1100. Development The idea that led to development of the satellite navigation systems dates back 1957 and first launch an artificial into orbit Russia’s Sputnik I. Dr. William H. Guier George C. Wieffenbach at Applied Physics Laboratory Johns Hopkins University were monitoring famous “beeps” transmitted by passing satellite. They plotted received signals precise intervals noticed a characteristic Doppler curve emerged. Since satellites generally follow fixed orbits they reasoned this could be used describe satellite’s orbit. then demonstrated determine all orbital parameters for observation single pass from station. shift apparent while receiving transmission proved effective measuring device establishing Frank T. McClure also Laboratory in reverse: If orbit was known measurements one’s position on Earth. His studies support hypothesis earned him National Aeronautics Space Administration award important contributions space development. In 1958 proposed exploring possibility operational system. Chief Naval Operations set forth requirements such successful launching prototype system April 1960 system’s feasibility. Navy Navigation Satellite System (NAVSAT known as TRANSIT) accuracy better than 0.1 nautical mile anywhere world though its availability somewhat limited. It primarily surface ships submarines but it had some applications air navigation. hydrographic surveying geodetic determination. transit program ended 1988 disestablished when Global Positioning became 1996. THE GLOBAL POSITIONING SYSTEM 1101. Description Federal Radionavigation Plan has designated NAVigation using Timing And Ranging (NAVSTAR) (GPS) primary U.S. government. GPS is spaced-based radio positioning which provides suitably equipped users with highly accurate position velocity time data. consists three major segments: segment control user segment. segment comprises 24 satellites. Spacing their orbits arranged so least four are view any time Each transmits two frequencies superimposed Included data predicted ephemeris atmospheric propagation correction data clock error information health This normally 21 orbiting active spares. altitude 20200 km six separate planes each plane inclined 55° relative equator. complete approximately once every 12 hours. transmit pseudorandom noise (PRN) sequence-modulated L1 (1575.42 MHz) L2 (1227.60 MHz). both Coarse Acquisition Code (C/A code) Precision (P code). Both P C/A codes carrier only code carrier. Superimposed message. message contains ephemeris bias. assigns unique practice division multiple access (CDMA) allows use common carrier frequency still allowing receiver transmitting. CDMA easy identification broadcasts own combination can assigned PRN sequence number. number how identified communicates about particular includes master station (MCS) monitor stations ground antennas located throughout world. station Colorado Springs Colorado equipment facilities required monitoring telemetry tracking commanding control uploading generation. Hawaii Kwajalein Diego Garcia Ascension Island passively track satellites accumulating ranging satellites’ relaying them MCS. MCS processes information signal accuracy updates relays antennas. antennas transmitting information. designed receive process or more either simultaneously sequentially. processor converts these wide variety applications marine land surveying receivers vary greatly function design. 1102. Capabilities accurate continuous worldwide all-weather common-grid threedimensional To obtain solution (latitude longitude altitude) (four unknowns) must used. measures pseudorange rate synchronizing tracking selected Pseudorange true distance between plus offset due user’s slant range clock. By decoding timing signal receiver/processor convert three-dimensional velocity. Four necessary solve unknown components (or velocity) frequency) achieved depends variability errors making measurements instantaneous geometry seen location Earth presence Selective Availability(SA). Availability discussed further below. 1103. Concepts distances computes spheres those distances. intersections receiver’s position. described above done comparing generated internal clocks. These signals characterized special wave form pseudo-random code phase other. arrives following delay proportional traveled. detected receiver. Knowing reach calculate receiver therefore sphere centered radius equal measurement. intersection yields possible points One disregarded since hundreds miles Theoretically then fix GPS. however fourth measurement error. travel speed light even extremely slight clocks will lead tremendous errors. atomic 10-9 seconds installing would make prohibitively expensive. Therefore sacrificed additional made. caused inaccuracies reduced subtracting constant until pinpoint reached. Assuming perfectly synchronized clock’s constant subtraction resulting determinations reduce “pinpoint” obtained. note here lines employ technique fix. determines dimensions adds unknown. unknowns. 1104. Signal Coding Two frequencies carry (L1) 1575.42 MHz second (L2) 1227.60 MHz. messages: P-code L2 only messages divided individual bits chips. sent type chipping rate. P-code 10.23 MHz (10.23 × 106 per second) 1.023 (1.023 message 50 Hz (50 second). modulate carriers angle 90° code. periods repetition differ. repeats millisecond seven days. As stated modulated. simply another way saying digital “1’s” “0’s” contained indicated along phase. analogous sending same varying amplitude (amplitude modulation AM) (frequency FM). See Figure 1104a. unchanged “information signal” shifting carrier’s modulation employed bi-phase keying (BPSK). APN2002-figure1104a.jpg Digital amplitude modulation. APN2002-figure1104b.jpg 1104b. Modulation signals. APN2002-figure1104c.jpg 1104c. spreading recovery Due BPSK “spread” center amount twice “chipping rate” modulating signal. case (2 = 20.46 For 2.046 Note modulated shaped differently carrier lowers total strength below thermal threshold present effect When multiplied collapsed original band. power raised level. Hz.) 25 frames frame consisting 1500 bits. five subframes 300 each. will take 30 seconds one 12.5 minutes frames. transmission handover word (HOW) transition code being tracked almanac coefficients ionospheric models Universal Coordinated Time (UTC). 1105. Correlation Process correlation compares square computer logic recognizes +1 depending whether “on” “off.” processed matched autocorrelation function. defines necessity “pseudorandom code.” repeatable (i.e. nonrandom) because makes calculations. At random work randomness matching excludes combinations except combination occurs shifted signal’s delay. simultaneous (non-random) give rise description “pseudo-random” enough repeatability enable retaining ensure incorrect calculations excluded. 1106. Precise Service Standard levels navigational provided GPS: (PPS) (SPS). designed foremost Department Defense United States military asset capability asset may need limit during war prevent enemies. PPS available authorized users mainly allies. SPS other hand worldwide anyone possessing Therefore does SPS. cryptographic methods deny civilian users: selective (SA) anti-spoofing (A-S). SA operates introducing controlled programmed degrade signals’ war denying potential adversary ability nominal SPS accuracy. introduces signal: (1) epsilon error: message (2) dither: introduced clocks’ timing. largest source status SA off on checked USCG’s NAVCEN Web site: http://www.navcen.uscg.gov Anti-spoofing negate hostile imitation alters Y remains unaffected. employs times without warning unaware transformation takes place. applied not protected spoofed. Only employing proper devices defeat anti-spoofing. Without devices subject degradation unable both determining Maximum obtained L2. difference corrections. acquire approximate Then locks (subject if cryptographically equipped). Some possess lock initially disregard entirely. frequency dual ionosphere methodology unavailable modeling procedure receivers mentioned above provide positions degraded A-S feature denies transformed cannot rely measure delays compute Consequently typical uses unaffected A-S. L1 method calculating corrections unavailable less method accounted 100-meter calculation. 1106 presents different types measurements. SA/A-S Configuration SIS Interface Conditions PPS Users SPS Users Set Zero Off P-Code no C/A-Code errors Full spoofable Full accuracy* spoofable Non-Zero Value spoofable Limited On Y-Code Not spoofable** Full accuracy*** spoofable** Limited ??* “Full accuracy” defined equivalent PPS-capable UE operated similar manner. ?** Certain do have P- Y-code abilities remain despite protection applied. *** negligible C/A-code operation (but susceptible jamming). Effect 1107. Receiver order navigate collect A begins track. visibility determined user-entered predictions stored exists attempt locate onto view. locked satellite demodulate read constellation. loop tracks loops operate together iterative locally generate differs produced frequency. velocity line sight adjusts receiver-generated matches incoming aid loop. replica targeted estimated match internally replica things done: 1) adjusted signal 2) lined up Doppler-estimated output slew millisecond search window correlate tracking. Once been stripped demodulated read. gives crucial word long (seven days) millisecond relatively small window. day repeat period requires given narrow manageable time. repeated subframe bit block subframe. unnecessary directly. whose approaches increases cost most non-military capability. acquired ready begin Recall termed inaccurate is represent biased bias equations (the x coordinate y z coordinate position). discussion Article 1101 solves problem measurement equation allow solving solved estimate dimensions coordinates referenced Earth model based World Geodetic (1984). 1108. User Range Errors Geometric Dilution There formal spherical shall 16 meters SEP (spherical probable) better. dimensional 100 2 drms Assume universal results conform normal (i.e. values symmetrically distributed around mean zero) probability factors affecting geometric dilution precision (GDOP) (UERE) continuously variable. UERE pseudoranges user. product several factors including stability predictability process distortion compensate it quality UERE GDOP relation independent broadcast Generally speaking optimum directly overhead spaced 120° horizon. worst occur closely overhead. DOP’s combine GDOP. vertical dimension (VDOP) describes horizontal (HDOP) geometry’s (latitude longitude) (PDOP). PDOP combined (TDOP) APN2002-figure1108.png Position computations. 1109. Ionospheric Delay 1108 covered inherent (UERE) constellation (GDOP). Another cause comprise Earth’s atmosphere useful understanding concept. Consider surrounded layers atmosphere. layer extending 10 troposphere. Above troposphere km stratosphere. Finally stratosphere varies ionosphere. Though subjected effects model significant operation. ionosphere name implies region large ionized molecules correspondingly high free electrons. charged lost No atom loose electron input energy energy causes ions formed comes ultraviolet (U-V) radiation Sun. intense Sun’s rays larger electrons exist phenomenon group through distance. Obviously relies noticeable magnitude elements. inversely transmits atmosphere’s content (TEC) degree ionization. general describing is: {\displaystyle \Delta t={\frac {(K\times {\text{TEC}})}{f\^{2}}}}{\displaystyle {\text{TEC}})}{f\^{2}}}} where ?t = f = operating K = U-V ionizes upper atmosphere stands reason value highest Sun shining lowest night. Experimental evidence borne out showing TEC local 0500 daylight operations. addition daily variations seasons vernal equinox. shows solar cycle dependence. greater sunspots higher effect. typically follows eleven year pattern. next minimum 2006 peak again 2010. Given serious system account it? used: technique method. 1110. Dual Frequency Correction Technique term 1103 operators DOD material able copy follows ranges different. dependence delay shown equation: R(f) Ractual + actual R(L1) R(L2). combining derived eliminated left measured advantages over calculates real-time data accurate. alleviates include portion devoted capable vast majority maritime them 1111. Model mathematically diurnal variation. cosinusoidal representing maximum cosine function) day variation introduced. maximum. obviously reasonable design consideration appears. 1110 user available. 1112. Multipath Reflection reflection detects parts times. reception direct path reception reflective surface. first reflected having longer later. minimize multipath demonstrate diffuse pattern lowering addition antenna reject reflection. properties good propagation. features expected 20 feet. DIFFERENTIAL 1113. Differential Concept discussions clear navigators today. should fraction population: allied military. open ocean inability hazard mariner seldom ever needs meter middle ocean. situation shore. Typically harbor piloting visual piloting. increase ship’s safety. sufficient. Any who groped his restricted channel thick fog certainly appreciate fact plot. However sufficient safety piloting situations. situation becomes regard legitimate security concerns answer seeming dilemma lies concept (DGPS). accurately surveyed utilizes powerful contribute degradation availability experienced DGPS close means that within 100-200 advantage theory behind straightforward. Located site already knows location. receives tell is. locations theoretical it. actually arrive. basis computation subjected errors error subject. correct sources share common. obtainable Service. limited displaying navigator electronic chart providing readout pilot safely channel. WAAS AND LAAS IN MARINE NAVIGATION 1114. WAAS/LAAS Aeronautical Use 1994 Telecommunications Information (NTIA) technical report Transportation concluded mix enhanced overall consist terrestrial Wide Area Augmentation (WAAS) concept correctional geostationary via HF eliminates antenna DGPS. intended enroute navigation reference stations widely across States coverage entire Mexico Canada. Local (LAAS) airport approaches airports broadcasting VHF frequencies. While many incorporate circuitry shorter-range LAAS) optimized line-of-sight low angles horizon blocked significantly warning. terrain-following objects path. flows continues unblocked. comparable few meters. 7 95% expect 1-3 he transmitter. Over miles commonly 1 transmitter site. found accuracies designed. certified environment DGPS very tool limitations understood. waters continental U.S. useful Outside area tall buildings trees obstructions blocked confined waters extreme caution areas. enhance navigator’s situational awareness available assured. Further indication part Aircraft contain Autonomous Integrity Monitoring (RAIM) software warning failure removes affected solution. VHF blocking areas near airports. Its miles. therefore suitable use. NON-U.S. SATELLITE SYSTEMS 1115. Galileo GPS various European councils commissions expressed Economic emphasized need technological Agency years proven early 2002 Union (EU) decided fund new great deal preliminary scientific work accomplished full deployment years. Several influenced decision develop Galileo Government (as Russian GLONASS). Disruption might leave critical contrast under dedicated operational proliferating far rapidly anticipated point planners developing enhancements (WAAS turned May 1 2000) size plummeted. Plans call (27 usable spares) 56 degrees 23616 (about 12750 nm). serve latitudes factor EU decision Scandinavian participation. launched miniaturization techniques rocket cost-efficient place maintain not: integrity monitoring. Currently unit there provision notification alerting improperly. issue compatibility addressed ongoing sharing discussion assume operational. Manufacturers undoubtedly offer exploit best technologies Galileo. Integration existing shipboard ECDIS ECS ensured. benefit redundancy increased (for integrate systems’ signals). 2005 scheduled 2008. 1116. GLONASS (GLONASS) military 1993 principles planes separated 120 45 degrees. equator 64.8 degrees 11hours 15minutes 19100 (10313 (95%) 150 vertical 15 cm/sec. Military 10-20 horizontal. entirely former Soviet Union. Reliability designs life spans addressing concerns. L-band channels 0.5625 separation bands: 1602.5625 1615.5 MHz 1240 1260 ?Chapter 10Chapter 12?