Global Navigation Satellite Systems (GNSS)

2. How GNSS Works: Constellations & Signals

At its core, GNSS operates on a fundamental principle: measuring the time it takes for signals to travel from satellites in space to a [cite_start]receiver on Earth[cite: 490]. By precisely calculating these travel times from multiple satellites, a receiver can determine its [cite_start]three-dimensional position[cite: 491]. This section explores the space-based infrastructure that makes this possible and the intricate [cite_start]structure of the signals themselves[cite: 492].

2.1 GNSS Constellations

A Global Navigation Satellite System comprises a constellation of satellites orbiting Earth, continuously transmitting signals that enable users to determine their three-dimensional position with global [cite_start]coverage[cite: 494].

GPS (Global Positioning System)

The Global Positioning System, developed by the U.S. Department of Defense, achieved full operational capability in 1993, with civilian [cite_start]access granted from the 1980s[cite: 496]. The GPS constellation nominally consists of 32 satellites, with 24 typically operational, [cite_start]distributed across six distinct orbital planes[cite: 497]. These satellites orbit at an altitude of approximately 20,200 kilometers (10,900 nautical miles) with an orbital period of about 12 hours, [cite_start]completing two Earth orbits per day[cite: 498].

GLONASS (Global Navigation Satellite System)

GLONASS, the Russian Federation's global navigation satellite system, [cite_start]operates on principles similar to GPS[cite: 501]. The GLONASS constellation is composed of 24 satellites, positioned in three orbital [cite_start]planes[cite: 502]. These satellites orbit at an altitude of approximately 19,100 kilometers (11,900 miles) with an inclination of [cite_start]64.8 degrees[cite: 503]. This specific orbital configuration makes GLONASS particularly well-suited for providing robust positioning services in high latitudes, both north and south, where GPS signal [cite_start]availability can sometimes be challenging[cite: 504].

Galileo (European Union)

Galileo is Europe's independently developed and civilian-controlled [cite_start]GNSS, which became operational in December 2016[cite: 507]. The system is fully funded and owned by the European Union, emphasizing its [cite_start]autonomy and public service orientation[cite: 508]. Galileo is designed to offer superior accuracy and reliability, with reported capabilities of achieving 1-meter accuracy, making it three to four [cite_start]times more accurate than standard GPS services[cite: 509].

BeiDou (China)

The BeiDou Navigation Satellite System (BDS) has been independently developed and operated by China to meet its national security and economic development needs, providing comprehensive PNT services to [cite_start]global users[cite: 513]. BeiDou is unique among global GNSS constellations due to its hybrid constellation architecture, which combines satellites in three distinct types of orbits: Geostationary Earth Orbit (GEO), Inclined Geo-Synchronous Orbit (IGSO), and Medium [cite_start]Earth Orbit (MEO)[cite: 514, 515].

Regional Systems (QZSS, NavIC)

In addition to the global constellations, several regional GNSS systems exist, designed to complement global systems and provide enhanced [cite_start]accuracy and reliability within specific geographic areas[cite: 520].

• QZSS (Quasi-Zenith Satellite System): Japan's regional GNSS, QZSS, is designed to augment GPS and improve positioning accuracy, especially in urban and mountainous areas where GPS signals may be [cite_start]obstructed[cite: 521].
• NavIC (Navigation with Indian Constellation): India's regional navigation system, formerly known as IRNSS, focuses on providing positioning services within India and its surrounding areas [cite_start][cite: 524, 525].

2.2 GNSS Signal Structure

GNSS satellites continuously transmit radio waves that carry the [cite_start]essential information for positioning[cite: 531]. These signals are complex, consisting of three main components: a carrier wave, a [cite_start]pseudorandom noise (PRN) code, and a navigation message[cite: 532].

Carrier Waves

The carrier wave is a high-frequency sinusoidal wave that serves as the [cite_start]foundation for transmitting the other signal components[cite: 533]. For GPS, the original design utilized two primary frequencies: L1 at [cite_start]1575.42 MHz and L2 at 1227.60 MHz[cite: 534].

Pseudorandom Noise (PRN) Codes

PRN codes are unique binary sequences (strings of zeros and ones) that [cite_start]appear random but are, in fact, deterministic and repeatable[cite: 537]. Each satellite transmits a unique PRN code, enabling a receiver to distinguish signals from different satellites even though they may share the same carrier frequency, a technique known as Code-Division [cite_start]Multiple Access (CDMA)[cite: 538].

Navigation Messages

Superimposed on the carrier wave and PRN code is the navigation message, a crucial component that provides vital information for [cite_start]position calculation[cite: 543]. This message is transmitted at a relatively low data rate (e.g., 50 bits per second for GPS C/A code) [cite_start][cite: 544].

• Ephemeris Data: Precise orbital information for the [cite_start]transmitting satellite[cite: 546].
• Almanac Data: Lower-resolution orbital information and status [cite_start]for all satellites in the constellation[cite: 548, 549].
• Satellite Clock Data and Corrections: Information about the [cite_start]precise atomic clock on board the satellite[cite: 550].
• Ionospheric Data: Parameters for ionospheric models, which [cite_start]help receivers estimate and correct for signal delays[cite: 552].
• GPS Date and Time: Provides the precise time reference from [cite_start]the satellite[cite: 553].

2.3 GNSS Receiver Fundamentals

GNSS receivers are sophisticated devices designed to capture the weak radio signals from satellites and convert them into precise [cite_start]positioning and timing information[cite: 557]. This conversion involves several critical signal processing stages, including signal acquisition, signal tracking, decoding navigation data, and ultimately [cite_start]computing the Position/Velocity/Time (PVT) solution[cite: 558].

GNSS Receiver Architecture

The architecture of a generic GNSS receiver typically includes several [cite_start]key building blocks[cite: 559]. The antenna captures the weak [cite_start]incoming signals[cite: 560, 561]. The RF Front-End down-converts and [cite_start]digitizes the signals[cite: 563]. The Baseband Signal Processing section contains multiple parallel channels, each dedicated to [cite_start]acquiring and tracking a single satellite signal[cite: 568, 569]. Finally, the Navigation/Applications Processor uses the measured data to compute the user’s Position, Velocity, and Time (PVT) solution [cite_start][cite: 572].

GNSS Receiver Operations

Regardless of the architecture, all GNSS receivers perform two [cite_start]fundamental processing steps: acquisition and tracking[cite: 581].

• Signal Acquisition: This is the initial process of detecting a satellite’s signal and coarsely aligning its code and frequency [cite_start][cite: 582, 583].
• Signal Tracking: Once acquired, the receiver continuously fine-tunes the alignment of its local signal replica to remain locked [cite_start]onto the satellite’s signal[cite: 585]. This is achieved using feedback loops such as the Delay-Locked Loop (DLL) for pseudorange measurement and the Phase-Locked Loop (PLL) for carrier phase [cite_start]measurements[cite: 587, 590].

These loops continuously produce two types of measurements: [cite_start]pseudorange measurements and carrier phase measurements[cite: 594]. The pseudorange is the apparent distance from the satellite to the receiver, calculated by multiplying the time it takes for the signal to [cite_start]travel by the speed of light[cite: 595]. Carrier phase tracking offers a significantly higher level of precision compared to pseudorange measurements, often by two to three orders of magnitude [cite_start][cite: 600].

3. GNSS Accuracy Factors

The accuracy of a GNSS-derived position is influenced by a variety of error sources, which can be broadly categorized into satellite-related, [cite_start]atmospheric, and receiver-related factors[cite: 606].

3.1 Satellite-Related Errors

Satellite Clock Errors: Even a small inaccuracy in a satellite's atomic clock can translate into a significant error in the calculated position; for example, a 10-nanosecond clock error can result in a 3-meter [cite_start]position error[cite: 610, 611].

Orbit Errors (Ephemeris Errors): Minor variations in a satellite’s orbit can lead to significant position inaccuracies, [cite_start]potentially up to ±2.5 meters[cite: 614, 615].

3.2 Atmospheric Delays

The Earth's atmosphere significantly affects GNSS signals as they propagate from satellites to receivers, causing delays and distortions [cite_start]that introduce positioning errors[cite: 619].

Ionospheric Delay and Mitigation Techniques

The ionosphere is a layer of electrically charged particles in the [cite_start]Earth's upper atmosphere[cite: 621]. As GNSS signals pass through this layer, the charged particles cause a delay, which can be substantial, [cite_start]typically around ±5 meters[cite: 622, 623]. The most effective method for mitigating this is the use of dual-frequency GNSS receivers, which [cite_start]can directly calculate and remove the delay[cite: 626, 627].

Tropospheric Delay and Mitigation Techniques

The troposphere is the lowest layer of the Earth's atmosphere, and it manifests as an extra delay in the signal's travel time due to changes [cite_start]in temperature, pressure, and humidity[cite: 635, 637]. Since the tropospheric delay is not frequency-dependent, it cannot be removed by dual-frequency combinations, so mathematical models are employed [cite_start]to estimate and correct for it[cite: 639, 640].

3.3 Receiver-Related Errors

The GNSS receiver itself can introduce errors that affect positioning [cite_start]accuracy[cite: 647].

• Receiver Noise: An inherent, uncorrelated error source related [cite_start]to the receiver's electronic components[cite: 648].
• Multipath: Occurs when GNSS signals reflect off surfaces such as [cite_start]buildings or water before reaching the antenna[cite: 652]. These delayed signals interfere with the direct signal, causing timing [cite_start]errors and inaccuracies[cite: 653, 654].

3.4 Geometric Dilution of Precision (GDOP)

Geometric Dilution of Precision (GDOP) is not an error source itself, but rather a factor that amplifies the impact of existing errors based on the geometric arrangement of the satellites visible to the [cite_start]receiver[cite: 670]. When satellites are widely spaced in the sky, the GDOP value is low, indicating a strong geometry and a smaller [cite_start]amplification of measurement errors[cite: 672].

4. Differential GNSS (DGNSS) and SBAS (WAAS, EGNOS)

Differential GNSS (DGNSS) and Satellite-Based Augmentation Systems (SBAS) are two primary techniques developed to significantly improve positioning accuracy and integrity by providing real-time corrections [cite_start][cite: 680].

4.1 Principles of Differential GNSS

DGNSS enhances GNSS accuracy by utilizing one or more stationary reference receivers to generate and provide correction data to a [cite_start]user's mobile receiver[cite: 682]. This method exploits the principle that GNSS errors are spatially correlated, meaning two receivers in close proximity will experience very similar errors from the same [cite_start]satellites[cite: 683, 684].

4.2 Satellite-Based Augmentation Systems (SBAS)

SBAS represents a wide-area form of differential GNSS, leveraging geostationary satellites to broadcast corrections and integrity [cite_start]information over vast geographical regions[cite: 704, 705]. WAAS, EGNOS, MSAS, and GAGAN are some of the key SBAS systems [cite_start]globally[cite: 713, 715, 717, 718].

5. High-Precision GNSS Techniques

For applications demanding centimeter-level or even millimeter-level positioning accuracy, standard GNSS and DGNSS are often [cite_start]insufficient[cite: 727].

5.1 Real-Time Kinematic (RTK) – OSR Corrections (Detailed)

Real-Time Kinematic (RTK) is a high-precision differential positioning technique that leverages the phase of the GNSS signal's carrier wave [cite_start]to achieve centimeter-level accuracy in real-time[cite: 730]. The core of RTK's high precision lies in accurately resolving the integer [cite_start]ambiguities associated with carrier phase measurements[cite: 735].

5.2 Precise Point Positioning (PPP) – SSR Corrections (Detailed)

Precise Point Positioning (PPP) is a GNSS method that allows a single GNSS receiver to achieve high-precision positions without the need for [cite_start]a local base station or network[cite: 781]. This unique characteristic means PPP is not limited by baseline length and can provide full accuracy anywhere in the world where GNSS signals are [cite_start]available and correction data can be received[cite: 785]. The high accuracy of PPP is fundamentally dependent on a continuous stream of precise correction data, typically delivered via satellite or over the [cite_start]Internet[cite: 787].

6. Integrity Monitoring & Assured PNT (A-PNT)

Integrity is a paramount performance criterion for GNSS, particularly [cite_start]in applications where human life and property are at stake[cite: 823]. It is defined as the ability of a navigation system to alert the user in a timely manner when the provided navigation information fails to meet [cite_start]specified accuracy requirements[cite: 824].

6.2 Receiver Autonomous Integrity Monitoring (RAIM)

Receiver Autonomous Integrity Monitoring (RAIM) is a technology developed to assess the integrity of GNSS signals directly within the receiver system, without relying on external information or facilities [cite_start][cite: 833]. RAIM detects faults by leveraging redundant GNSS pseudorange measurements when more satellites are visible than the [cite_start]minimum required for a position fix[cite: 835].

6.3 System-Level Integrity

In addition to user-level integrity monitoring like RAIM, system-level [cite_start]approaches provide integrity information from external sources[cite: 842]. Satellite-Based Augmentation Systems (SBAS) are prime examples of system-level integrity providers that broadcast integrity messages from [cite_start]geostationary satellites[cite: 843, 844].

6.4 Assured PNT (A-PNT) Strategies

Given the inherent vulnerabilities of GNSS signals, the concept of [cite_start]Assured PNT (A-PNT) has gained significant prominence[cite: 848, 849]. A-PNT aims to provide continuous, accurate, and reliable PNT services even in environments where GNSS signals are degraded, denied, or [cite_start]deliberately interfered with[cite: 849]. The core of A-PNT lies in sensor fusion, which integrates data from multiple, diverse PNT [cite_start]sources to overcome the limitations of any single system[cite: 851].

Multi-Sensor Fusion (GNSS/INS, Vision-based, Terrestrial Systems)

The most powerful fusion strategy is the integration of Inertial Navigation Systems (INS) with GNSS. INS is immune to external signal interference but suffers from cumulative drift errors over time, which [cite_start]are corrected by GNSS data[cite: 853, 854, 855].

Additional sensor inputs like wheel odometers, magnetometers, or altimeters can further aid the PNT solution, especially in land [cite_start]vehicles[cite: 862].

Vision-Based Navigation Techniques

These methods employ cameras and LiDAR in conjunction with Simultaneous Localization and Mapping (SLAM) algorithms to provide situational [cite_start]awareness and positioning in GNSS-denied environments[cite: 863]. SLAM allows a robot or autonomous vehicle to build a map of an unknown environment while simultaneously keeping track of its own location [cite_start][cite: 864].