GNSS Principles and Observables for Positioning Using Pseudo-Ranges Lecture 1

GNSS Principles and Observables for Positioning Using Pseudo-Ranges (Stand-Alone) Prepared by Lecturer: Muhsin Khalid 1 GNSS Basics Global Navigation Satellite Systems A space-based satellite navigation system that provides auto...

GNSS Principles and Observables for Positioning Using Pseudo-Ranges (Stand-Alone) Prepared by Lecturer: Muhsin Khalid 1 GNSS Basics Global Navigation Satellite Systems A space-based satellite navigation system that provides autonomous positioning and timing information with a global coverage. Global Navigation Satellite System (GNSS) is the standard generic term for all navigation satellites systems like GPS, GLONASS, GALILEO, BeiDou, QZSS, NAVIC 2 GNSS Basics 3  Autonomous (Pseudo-Ranges) (Stand- Alone) GPS Positioning Measure position by measuring ranges to satellites by using Low cost receiver. A few satellites can serve an unlimited number of users on the ground, anywhere in the world Measured ranges are called pseudo-ranges pseudo‐random (PRN) uses L1 frequency only. Why call it a “pseudorange”? Range is the distance from satellite to receiver, plus path delays. Pseudo-range is distance plus effects of clock error 4 GPS Space Segment: Curent & Future Constellation 5 Codes in GPS System C/A Code (Coarse/Acquisition Code): 1. Purpose: Primarily used for civilian GPS applications. 2. Structure: The C/A code is a binary code that is 1,023 bits long, repeating every millisecond. It is generated using a specific algorithm based on a Gold code. 3. Frequency: The C/A code is transmitted on the L1 frequency (1575.42 MHz). 4. Functionality: It allows civilian GPS receivers to acquire and track the GPS signals. The C/A code enables the receiver to determine its position, velocity, and time (PVT) information. 5. Accuracy: The accuracy of the C/A code is generally sufficient for most civilian applications, providing a positioning accuracy of about 5 to 10 meters. 6 P-Code (Precise Code) 1. Purpose: Used for military applications and provides higher accuracy compared to the C/A code. 2. Structure: The P code is much longer, with a length of 2^10 weeks (approximately 19.7 years) and is generated using a more complex algorithm. It can be encrypted (as Y code) for secure military use. 3. Frequency: The P code is transmitted on both L1 and L2 frequencies (1227.60 MHz). 4. Functionality: The P code provides more precise positioning information and is used in military and authorized applications. It improves accuracy by mitigating errors caused by atmospheric conditions and other factors. 5. Accuracy: The P code can achieve positioning accuracies of about 1 meter or better. 7 GNSS: How does it work? Determine the Distance using Radio Wave 8  Distance = Velocity x Time Distance = c x ∆𝒕 Distance =𝐜 𝐱 (∆𝒕 − 𝑬𝒕) 𝑫 + ∆𝑫 = [ 𝑿𝒔 − 𝑿𝒓 𝟐 + 𝒀𝒔 − 𝒀𝒓 𝟐 + 𝒁𝒔 − 𝒁𝒓 𝟐] 9 10 11  Error Sources These are the factors that make it difficult for a GNSS receiver to calculate an exact position. there are several sources of error that degrade the GNSS position from a theoretical few metres to tens of metres. These error sources are: 1. Satellite. 2. Signal Propagation. 3. Receiver. 4. Satellite Geometry. 5. Selective Availability (S/A). 6. Anti Spoofing (A-S). 12  Error Sources 1) Satellite Clock : The atomic clocks in the GNSS satellites are very accurate, but they do drift a small amount. Unfortunately, a small inaccuracy in the satellite clock results in a significant error in the position calculated by the receiver. For example, 10 nanoseconds of clock error results in 3 meters of position error. 13  Error Sources 1) Satellite Orbit Errors: GNSS satellites travel in very precise, well known orbits. However, like the satellite clock, the orbits do vary a small amount. Also, like the satellite clocks, a small variation in the orbit results in a significant error in the position calculated. Even with the corrections from the GNSS ground control system, there are still small errors in the orbit that can result in up to ±2.5 meters of position error. 14  Error Sources 2) Atmospheric In satellite positioning, the Earth’s atmosphere is normally treated as two separate sections, namely the ionosphere and the troposphere. The ionosphere is considered to extend from approximately 50to 1000km above sea level. It is a region comprising charged particles, the density of which is non-uniform and related to the level of solar activity. The troposphere, which extends from approximately 0 to 50km above sea level, is an electrically neutral medium, in which the state of the lower troposphere is related to the weather. 15  Error Sources Ionospheric error The ionosphere is a dispersive medium at GPS signal frequencies. It delays the pseudo ranges, It affects speed of electromagnetic energy. The amount of affect depends on: 1. frequency differences in L1 and L2 (need “dual-frequency” receivers to correct). 2. Total electron content. 3. Elevation angle. This leads to ~10m errors in stand. Alone observations. 16  Error Sources Tropospheric error The troposphere delays the pseudo-range measurments. The magnitude of this delay is a function of atmospheric pressure (and temperature) and elevation angle, Hydrostatic component which typically accounts for about 80 to 90% of the total tropospheric bias. Wet component which is the distribution of water vapour in the atmosphere accounts for 10 to 20% of the total tropospheric errors. 17  Error Sources 3) Multipath Error Multipath is the problem whereby a signal propagates from the satellite to the receiver by more than one path, due to reflective bodies. The bias due to multipath is a function of the wavelength. For stand-alone positioning using pseudo-ranges, multipath can typically result in an error in receiver coordinates of up to 1m. 18  Error Sources 4) Satellite Geometry Satellite geometry can also affect the accuracy of GPS positioning. This effect is called Geometric Dilution of Precision (GDOP). In general, the wider the angle between satellites, the better the measurement. A low DOP indicates a higher accuracy, and a high DOP indicates a lower accuracy. 19  Satellite Geometry Satellites close to each other have larger uncertainty 20  Error Sources 5) Selective Availability (S/A) In the early days of GPS, the U.S. Government used Selective Availability (S/A) to degrade system accuracy for civilian users. The idea was to prevent a hostile force from using GPS accurately for their own purposes. S/A caused continuously varying position errors that could reach as much as 100 meters. S/A was turned off in 2000 by Presidential Decision Directive. 6) Anti Spoofing (A-S) Spoofing is the transmission of a GPS-like signal intended to “fool” GPS receivers so they compute erroneous times or locations. The military signals are encrypted for anti-spoofing operations. 21 22 1 2 GNSS :(Global Navigation Satellite System) is a satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Some of GNSS systems are currently in operation: ⦁ The United States' Global Positioning System (GPS) ⦁ The Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). ⦁ The Europe's (Galileo) , is slated to reach full operational capacity in 2020. ⦁ Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a network of ground stations. ⦁ There are also some local systems which are launched by independent country such as China (BieDou or Compass), Japan (QZSS), IRNSS – Indian Regional Navigation Satellite System 3 4  History GPS The United States' Global Positioning System (GPS) reached Full Operational Capability on 17 July 1995, completing its original design goals. Advances in technology and new demands on the existing system led to the effort to modernize the GPS system. In 2000, the U.S. Congress authorized the effort, referred to as GPS III. The project involves new ground stations and new satellites, with additional navigation signals for both civilian and military users, and aims to improve the accuracy and availability for all users. Jan. 18, 2023 The sixth Global Positioning System III (GPS III) satellite designed and built by Lockheed Martin (NYSE: LMT) has been launched and is propelling to its operational orbit approximately 12,550 miles above Earth, where it will contribute to the ongoing modernization of the U.S. Space Force's GPS constellation. 5 6 7 ⦁ The GPS system consists of three segments: 1) The space segment: the GPS satellites themselves. 2) The control system, operated by the U.S. military. 3)The user segment, which includes both military and civilian users and their GPS equipment. 1-Space segment The first GPS satellite was launched by the U.S. Air Force in 1978. ⦁ The current GPS constellation includes 24 satellites, each traveling in a 12-hour, circular orbit, 20,200 kilometres above the Earth. The satellites are positioned so that six are observable nearly 100 percent of the time from any point on Earth. ⦁ The GPS satellites are powered primarily by sun-seeking solar panels , with nicad batteries providing secondary power. ⦁ 4 Atomic clocks on board GPS satellites are stable "GPS time itself is designed to be kept within one microsecond, 1 µsec or one-millionth of a second. 8 9 ⦁ The main functions of the Space Segment are to transmit radio-navigation signals, and to store and retransmit the navigation message sent by the Control Segment. ⦁ The GPS Space Segment is formed by a satellite constellation with enough satellites to ensure that the users will have, at least, 4 simultaneous satellites in view from any point at the Earth surface at any time. ⦁ GPS system contain 6 orbital planes with the inclination of 55 degree to equator, therefore, more satellites are visible, on average, at high latitudes, since receivers can track satellites visible over the pole ⦁ Each orbit contain 4 satellites. ⦁ The orbit period of each satellite is approximately 12 hours at an altitude of 20,233km 10 2-GPS Control Segment: The GPS Control Segment consists of:  Master Control Station (MCS) (1 Main + 1 alternate )  Monitor Stations (MS) (17 numbers)  Ground Antenna (GA) (12 numbers ) 11 12 Master Control Station MCS 1. It is located in USA-Colorado and is operated by the United States Air Force. 2. Provides command and control of the GPS constellation. 3. Uses global monitor station data to compute the precise locations of the satellites. 4. Generates navigation messages for upload to the satellites. 5. Monitors satellite broadcast and system integrity to ensure constellation health and accuracy. 6. Routine satellite bus and payload status monitoring. 7. Performs satellite maintenance and anomaly resolution, including repositioning satellites to maintain optimal constellation. 8. Currently uses separate systems (AEP and LADO) to control operational and non-operational satellites. 13 Monitor Stations they are distributed around the world and equipped with atomic clocks standards. 1. Track GPS satellites as they pass overhead. 2. Collect navigation signals, range/carrier measurements, and atmospheric data. 3. Feed observations to the master control station. 4. Utilize sophisticated GPS receivers. 5. Provide global coverage via 17 sites: 6 from the Air Force plus 11 from The National Geospatial-Intelligence Agency (NGA). 14 Ground Antennas 1. Send commands, navigation data uploads, and processor program loads to the satellites. 2. Collect telemetry. 3. Communicate via S-band and perform S-band ranging to provide anomaly resolution and early orbit support. 4. Consist of 4 dedicated GPS ground antennas plus 8 Air Force Satellite Control Network (AFSCN) remote tracking stations. 15 3-User segment: (GPS receivers) A GPS Receiver is a device capable of processing the signal of the GPS satellites and determining the user position, velocity and precise time (PVT) by processing the signal broadcasted by satellites. Any navigation solution provided by a GNSS Receiver is based on the computation of its distance to a set of satellites, by means of extracting the propagation time of the incoming signals traveling through space at the speed of light, according to the satellite and receiver local clocks. 16  Range Determination ⦁ The signals transmitted by each satellite is also generated in the receiver. The receiver uses code matching techniques to determine the time it took the signal to travel from the satellite to the receiver. ⦁ The speed of the signal is closely approximated by the speed of light (299 792 458 m/s), with variations resulting from ionospheric and atmospheric effects modelled from parameters contained in the calendar. ⦁ The distance from the receiver to the satellite, referred to as a pseudorange, is computed by multiplying the signal travel time and the average speed of the signal 17  GPS satellites transmit three signals: 1. The L1 signal is the oldest GPS signal. The GPS L1 band (1575.42 MHz)frequency with Wave lengths L1~190 mm. Since the L1 is the oldest and most established signal, even the cheapest GPS units are capable of receiving it. However, because its frequency is relatively slow it is not very effective at traveling through obstacles. 2. The L2 frequency was implemented after the L1.The L2 band (1227.60 MHz) frequency with wave length L2~244 mm. This allows the signal to better travel through obstacles such as cloud cover, trees, and buildings. 3. L5 is the third GPS signal band with (1176.45 MHz) It is the most advanced GNSS signal yet. 18 Code types of GPS ⦁ Each GPS satellite transmits unique ranging code signals on two frequencies: 1575.42 MHz (L1) and 1227.60 MHz (L2). ⦁ The Coarse Acquisition (C/A) code is transmitted on L1 and can be received by any type of GPS receiver. C/A code, for civilian use, transmits data at 1.023 million chips per second. ⦁ The Precision (P-code) code is transmitted on L1 and L2. P-code is encrypted and available only to users with appropriate decryption equipment provided by the USA Department of Defence. the P code, for U.S. military use, transmits at 10.23 million chips per second. ⦁ Both codes are synchronized to the satellite’s atomic clocks. 19 GPS Navigation Data Message ⦁ Satellites transmit a navigation message that contain: 1-Ephemeris Data ⦁ contains information on week number, satellite accuracy and health, age of data, satellite clock correction coefficients, orbital parameters ⦁ Used for real time satellite coordinate computation which is required in position computation 20 2-Almanac Data ⦁ The satellites also transmit almanac data, which contains an indicator of the health of all the satellites, satellite clock corrections and coarse orbital data, atmospheric delay parameters, and the current GPS time and offset from UTC time. 21 Computation of Position: ⦁ GPS receiver calculates its distance from a satellite by measuring how long a signal from the satellite takes to reach it. It is implied that the receiver is located somewhere on the surface of an imaginary sphere centered at the satellite (about 20000km) ⦁ The distance to the other satellite will also be calculated by the receiver. Similarly a sphere centered at B (satellite2) with a radius R2 can be imagined on whose surface lies the receiver. Since the receiver is R1 distance from A (satellite1) and R2 distance from B (satellite2), it is clear that the receiver will be on either of the points of intersection of the two spheres 22 ⦁ The distance calculated from the third satellite will add one more sphere to be imagined on whose surface lies the receiver. This gives rise to only one valid intersection i.e. the point where the three spheres intersect is the position of the receiver in a two dimensional space. ⦁ A GPS receiver determines its position by using the signals that it receives from different satellites. Since the receiver must solve for its position (X,Y,Z) and the clock error (d). 23