Global Navigation Satellite Systems (GNSS) revolutionized positioning and navigation. These systems use orbiting satellites to provide accurate location data worldwide. Understanding GNSS is crucial for geospatial engineers working in surveying, mapping, and navigation.

GNSS consists of satellite constellations, ground control stations, and user receivers. Multiple systems like GPS, GLONASS, Galileo, and BeiDou offer global coverage. GNSS signals contain ranging codes and navigation messages, enabling precise positioning despite various error sources.

Overview of GNSS

• Global Navigation Satellite Systems (GNSS) provide accurate positioning, navigation, and timing services worldwide, enabling a wide range of applications in geospatial engineering
• GNSS relies on a network of satellites orbiting the Earth, transmitting radio signals that can be received and processed by users on the ground to determine their precise location and time
• Understanding the principles and components of GNSS is essential for geospatial engineers to effectively utilize these systems in various applications such as surveying, mapping, and navigation

Definition and purpose

• GNSS refers to the constellation of satellites that provide global coverage for positioning, navigation, and timing (PNT) services
• The primary purpose of GNSS is to enable users to determine their precise location, velocity, and time anywhere on or near the Earth's surface
• GNSS has revolutionized the field of geospatial engineering by providing a reliable and accurate means of positioning and navigation, supporting a wide range of applications (surveying, mapping, navigation, and geodynamics)

Key GNSS components

• GNSS consists of three main segments: the space segment (satellites), the control segment (ground stations), and the user segment (receivers)
• The space segment comprises a constellation of satellites orbiting the Earth at an altitude of approximately 20,000 km, transmitting radio signals containing ranging codes and navigation messages
• The control segment consists of a network of ground stations that monitor and manage the satellite constellation, ensuring the accuracy and integrity of the GNSS signals
• The user segment includes the GNSS receivers that capture and process the satellite signals to compute the user's position, velocity, and time

GNSS satellite constellations

• Multiple GNSS constellations have been developed by different countries and organizations to provide global or regional coverage for positioning and navigation services
• Each constellation has its own unique characteristics, such as the number of satellites, orbital configuration, signal structure, and performance specifications
• The existence of multiple GNSS constellations enhances the availability, reliability, and accuracy of positioning and navigation services, particularly in challenging environments (urban canyons, dense forests)

GPS (USA)

• The Global Positioning System (GPS) is the oldest and most widely used GNSS, developed and maintained by the United States Department of Defense
• GPS consists of a constellation of at least 24 satellites, orbiting the Earth in six orbital planes at an altitude of approximately 20,200 km
• GPS satellites transmit signals on two main carrier frequencies (L1 and L2) and offer a variety of ranging codes and navigation messages to support different positioning modes and applications

GLONASS (Russia)

• GLONASS (GLObal NAvigation Satellite System) is the Russian GNSS, developed and operated by the Russian Aerospace Defense Forces
• The GLONASS constellation consists of 24 satellites orbiting the Earth in three orbital planes at an altitude of approximately 19,100 km
• GLONASS satellites transmit signals on two main carrier frequencies (L1 and L2) using a frequency division multiple access (FDMA) technique, which distinguishes it from other GNSS

Galileo (European Union)

• Galileo is the European Union's GNSS, developed and operated by the European Space Agency (ESA) and the European GNSS Agency (GSA)
• The Galileo constellation is designed to consist of 30 satellites (24 operational and 6 spares) orbiting the Earth in three orbital planes at an altitude of approximately 23,222 km
• Galileo satellites transmit signals on three main carrier frequencies (E1, E5, and E6) and offer a range of services (Open Service, Commercial Service, Public Regulated Service, and Search and Rescue Service)

BeiDou (China)

• BeiDou is the Chinese GNSS, developed and operated by the China National Space Administration (CNSA)
• The BeiDou constellation consists of a combination of geostationary (GEO), inclined geosynchronous (IGSO), and medium Earth orbit (MEO) satellites, providing regional and global coverage
• BeiDou satellites transmit signals on three main carrier frequencies (B1, B2, and B3) and offer a variety of services (Open Service, Authorized Service, and Wide Area Differential Service)

Regional and augmentation systems

• In addition to the global GNSS constellations, several regional and augmentation systems have been developed to enhance the performance and coverage of GNSS in specific regions
• Examples of regional systems include the Indian Regional Navigation Satellite System (IRNSS) and the Japanese Quasi-Zenith Satellite System (QZSS)
• Augmentation systems, such as the Wide Area Augmentation System (WAAS) in North America and the European Geostationary Navigation Overlay Service (EGNOS), provide differential corrections and integrity information to improve the accuracy and reliability of GNSS positioning

GNSS signal characteristics

• GNSS satellites transmit radio signals in the L-band frequency range (1-2 GHz), which are used by receivers to determine the user's position, velocity, and time
• The signal structure and characteristics of each GNSS constellation vary, but they generally consist of carrier frequencies, ranging codes, and navigation messages
• Understanding the properties and behavior of GNSS signals is crucial for designing and operating GNSS receivers and for mitigating various error sources that affect the accuracy and reliability of positioning and navigation

Carrier frequencies and modulation

• GNSS signals are transmitted on specific carrier frequencies in the L-band, such as L1 (1575.42 MHz) and L2 (1227.60 MHz) for GPS, and E1 (1575.42 MHz) and E5 (1191.795 MHz) for Galileo
• The carrier frequencies are modulated with ranging codes and navigation messages using techniques such as binary phase-shift keying (BPSK) or binary offset carrier (BOC) modulation
• The choice of carrier frequencies and modulation schemes affects the signal's resistance to ionospheric delays, multipath, and interference, as well as the receiver's ability to track the signal under various conditions

Codes and navigation messages

• GNSS signals contain two main components: ranging codes and navigation messages
• Ranging codes are pseudorandom noise (PRN) sequences that enable receivers to measure the time of arrival (TOA) of the signal and compute the pseudorange between the satellite and the receiver
  • Examples of ranging codes include the Coarse/Acquisition (C/A) code and the Precision (P) code for GPS, and the Open Service (OS) code and the Pilot Channel (PL) code for Galileo
• Navigation messages contain information about the satellite's orbit, clock, health status, and other parameters that are essential for computing the user's position and time
  • Navigation messages are typically transmitted at a lower data rate than ranging codes and are used by receivers to obtain the satellite's ephemeris, almanac, and timing information

Signal propagation and errors

• As GNSS signals propagate from the satellites to the receivers, they are subject to various errors and disturbances that affect the accuracy and reliability of positioning and navigation
• The main sources of errors include ionospheric delays, tropospheric delays, multipath, and interference
  • Ionospheric delays are caused by the dispersive nature of the ionosphere, which affects the signal's group velocity and phase velocity differently, resulting in a range error that varies with the signal frequency and the total electron content (TEC) along the signal path
  • Tropospheric delays are caused by the non-dispersive nature of the troposphere, which affects the signal's propagation velocity and results in a range error that varies with the atmospheric pressure, temperature, and humidity along the signal path
• Other factors that can degrade the signal quality and affect the positioning accuracy include satellite clock errors, orbital errors, receiver clock errors, and receiver noise
• To mitigate these errors, various techniques are employed, such as dual-frequency measurements, differential corrections, and error modeling and estimation

GNSS receiver principles

• GNSS receivers are devices that capture and process the signals transmitted by GNSS satellites to determine the user's position, velocity, and time
• The main components of a GNSS receiver include the antenna, front-end, baseband processor, and navigation processor
• Understanding the principles and operations of GNSS receivers is essential for designing and using these devices effectively in various applications and environments

Antenna and front-end components

• The antenna is the first component of a GNSS receiver, responsible for capturing the incoming satellite signals and converting them into electrical signals
  • GNSS antennas are typically designed to have a hemispherical or near-hemispherical radiation pattern to ensure good visibility of satellites at various elevations and azimuths
  • The choice of antenna type (e.g., microstrip, helical, or choke ring) and its placement on the platform affect the receiver's performance in terms of signal strength, multipath rejection, and phase center stability
• The front-end is the next stage of the receiver, responsible for amplifying, filtering, and down-converting the incoming signals to a lower intermediate frequency (IF) or baseband
  • The front-end typically consists of a low-noise amplifier (LNA), bandpass filters, mixers, and local oscillators
  • The design of the front-end affects the receiver's sensitivity, selectivity, and dynamic range, which are critical factors in determining the receiver's ability to acquire and track weak signals in the presence of noise and interference

Signal acquisition and tracking

• Once the incoming signals are down-converted and digitized, the baseband processor performs the tasks of signal acquisition and tracking
• Signal acquisition is the process of detecting the presence of a satellite signal and determining its rough code phase and Doppler frequency
  • Acquisition is typically performed using a two-dimensional search in the code phase and Doppler frequency domain, where the received signal is correlated with locally generated replica codes at various code phases and Doppler frequencies
  • The acquisition process provides an initial estimate of the signal's parameters, which are then refined by the tracking process
• Signal tracking is the process of continuously updating the estimates of the signal's code phase, carrier phase, and Doppler frequency to maintain lock on the signal and extract the navigation data
  • Tracking is typically performed using a combination of delay lock loops (DLLs) for code tracking and phase lock loops (PLLs) or frequency lock loops (FLLs) for carrier tracking
  • The choice of tracking loop parameters (e.g., loop bandwidth, integration time) affects the receiver's ability to maintain lock on the signal under various dynamics and signal conditions

Positioning algorithms and solutions

• The navigation processor is responsible for computing the user's position, velocity, and time (PVT) based on the measurements obtained from the tracked satellite signals
• The main inputs to the positioning algorithms are the pseudorange and carrier phase measurements, which are used to form the observation equations relating the user's position and clock bias to the measured quantities
  • Pseudorange measurements are obtained by correlating the received signal with a locally generated replica code and measuring the time delay between the two, which is then converted to a range by multiplying with the speed of light
  • Carrier phase measurements are obtained by tracking the phase of the incoming carrier signal and counting the number of whole and fractional cycles, which provides a more precise but ambiguous measure of the range
• The positioning algorithms typically involve solving a set of nonlinear equations using techniques such as least squares estimation, Kalman filtering, or integer ambiguity resolution
  • The choice of positioning algorithm depends on the desired accuracy, availability, and computational complexity, as well as the specific application and environment
• The main types of positioning solutions include single-point positioning (SPP), differential GNSS (DGNSS), real-time kinematic (RTK), and precise point positioning (PPP), which offer different levels of accuracy and require different types of measurements and corrections

GNSS observables and measurements

• GNSS observables are the basic measurements that are obtained from the tracked satellite signals and used to compute the user's position, velocity, and time
• The main GNSS observables are pseudorange, carrier phase, Doppler shift, and signal-to-noise ratio (SNR)
• Understanding the characteristics and properties of these observables is essential for designing and implementing GNSS positioning algorithms and for assessing the quality and reliability of the resulting solutions

Pseudorange and carrier phase

• Pseudorange is the primary observable used in GNSS positioning, representing the apparent range between the satellite and the receiver
  • Pseudorange is obtained by measuring the time difference between the transmission of the signal from the satellite and its reception by the receiver, and multiplying it by the speed of light
  • Pseudorange measurements are affected by various errors, such as satellite clock errors, ionospheric delays, tropospheric delays, and multipath, which need to be modeled or estimated to obtain accurate positioning results
• Carrier phase is a more precise observable that represents the phase difference between the incoming carrier signal and a locally generated reference signal
  • Carrier phase measurements are obtained by tracking the phase of the incoming signal and counting the number of whole and fractional cycles
  • Carrier phase measurements have a much lower noise level than pseudorange measurements, typically on the order of a few millimeters, but they are ambiguous by an unknown number of whole cycles (integer ambiguity)
  • To use carrier phase measurements for precise positioning, the integer ambiguities need to be resolved using techniques such as double differencing or PPP-AR (PPP with ambiguity resolution)

Code and phase smoothing

• Code and phase smoothing are techniques used to reduce the noise and multipath errors in pseudorange measurements by combining them with carrier phase measurements
• Code smoothing involves using a low-pass filter to smooth the noisy pseudorange measurements with the precise but ambiguous carrier phase measurements
  • The smoothed pseudorange is obtained by adding the carrier phase measurement to the low-pass filtered difference between the pseudorange and carrier phase measurements
  • Code smoothing can reduce the noise level of pseudorange measurements by a factor of 10 or more, depending on the smoothing time constant and the signal conditions
• Phase smoothing involves using the precise carrier phase measurements to smooth the noisy Doppler measurements and obtain a more accurate estimate of the receiver's velocity
  • The smoothed Doppler is obtained by differentiating the carrier phase measurements and low-pass filtering the result
  • Phase smoothing can provide a more accurate and robust estimate of the receiver's velocity than using raw Doppler measurements, particularly in high-dynamic applications

Doppler shift and velocity

• Doppler shift is the change in the frequency of the incoming signal due to the relative motion between the satellite and the receiver
  • Doppler shift measurements are obtained by tracking the frequency of the incoming signal and comparing it with the nominal frequency of the transmitted signal
  • Doppler shift measurements can be used to estimate the receiver's velocity and to aid in signal acquisition and tracking
• Velocity is a derived observable that represents the receiver's speed and direction of motion
  • Velocity can be estimated from Doppler shift measurements using the Doppler equation, which relates the observed frequency shift to the relative velocity between the satellite and the receiver
  • Velocity measurements are used in various applications, such as navigation, tracking, and dynamic positioning, where the receiver's motion is of interest

GNSS positioning modes

• GNSS positioning modes refer to the different methods and techniques used to compute the user's position, velocity, and time from the observed satellite signals
• The choice of positioning mode depends on the desired accuracy, availability, and computational complexity, as well as the specific application and environment
• The main GNSS positioning modes are single-point positioning (SPP), differential GNSS (DGNSS), real-time kinematic (RTK), and precise point positioning (PPP)

Single-point positioning (SPP)

• Single-point positioning (SPP) is the most basic and widely used GNSS positioning mode, where the user's position is computed using pseudorange measurements from a single receiver
  • SPP involves solving a set of nonlinear equations relating the user's position and clock bias to the observed pseudoranges, typically using a least-squares estimation method
  • SPP can provide a positioning accuracy of around 5-10 meters (95% confidence level) under good signal conditions, but it is subject to various errors, such as ionospheric delays, tropospheric delays, and satellite clock and orbit errors
• To improve the accuracy of SPP, various techniques can be applied, such as using dual-frequency measurements to estimate and correct for ionospheric delays, using precise satellite orbit and clock products, and applying empirical correction models for tropospheric delays

Differential GNSS (DGNSS)

• Differential GNSS (DGNSS) is a positioning mode that uses pseudorange corrections from a reference station to improve the accuracy of the user's position
  • DGNSS involves using a reference station at a known location to compute the pseudorange errors for each visible satellite and transmitting these corrections to the user's receiver
  • The user's receiver applies the pseudorange corrections to its own measurements to remove the common errors, such as satellite clock errors, ionospheric delays, and tropospheric delays
• DGNSS can provide a positioning accuracy of around 1-3 meters (95% confidence level) under good signal conditions, depending on the quality of the reference station and the baseline length between the reference station and the user's receiver
  • The accuracy of DGNSS degrades with increasing baseline length due to the spatial decorrelation of the ionospheric and tropospheric errors
  • To extend the coverage area of DGNSS, networks of reference stations can be used to generate and broadcast wide-area differential corrections, such as the Wide Area Augmentation System (WAAS) and the European Geostationary Navigation Overlay Service (EGNOS)

Real-time kinematic (RTK)

• Real-time kinematic (RTK) is a high-precision positioning mode that uses carrier phase measurements from a reference station and a user's receiver to compute the user's position with centimeter-level accuracy
  • RTK involves using a reference station at a known