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Basic concept, System Architecture, GPS and GLONASS Overview, Satellite Navigation, Time and GPS, User Position and Velocity Calculations, GPS Satellite Constellation, Operation Segment, User Receiving Equipment, Space Segment Phased Development, GPS Aided Geo augmented Navigation (GAGAN) Architecture.

GVNSK SravyaSeguir

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- Global Positioning System UNIT-I Introduction -By GVNSK Sravya Assistant Professor ECE Dept. GNITS
- Contents ▶ Basic concept ▶ SystemArchitecture ▶ GPS and GLONASS Overview ▶ Satellite Navigation ▶ Time and GPS ▶ User Position and V elocity Calculations ▶ GPS Satellite Constellation ▶ Operation Segment ▶ User Receiving Equipment ▶ Space Segment Phased Development ▶ GPSAided Geoaugmented Navigation (GAGAN) Architecture. 2
- Basic Concept of GPS ▶Navigation refers to the art of determining the current location of an object which could be in space, in the air, on land, on or under the surface of a body of water, or underground. 3
- Contd… ▶ Navigation Modes In order to get from A to B there are basically five basic navigation modes: ▶ Pilotage ▶ Celestial Navigation ▶ Dead Reckoning ▶ Radio Navigation ▶ Inertial Navigation 4
- Contd… ▶ Satellite Navigation or Satnav System is a system that uses satellites to provide autonomous geo-spatial positioning. ▶ It allows small electronic receivers to determine their location to high precision using time signals transmitted along a line of sight by radio from satellites. 5
- Contd… ▶ GNSS stands for Global Navigation Satellite System, and is the standard generic term for satellite navigation systems that provide autonomous geo- spatial positioning with global coverage. This term includes e.g. the GPS, GLONASS, Galileo, Beidou, IRNSS and other regional navigation systems. 6
- GPS ▶GPS The GPS is part of a satellite-based navigation system developed by the U.S. Department of Defense in 1995. 7
- Contd… ▶ Official name of GPS is Navigational Satellite Timing And Ranging Global Positioning System (NA VSTAR GPS). ▶ Global Positioning Systems (GPS) is a form of Global Navigation Satellite System (GNSS). ▶ Consists of two dozen GPS satellites in Medium Earth Orbit (The region of space between 2000km and 35,786 km) 8
- Contd… known as a satellite ▶ T wo dozen satellites working in unison are constellation States Air ▶This constellation is currently controlled by the United Force 50th Space Wing ▶ It costs about $750 million to manage and maintain the system per year ▶ Mainly used for navigation, map-making and surveying 9
- Working of GPS ▶ Each satellites broadcast radio signals with their location and time. ▶ GPS receivers receives radio signals, and used these data to calculate its distance from at least four satellites. ▶ Distance=Speed * Travel time ▶ GPS radio signals are travel at the speed of light . ▶ Both satellites and receiver generate the same pseudocode signals. ▶ Difference b/w the 2 signals is the travel time. ▶ Then the receiver uses Trilateration method to define its exact location on Earth. 10
- TrilaterationProcess ▶ If we know the distance b/w the satellite and the receiver for: 1 satellite, the receiver’s location is known within a sphere. 11
- Contd… ▶ 2 satellite, the receiver’s location is known within 3D ring. 12
- Contd… ▶ 3 satellite, the receiver’s location is somewhere on at most two 3D regions. 13
- Contd… ▶ 4 satellite, the region gets smaller because of the sphere of the new satellite. 14
- Architecture of GPS ▶ Architecture of GPS consists of three segments: Space segment(SS) Sontrol segment(CS) User segment(US) 15
- Contd… 16
- Space Segment ▶ Minimum of 24 satellites (currently 32) in orbit around earth at altitude 20,000 km. ▶ It transmit radio- navigation signals, and store and retransmit the navigation message sent by the control segment. 17
- Control Segment control station, ▶ Combination of a master four dedicated ground antennas and six dedicated monitor stations. ▶ Responsible for the proper functioning of all the operation of GPS such as changing unhealthy satellite with a healthy one. 18
- Components of Control Segment 19
- User Segment ▶ Comprises of thousand of military users who uses the secure GPS Precise Positioning service, and millions of civil, commercial and scientific users . 20
- GLONASS GLONASS ▶ Asecond configuration for global positioning is the Global Orbiting Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union, and now maintained by the Russian Republic. 20
- GPS and GLONASS Overview GPS Orbits ▶ The fully operational GPS includes 32 or more active satellites approximately uniformly dispersed around six circular orbits with four or more satellites each. GLONASS Orbits ▶ GLONASS has 24 satellites, distributed approximately uniformly in three orbital planes (as opposed to six for GPS) of eight satellites each (four for GPS). 21
- Contd… GPS Orbits ▶ The orbits are inclined at an angle of 55° relative to the equator and are separated from each other by 60°. GLONASS Orbits ▶ Each orbital plane has a nominal inclination of 64.8° relative to the equator, and the three orbital planes are separated from each other by multiples of 120° right ascension. 22
- Contd… GPS Orbits ▶ The orbits are non geostationary and approximately circular, with radii of 26,560 km and orbital periods of one-half sidereal day (≈11.967 h). GLONASS Orbits ▶ GLONASS orbits have smaller radii than GPS orbits, about 25,510 km, and a satellite period of revolution of approximately 8/17 of a sidereal day. 23
- Contd… ▶ Theoretically, three or more GPS satellites will always be visible from most points on the earth’s surface. ▶ Four or more GPS satellites can be used to determine an observer’s position anywhere on the earth’s surface 24 h/day. 24
- Contd… GPS Signals ▶ The GPS system uses Code division multiplexing of independent satellite signals. GLONASS Signals ▶ The GLONASS system uses frequency- division multiplexing of independent satellite signals. 25
- Contd… GPS Signals Each GPS satellite transmits two spread spectrum, L-band carrier signals on two of the legacy L- band frequencies—an L1 signal with carrier frequency f1 = 1575.42 MHz and an L2 signal with carrier frequency f2 = 1227.6 MHz. GLONASS Signals ▶ Its two carrier signals corresponding to L1 and L2 have frequencies f1 = (1.602 + 9k/16) GHz and f2 = (1.246 + 7k/16) GHz, where k = −7, −6, . . . 5, 6 is the satellite number. These frequencies lie in two bands at 1.598–1.605 GHz (L1) and 1.242–1.248 GHz (L2). 26
- Contd… GPS Signals ▶ These two frequencies are integral multiples f1 = 1540f0 and f2 = 1200f0 of a base frequency f0 = 1.023 MHz ▶ The L1 signal from each satellite uses binary phase-shift keying (BPSK), modulated by two pseudorandom noise (PRN). GLONASS Signals ▶ The L1 code is modulated by a C/A- code (chip rate = 0.511 MHz) and by a P-code (chip rate = 5.11 MHz). L2 code is presently modulated only by the P-code. 27
- Contd… GPS Signals ▶ The initial satellite configuration used SA with pseudorandom dithering of the onboard time but this was discontinued on May 1, 2000. GLONASS Signals ▶ GLONASS does not use any form of SA. 28
- GPS Satellite Constellation 29 Fig. GPS Satellite Constellation
- Contd… 30 Fig. GPS Satellite Constellation Planar Projection
- Contd… Orbits and Satellite Constellation ▶ GPS Satellite Constellation consists of 24 operational satellites in 6 orbital planes. ▶ Orbital period is of 1 sidereal day(11 hr. 58 mins.). ▶ Angle of Inclination is 55 degrees w.r.t equator. ▶ Equally spaced around the equator at a 60 degrees separation. Orbital radius 26,600km.(distance from center of the earth to sat.) ▶ Satellite geometry to provide good observability to users throughout the world 31
- Contd… Orbits and Satellite Constellation ▶ This geometry is measured by a parameter called DoP(Dilution Of Precision). ▶ At least 4 SV’s in each orbit. ▶ Orbital period is 12 hours. ▶ Receives information from CS. ▶ Broadcasts one-way ranging signals. 32
- Space Segment Phased Development ▶ The continuing development of the control and space segments has been phased in over many years, starting in the mid-1970s. ▶ This development started with a concept validation phase and has progressed to several production phases. ▶ The satellites associated with each phase of development are called a block of satellites. 33
- Satellite Block Development ▶ Five satellite blocks have been developed to date. ▶ The initial concept validation satellites were called Block I. ▶ The last remaining prototype Block I satellite was disposed of in late 1995. ▶ Block II satellites are the initial production satellites, while Block IIA refers to upgraded production satellites. ▶ Block IIR satellites, denoted as the replenishment satellites. ▶ Modified Block IIR version of satellites is denoted as Block IIR-M. ▶ Block IIF satellites are referred to as the follow-on or sustainment satellites. ▶ Since satellites are launched only as replacements for a satellite failure, their scheduling is difficult to predict, especially when most satellites have far outlived their design lifetime. 34
- Block I, Navigation development Satellites ▶ Five satellite blocks have been developed to date. ▶ Eleven satellites of this kind were launched between 1978 and 1985. ▶ The SelectiveAvailability (S/A) was not implemented. ▶ They weighed about 845Kg and had a planned average life of 4.5 years, although some of them lasted up to 10. ▶ They were capable of giving positioning service for 3 or 4 days without any contact with the Control Centre. 35
- Block II and IIA, Operational Satellites ▶ They consist of 28 satellites in total that were launched from 1989 on and many are still operating. ▶ They weigh about 1500 Kg and have a planned average life of 7.5 years. ▶ Since 1990, an improved version was used, Block IIA (advanced), with capability of mutual communication. ▶ They are able to supply positioning service for 180 days with no contact with the control segment. ▶ However, under normal operating mode, they communicate daily. 36
- Block IIR, Replacement Operational Satellites ▶ From 1997, these satellites are being used as spares for Block II. ▶ Block IIR is formed by a set of 20 satellites, although it could be increased by 6 more. ▶ They weigh about 2000Kg and have a planned average lifespan of 10 years. ▶ These satellites can determine their orbits and compute their own navigation message autonomously. 37
- Block IIR-M, Modernized Satellites ▶ They include a new military signal and the more robust civil signal. ▶ There will be eight satellites in the Block IIR-M series. ▶ The first satellite of this block was launched on September 26, 2005. 38
- Block IIF, Follow-on Operational Satellites ▶ The first satellite (SVN62) was launched on May 28th 2010. ▶ These satellites include the third civil signal on the L5 band. ▶ Their theoretical average life is about 12 years, and they will have inertial navigation. 39
- Block III (GPS III)systems ▶ The new generation of GPS satellites introduces significant enhancements in navigation capabilities, by improving interoperability. ▶ They provide the fourth civil signal on L1 band. ▶ The first launch is expected as of 2017. 40
- Navigation Payload Overview ▶ The navigation payload is responsible for the generation and transmission of ranging codes and navigation data on the L1, L2 and L5 frequencies to the user segment. ▶ Control of the navigation payload is taken from reception of the predicted navigation data and other control data from the CS via the tracking, telemetry, and control (TT&C) links. 41
- Satellite Navigation Payload 42
- Contd… ▶ Atomic frequency standards (AFSs) are used as the basis for generating the extremely stable ranging codes and carrier frequencies transmitted by the payload. ▶ The navigation data unit (NDU), known as the mission data unit contains the ranging code generators that generate the C/Acode and P(Y) codes. 43
- Contd… ▶ The combined baseband ranging signals are then sent to the L-band subsystem where they are modulated onto the L-band carrier frequencies and amplified for transmission to the user. 44
- Contd… ▶ The L-band subsystem contains numerous components, including the L1 and L2 transmitters and associated antenna. ▶ The NDU processor also interfaces to the crosslink receiver/transmitter for intersatellite communication, as well as ranging on Block IIR and later versions. ▶ This crosslink receiver/transmitter uses a separate antenna and feed system. 45
- Control Segment (Operating Segment) ▶ CS Comprises of 2 Master control stations(MCS), 16 monitoring stations and 12 ground antennas. Monitoring Station and their ▶ Check position, speed, altitude and health of tracked SV’s. ▶ Collects GPS signals, navigation data, atmospheric data variations ▶ Sends collected information to MCS. 46
- Contd… Master Control Station ▶ Computes space coordinates of SV’s. ▶ Evaluates health of SV’s. ▶ Generates navigation data. ▶ Performs satellite maintenance. ▶ Resolves satellite performances. ▶ Maintains GPS constellation. ▶ Through Ground Antennas, MCS provides commands, keeps control and uploads navigation messages and other data to the GPS satellites. 47
- Contd… GroundAntennas ▶ Collects, stores data from MCS. ▶ CS provides commands and keeps control on satellites. ▶ Uploads to the GPS satellites using S band signals(2-4 GHz)Computes space coordinates of SV’s. 48
- User Segment(User Receiver Equipment) ▶ The user segment of GPS system consists of a GPS receiver. ▶ Processes the L band signals transmitted from satellites to determine PVT. ▶ GPS receivers fundamentally consist of 3 basic constituents- antenna, GPS receiver and a controller. 49
- GAGAN(GPS aided GEO Augmented Navigation) Architecture 50 Fig.Architecture of GAGAN
- Contd… 51 ▶ It is an implementation of regional satellite based augmentation system(SBAS) jointly developed by ISRO and AAI to provide the best possible navigational services over Indian FIR (Flight Information Region) with the capability of expanding to neighboring FIRs. Ground Segment ▶ Developed to provide accuracy of GNSS receiver ▶ GAGAN consists of 3 basic components- Space segment and User Segment
- Contd… 52 Space segment consists of (i) 3 GEO satellites (ii) GPS constellation
- Contd… 53 Ground segment consists of ▶ INRES(Indian Reference Stations) ▶ INMCC(Indian Master Control Stations) ▶ INLUS(Indian Navigation land earth uplink stations)
- Contd… 54 GAGAN User Segment Consists of ▶ GAGAN-enabled GPS receivers, with the same technology as WAAS receivers, capable to use the GAGAN Signal-in-Space (SIS).
- Satellites of GAGAN 55 ▶ GSAT-8 is an Indian geostationary satellites, which was successfully launched on 21 May 2011 and is positioned in geosynchronous orbit at 55 degrees E longitude. ▶ GSAT-10 is augmented to carry 12 Ku Band, 12 C Band and 12 Extended C Band transponders and a GAGAN payload.
- Contd… 56 ▶ The spacecraft employs power handling capability of around 6kW with a lift off mass of 3400kg. GSAT-10 was successfully launched on 29 September 2012. ▶ GSAT-15carries 24 Ku band transponders with India coverage beam and a GAGAN payload, was successfully launched on 10 November 2015.
- Time and GPS Coordinated Universal Time Generation ▶ Coordinated Universal Time (UTC) is the time scale based on the atomic second, but occasionally corrected by the insertion of leap seconds, so as to keep it approximately synchronized with the earth’s rotation. ▶ The leap second adjustments keep UTC within 0.9 s of UT1, which is a time scale based on the earth’s axial spin. ▶ UT1 is a measure of the true angular orientation of the earth in space. Because the earth does not spin at exactly a constant rate, UT1 is not a uniform time scale. 57
- Time and GPS GPS System Time ▶ The timescale to which GPS signals are referenced is referred to as GPS time. ▶ GPS time is derived from a composite or “paper” clock that consists of all operational monitor station and satellite atomic clocks. 58
- Time and GPS GPS System Time ▶ Over the long run, it is steered to keep it within about 1 µs of UTC, as maintained by the master clock at the U.S. Naval Observatory, ignoring the UTC leap seconds. ▶ At the integer second level, GPS time equaled UTC in 1980. However, due to the leap seconds that have been inserted into UTC, GPS time was ahead of UTC by 14 s in February 2006. 59
- Time and GPS Receiver Computation Of UTC ▶ The parameters needed to calculate UTC from GPS time are found in subframe 4 of the navigation data message. ▶ These data include a notice to the user regarding the scheduled future or recent past (relative to the navigation message upload) value of the delta time due to leap seconds, together with the week number WNLSF(Week number Leap second future) and the day number DN at the end of which the leap second becomes effective. 60
- Time and GPS Receiver Computation Of UTC ▶ The latter two quantities are known as the effectivity time of the leap second. “Day 1” is defined as the first day relative to the end/start of a week and the WNLSF value consists of the eight least significant bits (LSBs) of the full week number. 61
- Satellite Navigation ▶ The GPS is widely used in navigation. Its augmentation with other space-based satellites is the future of navigation. Navigation Solution (Two-Dimensional Example) Antenna location in two dimensions can be calculated by using range measurements. 62
- Symmetric Solution Using Two Transmitters on Land. In this case, the receiver and two transmitters are located in the same plane, as shown in Fig. with known positions x1, y1 and x2, y2. 63 Fig. Two transmitters with known two dimensional positions
- Contd… Ranges R1 and R2 of two transmitters from the user position are calculated as R1 = c AT1 R2 = c AT2 Where c = speed of light (0.299792458 m/ns) AT1 = time taken for the radio wave to travel from transmitter 1 to the user AT2 = time taken for the radio wave to travel from transmitter 2 to the user (X , Y )= user position 64
- Dilution of Precision The accuracy with which a position can be determined using GPS depends on the accuracy of the individual pseudo range measurements and on the other hand it also depends up on the GPS satellite geometry. This is termed as the Dilution of Precision(DOP). Satellite Geometry can affect the quality of GPS signals and accuracy of receiver trilateration. 65
- Contd… There are 5 distinct kinds of DOP. • GDOP Geometric Dilution of Precision • PDOP Position Dilution of Precision • TDOP Time Dilution of Precision • VDOP Vertical Dilution of Precision • HDOP Horizontal Dilution of Precision 66
- Contd… There are 5 distinct kinds of DOP. • GDOP Geometric Dilution of Precision (Latitude, Longitude, Altitude and clock) • PDOP Position Dilution of Precision (Latitude, Longitude, Altitude ) • TDOP Time Dilution of Precision (Clock) • VDOP Vertical Dilution of Precision (Altitude) • HDOP Horizontal Dilution of Precision (Latitude and Longitude Positions) 67
- Contd… 68
- Computation of DOP Values As a first step in computing DOP, consider the unit vectors from the receiver to satellite i Where, (x,y,z) represents unknown position of the receiver (xi,yi,zi) represents known positions of the satellite 69
- Computation of DOP Values Formulate the matrix, A, which (for 4 pseudo range measurement residual equations) is 70
- Computation of DOP Values The first three elements of each row of A are the components of a unit vector from the receiver to the indicated satellite. The last element of each row refers to the partial derivative of pseudo range w.r.t. receiver's clock bias. Q, as the covariance matrix resulting from the least-squares normal matrix elements of Q are 71
- Computation of DOP Values 72 PDOP, TDOP and GDOP are given by GDOP is the square root of the diagonal elements of the matrix Q.
- Contd… of DOP 73 DOP gives the geometric orientation of the satellites w.r.t the antennas. Values of the DOPs are used for the GPS measurement quality Smaller values of DOP gives the better satellite geometry and accurate user positions, values greater than 5 suggest poor satellite geometry and least accurate user positions. Ideal satellite geometry has one satellite directly above the antenna and remaining three satellites are spread by 120 degree apart.
- Satellite Geometry 74 a) Ideal Satellite Geometry
- Satellite Geometry 75 b) Poor Satellite Geometry
- User Position Calculations With No Errors Position calculation with no errors is given as ρr = pseudorange (known), x, y, z = satellite position coordinates (known), X, Y, Z = user position coordinates (unknown), where x, y, z, X, Y, Z are in the earth-centered, earth-fixed (ECEF) coordinate system. 76
- Contd… Squaring both sides yields where r equals the radius of earth and Crr is the clock bias correction. 77
- Contd… The four unknowns are (X, Y, Z, Crr ). position (x, y, z) is calculated from ephemeris data. For four satellites, the above Eq becomes 78
- Contd… With four unknown state vectors X, Y, Z and Crr We can rewrite the four equations in matrix form as 79
- Contd… Then we pre multiply both sides of above equation with M-1 80
- User Velocity Calculations With No Errors The equation in this case is given as 81
- Contd… 82
- Contd… The above equation is w.r.t one satellite, Similarly the equation for 3 satellites is given as 83
- Contd… Uv is the User velocity N is the Matrix Pseudo range rate 84

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