GPS Satellite Constellation and Signal Structure INTRODUCTION a The Global Positioning System (GPS) i a world wide satelite-based navigation , Pe ie i . jation system. GPS provides hi: Sretone of objects then wclocty ard Give data. Twas eral tleaied bo mtited eae ee 4980s, the US government made the system available for civilian use. GPS consists ofthe following three seqmonts - we rents: Space segment = Control segment User segment The spoce segment consists of a nominal constellation of 24 orbiting satellites. These satellites transmit one-way signals that give the current GPS satellite position and time. ‘Each GPS satellite orbits at an altitude of 20,200 kilometres above the earth. The control segment consists of worldwide monitor and control stations that manta Sect thei orbits. It tracks the GPS satellites, uploads the updated navigational data, crt mien fe satelites in their proper satellite constellation. s health and status of the The user segment consists of the GPS receiver equipment, which receives the si f ves the ; Each satelite transmits signals on two frequencies, LI (1575.42 MHz) and L2 (1227.60 Miz), which can be detected by receivers on the ground.GPS Sotellite Consteliation and ‘Signo Structure S8) ‘he satellites are positioned in six earth-centered orbital planes with four satellites in each plane. The orbits are arranged so that atleast six satellites are always within line of sight from almost everywhere on the conte surface. GPS satellite signals are generated using a process known as Direct. Sequence Spread Spectrum (DSSS) modula- bon. GFS satellites are equipped with four extremely stable atomic clocks. The atomic clocs used or of rabidioms (R) ‘and cesium (Cs). The stability of the atomic clock is up to an order of 2 parts in 10%, ot one second in 1,400,000 years. The fur damental frequency of 10.23 MHZ is produced from the resonant frequency of one of these onboard! clocks The camer frequencies (L1 and 12), data pulse frequency, C/A (Coarse/Acquistion) and P (Precision) codes are all derived from this nominal reference frequency. Since oll the GPS satelite trnsmit on the same frequencies (575.82 ‘Mile ond 1227.46 Mhz); a process known as a Code Division Multiple Access (CDMA) is used. Ihe (7A code plays an inpertant role in the multiplexing and modulation. It isa constantly repeated sequence of 1023 bits known as a Pseudo Random Noise (PRN) code. This code is unique to each satellite and serves as its identifying signature. The principle of operation of GPS is described in Chapter 1. In this chapter, the GPS satellite constellation, signal generation, DSSS modulation, C/A and P code generations, and navigation message details are described in’ deteil for the convenience of the reader, fundamentals of the GPS signal processing such as introduction to Fourier trang, forms, autocorrelation and cross-correlation of the GPS signals are also described in detail, —— ee 3.1. Gps sysTEM SEGMENTS —_—_ The Global Positioning System is comprised of three segments: satellite constellation, ground control/ monitoring nenwork and user-receiving equipment, The formal terms for these components are space, Sperational control and user equipment segments respectively (Fig. 3.1) [1]. * The satettite constellation contains the satellites in orbit that provide the ranging signals and data messages to the user equipment. * The Operational Control Segment (OCS) tracks and maintains the satellites in space. The OCS monitors the satellite's health and signal integrity and maintains the orbital configuration of the Satellites. Furthermore, the OCS updates the satellite clock corrections and ephemerides as well as humerous other parameters essential for determining user Position, Velocity. and Time (PVT), {Fs R User segment Control segment FIG'34) GPS sogmonteGlobal Navigation Sotellite Systems 70 + Lastly, the user-receiver equipment performs the navigation, timing or other related functions (c surveying). The following subsections describe the details of the three GPS system segments 3.1.1. Space Segment GPS satellite constellation design consists of 24 satellites (Fig. 3.2a). The first Block 1 GPS satellite was Iaunched in 1978 and the first Block II satellite was launched in 1989 (Figure 3.2b). Block I Satellite Vehicle (SV) Numbers I to 12 were launched in between the year 1978-1985. Block II SV Numbers 13 to 21 were launched in between the years 1989-90 and Block IIA SV numbers 22-40 were Jaunched in between 1990. 1996. Block I and Block II satellites have become nonoperational now. The operational GPS satellites are designated as BLOCK I, BLOCK HA, BLOCK IIR and BLOCK UR-M. GPS satellites are placed in six orbits and each orbit has four satellites. The angle made by cach orbit with the equator is 55°. This angle is referred to as an inclination angle. The orbits are separated by 60” to cover the complete 360° (see Fig. 3.3) The radius of a GPS satellite orbit is 26,560 km and the satellite rotates around the earth twice day. The four satellites in an orbit are not equally spaced. Two satellites are separated by 30,0?~32.1°. The other two make three angles with the first two satellites and these angles range from 92,38" -130.98". (b) Fig.3.2 (a) GPS satellite constellation (b) Block | (SV Nos 1-12) and Il GPS satellites To minimise the single satcllite failure effects on system performance, the spacing has been optimised At any time and from any location on the earth's surface, a GPS receiver should have a ditect line of sie and receives signals {rom 4 to 11 satellites. But for a majority of the time, a GPS receiver receives signal’ from more than four satellites [2), Since four sa 7 . nd h ellites are the minimum nuinber of satellites required (0 HM the user position, the constellation arrangement can provide user position at any time and any location. constellation in planar projection is illustrated in FGPS Satellite Constellation and Signal Structure a Fig. 3.3(a) GPS satelite constellation with respective orbital plane, Right Ascension of Ascending (RAN) Node and mean anomaly Flg-3.2(b) GPS satelite orbits with inclination angles Ast 24© 3.3) illustrates the GPS satellite constellation with respective orbital plane, Right Ascension of cending Node (RAAN) and mean anomaly, and Fig, 3.3(b) represents the number of GPS satellite orbital os 4nd the inclination angle, The inclination angle is the angle between the equatorial plane and the ital plane. Several notations are used to refer tothe satellites in their orbits, One nomenclature assigns a letter to each "al Plane (ice. A, B,C, D, Band F) with each satellite within a plane assigned a number from | to 4. ee “cond notation represents the configuration of the Pseudo Random Number (PRN) code generator "the satellite. These PRN code generators are configured uniquely on each satellite, thereby producing Unique MBE Versions of both C/A code and PCY) code. “Sa GPS satellite can be identified by the PRN code that it generates,nR Globo! Navigation Satellite Systems Orbital plane 7 137 257° Tr 197° 317° Right ascension of ascending node Fig. 3.3(€) GPS satellite orbital planes with RAAN and mean anomaly Block IA, A Block IIR, ° A Block IRM A B c D E F Fig. 3.3(d) Block IIA, JIR and IIR-M satellite constellation in planar projection For example, a satellite referenced as D2 refers to a satellite number 2 in the orbital plane D. & Figure 3.3(c) illustrates the satellite orbital planes with RAAN and mean anomaly. For examp!* RAAN of the satellite Ad is 272.847°, and the argument of the latitude is 41.806". Figure 3.3(d) gives the constellation in planar projection for Block ILA, IIR and ITR-M satellites. Details of the satellites in the GPS constellation are depicted in Table 3.1 and the characteristics of GPS satellites are listed in Table 3.2.GPS Satellite Constellation and Signal Structure ® Table 3.1. GPS satellite constellation details Constellation design | | Number of satelite 2 r Number of orbital planes 6 Number of satellites per orbit 4 Orbital inclination ss Orbital radius 26,560 km_ Orbital period 11 hrs $7 min $7.26 sec Ground track repeat sidereal day” Sidereal Day The orbital period of the GPS satellite with an altitude of 20,200 km is equivalent to half of the sidereal day. A sidereal day is the time taken for the earth to rotate exactly 360°. This is not the same as a solar day. A solar day is the time taken for the earth to rotate once relative to the sun, which is slightly more than 360° because, while rotating on its axis, the earth is also orbiting the sun. So the earth is rotating a little more than 360° to return to the same angle relative to the sun. A solar day is of 24 hours, and a sidereal day is slightly shorter at 23 hours 56 minutes 4.1 seconds. The real significance of the GPS orbital period is that the visible positions of the satellites repeat themselves every day; that is, in one sidereal day, the satellites will have orbited exactly twice, the earth will have rotated exactly once, and all the satellites will be back in. exactly the same position relative to us. As of I March 2010, the space segment was built-up by 32 operational satellites: 12 usable Block HA satellites, 12 Block IIR satellites, 8 block IIR-Ms (Fig. 3.4). 0 2 4 6 8 10 12 14 16 18 20 2224 Time of the day in hours (GPS) FIg.3.4 GPS satellite visibility period over a period of 24 hours (22 July 2008) from Bangalore, India The emire Block I] satellites have became nonoperational, At present, the Block HA/IRAIR-M satellites are operational. satellite constellation (HA/UR/IR-M satellites), each satellite's With the current operational GPS Visibility period over a period of 24 hours and the number of satellites visible at any given instant of time are Plotted in Figs. 3.4 and 3.5 respectively [3] shows the position of a satellite relative t0 an ble constellation from any74 Global Navigation Satellite Systems 10 Number of satellites visible o Clt2 esto someone) 16 18 20 22 24 Time of the day in hours (GPS) Fig.3.5 Number of GPS satellites visible at any given instant of time seen from IISC, Bangalore, India If you are unfamiliar with azimuth-elevation plots, here is a brief explanation. The outer circle of the plot represents the horizon and the centre of the plot represents the point in the sky directly overhead. The top of the plot is north. A skyplot at a single time shows the position of one or more satellites, relative to you, at that time. A skyplot for some period of time shows lines representing the path of the satellites, relative to you, over the period of time. ‘The visibility of GPS satellites as seen from HSC, Bangalore, India, on 22 July 2009 is given in the figures. From the figures, it is clear that at any given instant of time the minimum number of satellites visible are 7 to 8, meeting the condition (minimum of 4 satellites availability) required for user-position determination. _F!8-3.6 GPS satellites locations observed fom Bangalore, India, at a particular epoch on 22” Juy 2009 yf BAA IAIN copatiines 8 AIK owe, Ore * CIA co enrad (L C/A) 7 ¢ : © 20a chvd wend (20) + Standasd posrtoeny pave * Hew aa + Precise ponieasiny tervice He Wagener HLTA LZ P(N) navigation on + Bed eh aigna (15) * Pica Ald power (+7063) W944 Capability phase 3.7, BLOCK IIR, BLOCK IIR AANIF and Block It satetitns * Rs. ww % lenpcoverd chal signal (L1C) 7 increased accuracy (48-12 ™) + Evatuating integnty Improvements + Navgaton survey + Increased AntJammung power (+ 20.08)GPS Satelite Constellation and Signal Structure 75 For example. a minimum of seven GPS satellites (PNR No. 7, 13, 23, 16, 6, 11, 19) seen with their locations at a particular epoch (instant of time) in the space in terms of azimuth and elevation angles observed from SC, Bangalore, India, Table 3.2 Curront GPS ational satellites re illustrated in Fig, 3.6. Launch order y SV No. Launch date Plane | | 2 > 26 NOV 19 BS | na-10 32 2B 26 NOV 1990 Rb =) | ast 24 24 04 JUL 1991 cs | jaa 25 25 23 FER 1992 Rb AS WA 26 26 07 JUL 1992 Rb rs | MAIS 27 21 09 SEP 1992 Cs Ad MA21 09 39 26JUN 1993 cs . WA-23 o4 4 26 OCT 1993 Rb be | Ma24 06 36 1OMAR 1994 Rb & 2 © MA-25 03 33 28 MAR 1996 cs ag 6 J. 1996 cs E3 A26 10 40 16JUL 1995 S S MA-27 30 30 12 SEP 1996 ey 38 06 NOV 1997 cs MA28 08 3 3 a3 | 3 43 23 JUL 197 RI a h Rb pz 46 07 OCT 1999 a * 2 Rb EI nes 20 SI 1 MAY 2000 2 a 2 “4 16 JUL 2000 ins 4 Rb FI a 10 NOV 2000 M6 4 - Rb BA Ra 18 34 30JAN 2001 i es . 56 29 JAN 2003 my s 4 Rb D3 TR-9 an 45 31 MAR 2003 ° UR, 7 21 DEC 2003 Rb mi * Rb 3 3 59 20 MAR 2004 3 mR 19 Ss 60 23 JUN 2004 Rb ie ; Rb DI mRa3, o 61 06 NOV 2004 Re eH mr. 3 26 SEP 2005 tn " a Rb Ad UR. 25 SEP 2006 | Rasy 31 32 a “ 17 NOV 2006 IR-16M 12 58 = WR-17M 15 55 17 OCT 2007 Rb UR-ARM 29 37 20 DEC 2007 Rb C6 u 7 15 MAR 2008 Rb A6 ia a is 2 Rb B2 UR-2094 a ” 24 March 2009 5 8 ug 2009 2 Lilla ts 50 17 Aug 2009 31.2 Control Segment Te pri Mtellte locations, syst tegrity, and behaviour of satel “lmanac. This information is uploaded into the GPS sate lary function of control segment is trv ing the GPS satellites in order to determine and Peon te atomic clocks, atmospheric data and the satellite ites through the S-band link.76 Global Navigation Satellite Systems al ground stations located around t To perform these functions, the control segment consists of several ground stati the world (see Fig. 3.8): ie ever Air Force Base in Colorado + A master control station at Schriever / : i : + Five monitor stations Hawaii 2nd Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; tic Ocean: and Colorado Springs. Colorado Ascension Island in the Atla ee + Four large ground-antenna stations that send commands and data up to the satellites and collect telemetry back from them Re, een ph Colorado , springs 2 an ‘Ascencion ? x Ea EE eo ee Fig. 3.8 Control segment locations “Tracking of satellites is done by unmanned monitoring stations. These unmanned monitoring stations track all GPS satellites visible to them at any given moment and collect the signal data from each satellite, THis information is passed to the master control station at Colorado via a secure defense satellite communication system, where satellite position, clock timing data, etc., are estimated and predicted. Then the master control Station periodically sends the corrected position and clock timing data to appropriate ground antennas, which then upload those data to each of the satellites, Now, the satellites use that corrected informat sequence of events occurs every few hours for ea creeping into satellite positions or their clos ion in the data transmission down to the end user. This ch of the satellites to en: ty of error s is minimised, sure that any possi 3.1.3 User Segment The user receiving equipment, typically referred toas a GP from the satcllites to determine Position, Velocity and Time (PVT). The basic structure of a receiver is the antenna, the receis de power supply. These receivers can be mounted in ships, information, regardless of weather conditions, : The hea, of ss users depends on the type of receivers lable today.(The diversity of GPS uset is attributed to the large variety of receivers available today.) Based on the type of observables (code pseud? ranges und carrier phases) and on the availability of codes (C/A-code, P-code or ¥-code), GPS receivers ee be elussified into four groups: C/A-code peudorange, CIA-code cage Phase, P-code carrier phase 2 receiver, processes the L band signals transmitted ver and processor, the display and a regulated Planes and cars, and provide an exact positio7 GP'S Satellite Consteltanan ond Signal Senuctsre (8) Aircratt navigatcn fo) Car navecgaton (9) Aircrat enroute navagation FIG.3.9 Professional uses of GPS Y-coue carr asuring instruments [4]. Today, every aircraft, ship. land vehicle incorporates GPS recive ace Yap 39 9), Also, GPS receivers are routinely beiog used wo condoct all types of land ct geodetic control surveys, 3.25 GPS SIGNALS enn Currenity, GPS. satellites of Block HA and HR continuously transmits the standard GPS signals, ie, CIA Code on the LI band (1575.42 MHz) and the PCY) code (only for DOD authorised users) om the 12 band (1227.60 Miz),78 Global Navigation Satellite Systems it igatic P-codo Satellite navigation s process known as (Direct Sequence Spread Spectrum) s 1000 feet yer 00 os 1 DSSS modulation. In DSSS, a nominal or baseband CT ]___L 1 Eid tt tt : jgnals are generated using a | frequency is deliberately spread out over a wider 9 Haeode : bandwidth through superimposing a higher frequency __1 see | signal. Each satellite transmitted signal consists of the [eel : following three parts (Fig. 3.10). {Navigation mossane : + Carrier * Pseudo Random Noise (PRN) code (C/A code and/or P code) + Navigation Message The details of these three components are given oe Fig. 3.10 Schematic of GPS transmitted signal @ Carrier Each GPS satellite broadcasts a components on L1 carrier modulated carrier on the L band frequency (between 1 GHz - 2 GHz). Currently, the two carrier frequencies are 1575.42 MHz referred to as Link 1 or Li and 1227.60 MHz referred to as Link 2 or L2. The corresponding carrier wavelengths are approximately 19 cm and 24.4 em respectively. ‘The advantage of having two carrier frequencies is that the ionospheric delay error can be removed. Gi) Pseudo Random Noise (PRN) Code Each satellite is assigned a unique sequence of 0's and I's Each ‘0° or ‘I’ is referred to as a chip. These sequences allow the receiver to determine the signal trans! time instantaneously. The sequences are called as Pseudo Random Noise (PRN) sequences or PRN codes (Fig. 3.11). 10111100011001101001110001110001011110001100110100111000111000 1 UU UUM Fig.3.11. A short repeating PRN code py code allows all the satellites to transmit at the same frequency without interfering with each other. : These sequences also allow the precise range measurements and mitigate the effects of reflected and interfering signals received by a GPS antenna. Spreading Sequence Fach satellite transmits two unique spreading sequences or PRN codes ‘The first one is Me e/ si ~y k (py) Ne the Coursed Acquisition (C/A) code and the other one is the encrypted precision c04° (a) Coarse/acquisition (C/A) code iach C/A-cole is a unique sequence of 1023 4 o nique sequence of 1023 bits, called chips, The sequence repeats every millisecon 8! she duration of each C/A cuate chip ix 1 [second (1 ms/1024 977.8 aoe | M et Fquivalently, the chip width or wavelength is about 300 mn, ji 7 is about 300 m. The rate of the C/A code chips, sate, is 1.023 MH¢ (or megachips/s (Meps)) (see Vig. 312) 0 of the C/A code chips, alled chippiP®GPS Satellite Constellation and Signal Structure 79 oJUULI PUL LAMA Fig.3.12 PRN code length and chip width (b) P-code The P code is a longer code. It is a unique segment of an extremely long (10" chips) PRN sequence. The chipping rate is 10.23 Mbps, and is ten times that for a C/A-code and the chip width is about 30 m. The smallest wavelength results in greater precision in the range measurements than that for the C/A- codes. It repeats itself each week starting at the beginning of the GPS week which is at Saturday/Sunday midnight. The C/A code is only modulated onto the L1 carrier while the P(Y) code is modulated onto both the LI and the L2 carriers. The P-code is not directly transmitted by the satellite, but it is modified (encrypted) by a ¥-code. Therefore. itis also called P(Y) code. Itis primarily used by the military to limit accessil available to civilians. Therefore, the P(Y) code is classified. The properties of the P(Y) code are same as that of the P-code. In order to receive the P(Y) code, one must have the classified code. More details are given in the following section about the P-code generation. (i) Navigation Data The navigation data contain information regarding satellite 01 igation data is a binary coded message consisting of data on the satellite health status, ephemeris (satellite position and velocity), clock bias parameters and an almanac data on all satellites in the constellation (5). This information is uploaded to all satellites from the ground stations in the GPS control segment. The navigation message is transmitted at 50 bits per second (bps), with a bit duration of 20 ms. It takes 12.5 minutes for the entire message to be received. The essential satellite ephemeris and clock parameters are repeated each thirty seconds. GPS satellites are each equipped with four extremely stable atomic clocks possessing stability in the order of 2 parts in 10", ‘The schematic description of a transmitted signal on the L1 carrier is shown in Fig. 3.13. It consists of LI cartier frequency, the characteristics of the C/A code and the data modulated on to this carrier. The GPS signal parameters are listed out in Table 3.3. Table 3.3. GPS signal parameters Parameter CAA signal P(¥) signal aa 1 Carried on Li only Li 1 Centre frequency 1575.42 MHz 1227.60 MHz | (Code length (chips) 1023 15,345,037 bits long | Chipping rate 1,023 Mas 10.23 MHzs Code type Gold code Pseudo Random | ition rate ims T week j Chip wigs 293m | Navigation message data rate 50 bitsls | Feature | Precise positioning and jamming resistantGlobal Navigation satellite Bystens 60 i] =| > edoes i } 20 code periods: fat —ttanimat Peete agro! >| [o> thi (0.0770 ym) One period (hme) Foal Llspollat> hea pac Lt Carson nem If Mfr h/t Aa Me aia \ 1NA0 eyeloalehip Fig.3.13) Schematic description of transmitted signal on LA eartinr with C/A code and data Figure A123 shows the thice patty forming the sjznal on the LE frequency, The C/A cade repeats itvell one navigation bit lasts 20 ns. Hence, for eaci: navigation bit, the signal contalay 20 complete every ins, C/A cadey Goo Vig. N14). ‘The LL and b2 sual modulations are Hlustiated in Bye 3,15, "Phe details af the P-code generation are described in the next section ‘ 1023 chip (1 ma) » Pan code Vehip 20 codon (20 ma) Data a i = mele folr fe] rte] 150 bp (20 e/a) FIG63.44 Navigation bit and the corresponding PRN coder 3.3 GPS SIGNAL GENERATION The carrier frequency, navigation data pulse frequency and C7A (Coarse/Aequisition) and P-code codes # all coherently derived from the nominal frequency, f, 10.2) M7 ‘The nominal frequency by produced from the resonant frequency UL ane of the onboard clocks Livery satellite sity analy centered On I? frequencies lying nthe F-hund. "These frequencies are derived tone a tundarentat freqneneys fv (wilt & relation 154/20), generated hy Hv atone clocks with watability about 104,at 1023 MHz i va L2eamer NS eee Lae PST 1227.60 Mz Fig.3.15 LI and L2 signal modulations The two carrier frequencies are related to the clock frequency as LI = 154 x 10.23 MHz = 1575.42 MHz L2 = 120 x 10.23 MHz = 1227.6 MHz At present, C/A-code is modulated onto the LI carrier only, while the P-code is modalated onto both Li 204 L2 carriers. ‘Tee carrier phase is shifted by 180° when the code value changes from zero to one oF from one to zero. Biphase or (Binary Phase Shift Keying) BPSK modulation is used (see Fiz. 3.16). one AA A AA me ) V | YA a | }__|__}_____-__. Code a |__| N |W ALA ae Rav wy Modulated Fig.336 Binary Phase Shit Keying82 Global Navigation Satelite Systems The signal structure of the GPS satellite is given below: — ‘As LI frequency contains both the C/A and PCY) codes in phase quadrature with each other, they can be written as Spy = Ap(P() @ D{1)) sin (27 f,1 + 0) + A (Cl) @ Di) cos (27 fit + 0) By =A, (Pit) Dit)) sin 2Rf:+ 0) ignal at LI frequency A, = Amplitude of the P-code P(t) = £1, which represents the phase of the P-code D(t) = £1, which represents navigation data code Jf, =L1 frequency @ = Initial phase A. = Amplitude of the C/A-code CU) = £1, which represents the phase of the C/A-code. Sig = Signal at L2 frequency fy = 12 frequency ‘The signal transmitted from the satellite *k* on both the frequencies (f, and f,) and can be described as $= A, (PO) ® DD) sin QF ft + 0) + A (CW @ Did) cos 2K f+ 9) +A, (PUD) ® Dit) sin Qefyt+ In the above equation, the C/A code (c(1)) ® data (Dyt)}and the P(Y) code(P(1) ® data (D(t)) signals are modulated onto the carrier signal L1 using the binary phase shift keying (BPSK) method. Note that the two codes are modulated in-phase and quadrature with each other on Ll, That i a 90° phase shift between the two codes. After the P(Y) added to form the resulting LI signal. The Standard Posi alone. there is Part is attenuated to 3 dB, these two LI signals are tioning Service (SPS) is based on C/A code signals The C/A and P-codes are combined with the binary navigation data using modulo-2 addition, In modulo-2 addition, if the code chip and the data are the sa 0; and if both are different, the result is 1 (see Fi the carrier in a process called modulation. The Keying (BPSK); a O-bit leaves the carrier signal equivalent to shiftin ime (both are 0's or both are 1's), the result is, g. 3.17). The composite binary signal is then impressed upon specific form of modulation used is called Binary Phase Shift e 1 unchanged; and a 1-bit multiplies the carrier by —1, which is alate g the phase of the sinusoidal signal by 180°. Ar code transmission from 0 to 1, or from | Fig 3.18 and the ne Carrier signal is shified by 180". General BPSK modulation for L1 signal is shown it "3.18 and the corresponding waveforms are illustrated in Fig. 3.19. The following time pulses and frequencies required are generated from the atomic clock: * The 50 Hz data pulse i . pee ca {Gourse/Acauisition) code (a PRN-Code broadcast at 1.023 MHz), which modulates the “ive-OR operation (EXOR) spreading the data over a 2 MHz bandwidth. * The frequency of the civil L1 cartier (1575.42 MHz) The data modulated by the C/A-code modulates th i : 1 e we LI carrier in turn by using Bi ht Keying (BPSK). With ever shang nthe malted date ona ane Binary Phase Sit KeyGPS Satellite Constellation and Signal Structure wees. | LULU UL Nav. bits | 50 Hz LULL ULL Fig.3.17 Modulo-?2 addition of C/A-code and navigation data 1575.42 MHz Antenna ior AM pee [roe Arcearee min jgenerator [1575.42 MH2| Ut carrier 1023MHz 49 1.023 MHz BPSK Base “Time pulse for] 1.923 [ Cia code LILI | 4 603 mez FIg.3.18 Satelite signal generation with BPSK modulation ie bans {HGingeneraor [iH Soncetor Frsquency {| Li.oes tis Poros ms SACOG 10.23 MHz 1023 chips «204600 Sone_P Escher ‘Data ‘50 Hz [Data aed 44 processir *| pulse: F ee ‘generator | Data, 50 bits's CiAcode 1 (PRN-18) 1.023 M bivs © 14 Data modulated byCiAcode © ete IN amcutes AMANITA WA AES Li carrier Fig.3.18 BPSK modulated GPS signal 8384 Global Navigation Satellite Systems 3.3.1 Generation of Codes ‘The signal transmitted by a GPS signal consists of three comy function. The three components are PRN code (C/A and/or P-code), carrier and the navigation message data. ‘The details of these codes are described briefly in Section 3.2. In this section, 1 and navigation data bits structure are described in detail. C/A Code The C/A code is BPSK modulated with a chip rate of 1.023 MHz The main lobe spectrum is 2.046 MHz. Each chip is about 977.5 ns long. The transmitting bandwidth of the GPS satellite in the L1 frequency is approximately 20 MHz to accommodate the P-code signal. Therefore, the transmitted C/A code has a main lobe and side lobes illustrated in Fig. 3.20). The total code period has 1,023 chips. With a ponents. Each of these components has a unique he generation of PRN codes chip rate of 1.023 MHz, 1,023 chips take 1 ms time for 2'Mie transmission. The C/A code belongs to the family of sl . Gold codes. 2oMie P-code P-code characteristics are Fig.3.20 Spectrum of C/A-code and P-code * P-code is BPSK modulated at 10.23 MHz * Main lobe of the spectrum is 20.46 MHz (see Fig. 3.20) * Chip length is about 97.8 ns (1/10.23 MHz) * Generates two pseudorandom noises (PRN) with the same chip rate: ~ One is 15,345,000 chips, period of 1.5 seconds — Other is 15,345,037 chips * code length is 23,017,555.5 (1.5 x 15,345,037) seconds, sli 345 » slightly longer th: But actual length of the P-code is I week (code reset every week). ace cm peamee Jong one can be divided into 37 different P-codes and each satellite can use adifferent portion |. The navigation data rate carried by the P-code through phase modulation i following subsections explain the generation of P-code, CVA cade and ae Seca amet 9 Eel P-code Generation Eoea een eee eet P-codes are non-linear codes, where. , whereas C/A-codes are linear code ee s codes. In P-code ing linear codes are short-eyeled before creating the product of the codes. This has the ante Seni eee The fact is that the 37 indivi 37 indivi l Reus ‘dual P-codes are simply a one-week piece of long code that is approximately 38 ‘The typical P-code Pour X1 and X2 code penerators Each of hee To ge nis there are four shift registers, two each for he short-cycled, either at 4092 or 4093 Py eau registers that have 2'? — | = 4095 possible states #7” Both of the X1 shift regi Peele sta re se ton X1 epochs, i.c., for every 1.5 seconds, while the X2 shift registers Aiteagl xl tnd xo ne eit ee cycles All shift registers are reset at the end of the week J FI peat Sand 1.5+ second, a a i at 1023 Miz, et modulo aon generac an eee ge ey ae anning asynehronosy) a le code with re:85 4098 chips 1 code generator 1023 MHz aA. OF PD = XOX (0-07) ‘Atomic | lock | i q eee f dt 15,345,037 _,{ Delay ichips | at or, i hips | forsatetite || 1FI9.3.21,P-code generator to X1 code an additional (j — 1) chips for the #* satellite provides codes for each satellite. The count of X1 ¢Pochs provides a Z-count that is used as basic timing for the system, to which the data message and the C/A Codes are synchronised. If the codes were not set at the end of the week, the code would eventually run into the code of the other satellites and return to the beginning almost 38 weeks later. C/A Code Generation GPS signals are generated from the product of wo 1,023-bit PRN sequences, GI and G2. Both GI and G2 are generated by a maximum-length linear shift register of 10 stages and are driven by a 1.023 MHz clock. The G1 and G2 generators are illustrated in Fig. 3.22. The basic operating principles of these two generators are similar. A Maximum Length Sequence (MLS) generator can be made from a shift register with Proper Feedback. Ifthe shift register has n bits, the length of the sequence generated is (2"~ 1). Both shift generators in G1 and G2 have 10-bits, thus, the sequence length is 1,023 (2'°— 1). The feedback circuit is accomplished through modulo-2 adders. When the two inputs are the same then output is 0, otherwise it is 1. The positions ofthe feedback circuit determine the output pattern of the sequence. The feedback of GI is from bits 3 and 10 and the corresponding polynomial is L+xr4xt? The feedback of G2 is from bits 2, 3, 6, 8, 9.10 and the corresponding polynomial is V4 era sterte rg06 Global Navigation Satelite Systems a G1 generator a CEP EL [lle]: 1 ] | hi vogistor | | Sy 1.023 MHz] { Rosot clock all ono Positions of these feedback dotormine the satelite ID Telefe) | G2 rogistor Fig.3.22/ C/A-code gonorator f code generator. The modulo-2 adder at the output adds the outputs from G! Figure 3.22 shows the C/A- ial values of the two shift registers G1 and G2 are all 1's and they must be and G2 shift registers. The in Joaded in the registers first. i ‘The satellite identification is determined by the two output positions of the G2 generator. There are 37 unique output positions, Among these 37 outputs, 32 are utilised forthe C/A codes of 32 satellites, but only 24 satellites are in orbit. The other five outputs are reserved for other applications such as ground transmissioo Navigation Data Bits Navigation message is a continuous stream of data transmitted at the rate of 50 bits per second. The navigation message is needed to calculate the current position of the satellites and to determine sigh2! travel times, The navigation message from each satellite carries the following information to earth: * Satellite time of transmission * Satellite position (determined from the broadcast orbital data (ephemeris)) * Satellite health * Satellite clock correction * Propagation delay effects ‘Time transfer to UTC Constellation status (approximate orbital data for all other satellites (almanae))GPS Satellite Constellation and Signal Structure 87 Navigation data has the following characteristics (7): + Data is a continuous stream of 50 bits per second. + Data is modulated to the carrier wave of each individual satellite, + Data is transmitted in logically grouped units known as frames or pages. + A complete navigation message consists of 25 frames (pages). + Each frame is 1500 bits long and takes 30 seconds to transmit, + The frames are divided into 5 subframes, + Each subframe is in turn divided into 10 words, each containing 30 bits. ~ Each subframe is 300 bits long and takes 6 seconds to transmit. ~ Each subframe begins with a telemetry word and a Hand Over Word (HOW). + Acomplete navigation message consists of 25 frames (pages). * To transmit a complete data 25 different frames are required. * Transmission time for the entire data is therefore 12.5 minutes, ‘The structure of the navigation message is illustrated in Fig. 3.23. Navigation message = 25 pages ~12, 5 min t«_Frame = 1500 bits ~ 30 s ~ 5 subframes 273 [[5 [61718 |9 |o FAC-3.23 Structure of the navigation message Word = 30 bits ~ 0, 6s bit ~ 20 ms ‘A fame is divided into five subframes, each subframe transmitting different information, * Subframe 1 contains the time values of the transmitting satellite including the parameters for SStecting signal wansit delay and onboard clock time as well as information on satellite health and a0 estimate of the positional accuracy of the satellite. Also, Subframe I transmits the 10-bit week NARPSt (GPS time began on Sunday, 6th January 1980 at 00:00:00 hours. Every 1024, weeks, the sac, umber restarts at. This event is called a “week rollover") |8}. ao ames 2 and 3 contain the ephemeris data of the transmitting satellite. This data provides on Accurate information on the satellite's orbit frame 4 contains the almanac data on satellite numbers 25 to 32 (cach subframe can transmit from one satellite only). The ditference between GPS and Universal Coonlinated Time (UTC) (e ae e i Se 0 UTC offset) and information regarding any measurement errors are caused by thetedite Systems + Subframe 5 contains the almanac data on satellite numbers 1 to 24 (each sub data from one satellite only). All 25 pa satellite numbers | to 24. me can transmit sate transmitted together with information on the health of icnals that we encounter can be divided into wo c and random signals. Deterministic signals are modeled by explicit = S cos/150:) and x1) = Se“ are examples of determin 2 signal about which there is some degree of uncertainty, An example of a random signal is a received GUS signal: the received signal contains beside the information-bearing signal also noise from disturbances in the atmosphere and noise from the internal circuitry of the GPS receiver [9], Signal analysis of GPS signals is of paramount importance, 4 of GPS signals, both in time and frequency domain. The two ma autocorrelation and power spectral densities. As described code, P-code, navigation message and two different carrier mainly govern the characteristics of the signals. Before proceeding to observing and comparing the various GPS sit section, one is required to understand how the power spectral densi GPS signals are derived. This section, therefore, mainly deals with basi above-mentioned properties, derivation of power correlation properties of the GPS signal. sof signals referred to as deterministic thematical expressions, The signals.a(t) A random signal, on the other hand, is Jo signal it helps one in studying the properties n characteristics that are of concem are previously, a GPS signal is constituted of C/A signals, of whieh C/A, PCY) code and carrier ignal power spectrums in the forthcoming y and the autocorrelation properties of the ics of signal processing concepts required to derive the spectrum of the GPS signal 4 nd autocorrelation and cuss 34.1 Time Domain Representation of GPS Signal Mirstly, we need to represent the C/A code in appropriate form in fhe eaves ‘er facilitating easy derivation of frequency domain saracteristics of the signal. As previously mentioned, C/A c Coarse/Acquisition code rey i ; es : Presented as C(t) is a periodic repetition of 1O25chip-pattern. One period of this code can be written aye 4. PUTT) N= 1023 chips where pi) is an elemental chip waveform (ree form (rectangular pulse ct a: below modified to have duration T, and delay ae neo The smpliude of 1 pulse is modulated by We. 3.24), element in the sequence fur tis satellite." ! NBICH Ay the a Naa Using unit impulse functions, we an Write one code ws # eOnVOlutOn Pstiod of the C/A FIG9,24) Pulse wavetoun and Senatruction of a unit inpulve fro . Pe rwotanguler pulse 0 @ P(F)* E4600 a)