GNSS Systems Introduction PDF

# UNIDAD N° 3 INTRODUCCION A LOS SISTEMAS GNSS ## INTRODUCCION - During the 1970s, the Global Positioning System (NAVSTAR - GPS) emerged, originating in US military applications. - By the mid-1990s, its use expanded to the civilian sector. - New constellations have been added over time, both for...

# UNIDAD N° 3 INTRODUCCION A LOS SISTEMAS GNSS ## INTRODUCCION - During the 1970s, the Global Positioning System (NAVSTAR - GPS) emerged, originating in US military applications. - By the mid-1990s, its use expanded to the civilian sector. - New constellations have been added over time, both for free and restricted use. - GNSS provides precise positioning and synchronization information globally, with high reliability and low cost. - GNSS can be used day and night, in all weather conditions, and on land, sea or air. - It does not require line-of-sight between topographic stations, unlike conventional systems that depend on distances and angles for determining coordinates. ## GNSS - GNSS are constellations of artificial satellites orbiting Earth at different altitudes. - They transmit signals enabling the determination of a receiver's three-dimensional position on Earth. - The calculation of a three-dimensional position involves measuring the distance from at least three satellites with known positions. - Each satellite emits its position to receivers on Earth through ephemeris. - This information allows the measurement of the distance between the receiver and the satellite. - Signal travel time is crucial for distance calculation, as errors can cause significant variations in measurements. - The GNSS satellite clocks are the most crucial elements for accurate measurements. ## COMPOSICION, CARACTERISTICAS Y SEGMENTOS - GNSS is structured into three segments: - Spatial Segment - Control Segment - User Segment ### SPATIAL SEGMENT (Satellites de navegación) - The Spatial Segment is responsible for transmitting coded signals at different frequencies. - It consists of a constellation of satellites that form the navigation and communication systems. - The segment needs sufficient satellites to guarantee global coverage at all times. - Spare satellites in orbit ensure rapid replacement if a primary satellite fails or additional coverage is required. - The navigation satellites are distributed in different orbital planes to ensure visibility from anywhere on Earth. - This distribution is not necessarily uniform. ### SEGMENTO ESPACIAL (satélites de comunicación) - Communication satellites support Satellite-Based Augmentation Systems (SBAS). - They retransmit corrected information from the Control Segment, enhancing the system's accuracy. ### SEGMENTO DE CONTROL - The Control Segment monitors and tracks satellites, transmitting the information to the Master Control Station in Colorado Springs, Colorado, the United States. This information is used for precise orbit predictions and clock corrections. - The Master Control Station downloads this data to satellites, which then transmit it to receivers. This data must be used by receivers to accurately predict satellite positions and clock biases. - The main roles of the control segment include: - Monitoring and controlling satellite orbital parameters. - Monitoring the health and status of satellite subsystems. - Updating navigation messages with ephemeris, almanac, and clock corrections. - Resolving satellite anomalies. - Implementing selective availability and anti-spoofing mechanisms. The Control Segment consists of four primary subsystems: - The Master Control Station - The Alternate Master Control Station - Ground Antennas - Globally distributed monitoring stations. - Each station generates information on system functionality, which is then sent to the master station for corrections. - This information is applied to the satellites or relayed to a geostationary satellite to form an augmentation system. - Satellite positions and timing are used for inverse navigation, determining satellite locations. ### SEGMENTO DEL USUARIO - The User Segment comprises GNSS receivers that obtain signals from the Spatial Segment. - These receivers are used in various sectors including military, mobile, and for individual applications. - This diverse user base is categorized by receiver type and usage. - Receivers acquire satellite signals to determine precise position, velocity, and time data. - Their communication capabilities determine their classification as Active, Passive, or both. - Active systems both receive and transmit signals - Passive systems only receive signals. ### SEGMENTO DE APOYO - This segment encompasses ground stations collecting satellite data and implementing error corrections. It comprises the Control and User segments. ### SATELITES GEOESTACIONARIOS (GEO, Geostationary Earth Orbit) - A significant portion of operational positioning satellites are located in GEO, approximately 35,848 km above Earth's surface. - These satellites follow circular orbits around Earth's equator, completing one revolution every 24 hours. - From an observer's perspective on Earth, the satellite appears stationary. ### SATELITES DE ORBITA MEDIA (ΜΕΟ, Mean Earth Orbit) - These satellites orbit between 19,180 and 28,000 km above Earth. ### SATELITES DE ORBITA BAJA (LEO, Low Earth Orbit) - LEO satellites orbit at an approximation of 800 km and are primarily used for observation. ## CALCULO DE POSICION - The trajectory of satellites is determined by their almanac, which provides orbit parameters valid for about 6 months, or by their ephemeris, which indicates their precise position. - Calculating position is based on the satellite position data and its clock information received from the satellite. - The receiver uses the ephemeris data to determine the satellite's position. - The receiver measures the distance to the satellite by calculating the signal travel time. - The signal travels at the speed of light. Based on this speed and the travel time, the distance between the receiver and the satellite can be calculated. - Each satellite indicates the receiver's position on the surface of a sphere with the satellite at the center and a radius representing their distance. - Three satellites are needed for three-dimensional positioning. - A fourth satellite is crucial to eliminating timing errors. ## MEDICIONES DE DISTANCIA Determining distances between satellites and the receiver is paramount for positioning. Several methods are used, including: - **The Doppler Effect:** The Doppler effect helps calculate the distance between the receiver and the satellite by analyzing the frequency shift of signals. - **Code Measurement:** Distance can be calculated by measuring the number of full wavelengths and the fractional wavelength of the signal. - **Phase Measurement:** The time elapsed between the transmission and reception of a signal can be measured. - **Phase Measurement (cont.)** Distance can be calculated by measuring the number of full wavelengths and the fractional wavelength of the signal. ## SEÑAL Satellites continually transmit data from two carrier waves, both traveling at the speed of light. These carrier waves are in the L band (radio frequencies). - Carrier waves are derived from the fundamental frequency generated by atomic clocks: - L1 - 1575.42 MHz (10.23 x 154) - L2 - 1227.60 MHz (10.23 x 120) - The L1 carrier is modulated with two codes: - The C/A or Coarse Acquisition code, modulating at 1.023 MHz (10.23/10). - The P or Precision code, modulating at 10.23 MHz. - The L2 carrier is modulated with only one code: - The P code, modulating at 10.23 MHz. ## METODOS DE POSICIONAMIENTO O DE TRABAJO - GNSS positioning methods are diverse and lack a universal classification. - Factors considered in classification include: - Observation Techniques - Instrumentation - Precision Requirements - Processing Methods ### SEGÚN LOS OBSERVABLES - **Code-Pseudodistances:** - Receivers track pseudodistances, which are the approximate distances to satellites. - Simple receivers rely only on the C/A code. - Other receivers can track both C/A and the more precise P code. - Tracking P code offers ten times higher precision. - P code access is restricted. - **Phase Measurement:** - Receivers track both pseudodistances and the carrier phase. - This method offers greater precision than pseudodistances. ### SEGÚN EL MOVIMIENTO DE LOS RECEPTORES - **Static:** The receiver remains stationary for an extended period. - **Kinematic:** The receiver is in constant motion. ### SEGÚN EL MOMENTO EN QUE SE EFECTUA EL CALCULO - **Post Processing:** Position and baseline calculations are performed after observation. - **Real Time:** Position and baseline calculations are performed concurrently with observation. ### ABSOLUTO - A single receiver is utilized. - Coordinates obtained are within the WGS84 geocentric system, whether cartesian or geographic. - These coordinates can be further transformed to a local reference system. - Positioning relies on either the C/A or P code. ### PPK - This technique leverages precise orbits and clocks from GNSS satellites to determine the position of a single receiver. ### RELATIVO O DIFERENCIAL - At least two receivers are required. - The final results are coordinate increments in the XYZ system. - Atmospheric and clock-related errors are largely minimized. ### SEGÚN EL TIEMPO DE OBSERVACION - **Static:** - Multiple static receivers at known locations are used for precise positioning and geodetic purposes. - The observation period is variable based on baseline length. - For short distances (under 20 km), one hour of observation is sufficient. - For medium distances (20-50 km), 2-3 hours are recommended. - Static methods are known for: - High precision - Efficiency - Cost-effectiveness compared to topographic methods - Replacing traditional triangulation methods - Static positioning's applications include: - Geodetic Control - National and International Networks - Tectonic Movement Monitoring - Construction and Engineering Replanting - Deformation monitoring in dams and structures. - Static observation times and expected accuracies are summarized in a table: - Baseline Length | Observation Time | Precision (Horizontal) - --------------------|-----------------------|-------------------------- - ≤ 20 km | 1 hour | 3 mm + 1 ppm - 20-50 km | 2 hours | 3 mm + 1 ppm - 50-100 km | Minimum 2 hours | 3 mm + 1 ppm - > 100 km | Minimum 3 hours | 3 m + 1 ppm - **Static Rapid:** - This method utilizes shorter observation periods and baselines for rapid location determination. - **Kinematic:** - This technique is crucial for positioning moving objects. - It begins with an initialization phase to resolve ambiguities. - Sustained ambiguity resolution is essential. - Kinematic positioning uses two receivers, one stationary for reference and one mobile for positioning. - Accuracy is dependent on continuous ambiguity maintenance. - If ambiguity is lost, the process must be re-initialized. - The method is well-suited for: - Control Surveys - Precise Positioning - replacing traditional polygonation methods. - **Kinematic-RTK:** - This differential method combines two GNSS receivers, one at a fixed known position (Reference Station) and one moving (Rover). - The Rover's position is determined relative to the Reference Station. - It helps mitigate specific errors in GNSS absolute methods. - Kinematic-RTK applications include: - Trajectory Determination - Precise Positioning of Moving Objects - Detailed surveying for construction projects - Hydrographic Surveys - Bathymetric Surveys ### TIPO DE APLICACION GNSS - GNSS applications span various fields: - Control Surveys - Topographic Surveys - Replanting - Precise Rapid Positioning - SIG (Geographic Information System) - Navigation - GNSS methods are used in: - RTK-NTRIP - Static Positioning - Differential Positioning - Autonomous Positioning. ## METODOS DE OBSERVACION - A graphical representation of GNSS positioning methods' classifications is provided: - Positional Methods: - Absolute - Relative - Timing of Observation: - Real Time - Post Processing - Method of Observation: - Static - Kinematic. ## CONSTELACIONES GNSS - Four main GNSS constellations exist: - **NAVSTAR-GPS** (United States) - **GLONASS** (Russia) - **GALILEO** (European Union) - **BEIDOU** (China) - Other regional systems include: - **QZSS** (Japan) - **IRNSS/NavIC** (India) ### NAVSTAR-GPS - The globally used navigation satellite system, operated by the United States. - Consists of 31 satellites, approximately 20,200 km in orbit around Earth. - Satellites are in six orbital planes inclined at 55° to the equator. ### GLONASS - A counterpart to GPS and Galileo. - Operated by Russia. - Features 24 Satellites (21 operational, 3 in reserve) in three orbital planes, each with 8 satellites. - Satellites orbit at an inclination of 64.8°. - Orbital altitude is slightly lower than GPS, at approximately 19,100 km. - The orbital period is about 11 hours and 15 minutes. - GLONASS is particularly effective in polar regions. ### GALILEO - Developed by European Union. - Consists of 26 satellites in three orbital planes. - Orbital altitude is about 23,222 km. - Offers five levels of service. ### BEIDU - A global satellite navigation system developed by China. - Consists of 35 satellites: - 5 geostationary satellites - 27 medium earth orbit (MEO) satellites - 3 inclined geosynchronous orbit (IGSO) satellites. - Orbital altitude is approximately 21,150 km. ### QZSS - A system operated by Japan. - Consists of four satellites. - 3 in highly elliptical orbit (HEO), at approximately 32,000 km altitude. - 1 geostationary orbit (GEO) satellite, at approximately 40,000 km altitude. - Includes a master control station, tracking stations, laser measurement stations, and monitoring stations. - Focuses on Asia. ### NAVIC IRNSS - A regional system operated by India. - Features a constellation of seven satellites: - 3 GEO satellites at 32.5°, 83°, and 131.5° longitude. - 4 Inclined GEO satellites crossing the equator at 55° and 111.5° longitude. - The four inclined GEO satellites follow a distinctive figure-eight orbital pattern. ## ESTADO GNSS - A table summarizes the operational status of these constellations: - Constellation | Satellites | Operational Satellites | Signals | Frequencies - -------------- | -------- | --------------------- | --------| -------- - GPS | 24 | 31 | 8 | 3 - GLONASS | 24 | 24 | 5 | 3 - BEIDOU | 27 MEO, 3 IGSO, 5 GEO | 27 MEO, 6 GEO, 10 IGSO | 6 | 4 - GALILEO | 30 | 22 | 5 | 4 - QZSS | 3 IGSO, 1 GEO | 3 IGSO, 1 GEO | 2 | 4 - IRNSS/NAVIC | 4 IGSO, 3 GEO | 4 IGSO, 3 GEO | 2 | 1 ## ERRORES - GNSS accuracy is essential for its use in fields like Topography, Geodesia, and Geomatics. - Error sources affect the system's precision in various ways: - Signal propagation - Satellite and receiver clocks - Environmental factors - Multipath effects. ### SOURCES OF ERROR - GNSS errors are caused by: - **Satellite and Receiver Clock Errors:** - Satellite clock errors (dts) arise from drifts and offsets compared to the GNSS system's time. These are mitigated by the system's clock correction process. - Receiver clock errors (dtr) occur when receiver clocks deviate from system time. This impacts the accuracy of signal time measurements and subsequent distance calculations. - **Atmospheric Refraction:** - Signal paths through the ionosphere and troposphere are affected by refraction. - Signal travel time changes due to variations in atmospheric conditions, affecting distance calculations. - Refraction is more pronounced at lower frequencies. - To mitigate these errors, measurements are taken at multiple frequencies, enabling atmospheric delay corrections. - **Multipath:** - Occurs when signals reflect off multiple surfaces and reach the receiver at different times with different strengths. - The result is a distorted signal, degrading navigation accuracy. - Multipath errors are exacerbated in urban environments with numerous reflecting surfaces. - **Ephemeris Errors:** - Ephemeris data predict the satellite's position. - However, factors like gravity, solar radiation pressure, and atmospheric drag influence the satellite's actual orbit. - These inaccuracies can lead to positioning errors. - **Other Errors:** - Improper Antenna Mounting: - Improper placement of the antenna during installation can result in significant positioning errors. - Ensuring the antenna is level and centered over the reference point is important to mitigate these errors. - **Geometric Dilution of Precision (GDOP):** - GDOP quantifies the geometric strength of satellite coverage. - It indicates the uncertainty of the receiver's position based on satellite positions relative to the receiver. - A higher GDOP indicates greater uncertainty in the calculated position. - GDOP is categorized into: - **HDOP (Horizontal Dilution of Precision):** Impacts horizontal positioning accuracy - **VDOP (Vertical Dilution of Precision):** Influences vertical positioning accuracy. - **PDOP (Position Dilution of Precision):** Impacts both horizontal and vertical positioning accuracy. - **Space Weather:** - Solar activity, especially solar flares, impact the ionosphere's density. - This impacts GNSS signal propagation and increases refraction errors. - GNSS accuracy is diminished during periods of high solar activity. ## ERROR CORRECTIONS - Various techniques are used to address GNSS errors: - **Differential Positioning:** - Uses two or more receivers, one fixed as a reference station and the other moving. - The fixed receiver provides corrections to the moving receiver's data, enhancing accuracy. - Differential positioning can be real-time or post-processed. - **Satellite-Based Augmentation Systems (SBAS):** - Enhance GNSS positioning accuracy by transmitting signal corrections from geostationary satellites. - **Real Time Kinematic (RTK):** - A precise and rapid method to quickly determine positions. - Real-time corrections are transmitted from a reference station to a rover, enabling centimeter-level accuracy. - **Precise Point Positioning (PPP):** - A method that uses high-precision orbital and clock data to determine the coordinates of a receiver directly. - It achieves accuracy comparable to RTK positioning. - GNSS positioning is a complex process, and understanding error sources and mitigation techniques is crucial for its effective application. **Note:** This summary is based on page 1-16 of the document. It omits the last page since it is missing.