver clock Signals are transmittedreceiver to the clock user segment at frequencies display: 0ms L1 = 1575:42 MHz, anddisplay:L2 67,3ms = 1227:60 MHz. These two 0ms 0ms III. GPS SYSTEM carriers L1 and L2 were selected to ensure that the signal 75ms 25ms 75ms 25ms is robust as possible against ionospheric effects and weather The US Department of Defense (DoD) started to50ms develop a 50ms conditions. They are generated as multiples of the fundamental global positioning system since 1973 for military purpose, the satellites clocks frequency (i.e., f = 10:23 MHz) [4]. The first satellite was launched in 1977. DoD’s NAVSTAR Global 0 satellites transmit L1 and L2 signals to the user, which are Positioning System (GPS) reached the full military operational encoded with information on their clock times and their capacity in 1995. The US congress took actions to make the positions. Both the civilians and USA military can access global positioning service available for civilians in 2000, since L1. However, L2 is only accessible by US government and that time DoD is offering unlimited access of GPS service to Signal military, as it carries encrypted signals in which only the the civilians in all over the world free of charge. military and US government receivers can decoded. There are two types of codes on the carrier signals: Coarse acquisition A. GPS constellation and segments Signal transmition (startcode time) (C/A) code and preciseSignal reception (P) code. (stop The time) C/A code is The space segment of GPS systemconsists of 24 active modulated on the L1 carrier. The L1 carrier can transmit satellites, which are placed in MEOFigure at an 4: Determining altitude ofthe 20,200transit time km the C/A code at 1:023 chips and repeats the code sequence above the earth. The satellites are spaced in orbits to ensure every 1ms, while the required bandwidth is 1 MHz. This that at any point of time there are at minimum 6 satellites chipping rate means that a single chip has a length of 300 in the receiver view. Currently there are 31 operational GPS m and that the entire C/A codes repeat themselves every 300 satellites were launched in the space by the US Air force plus km during transmission [4].