Introduction to Satellite Positioning Technology
Last updated
Last updated
Positioning Technology Overview
Positioning technology can be categorized into two main types based on the measured parameters: absolute positioning (which provides heading, latitude, longitude, or planar coordinates) and relative positioning (which involves 3-axis angular velocity and 3-axis acceleration). To meet basic navigation needs, mass-produced vehicles typically include systems like Global Positioning System (GPS), Inertial Measurement Unit (IMU), and wheel speed sensors, which cater to both absolute and relative positioning requirements (road-level). However, for L2+ autonomous driving applications, more accurate positioning is required, such as lane-level positioning, and the need for higher precision increases as the level of autonomy grows.
For absolute positioning, the addition of Real-Time Kinematic (RTK) technology, combined with vehicle-end sensors and HD maps for matched positioning, enables centimeter-level accuracy. For relative positioning, imaging sensors are utilized for inter-frame pose estimation techniques like Visual-Inertial Odometry (VIO) and Laser-Inertial Odometry (LIO).
Satellite-based positioning technologies include a range of systems such as Global Navigation Satellite Systems (GNSS), Differential Global Positioning System (DGPS), Real-Time Kinematic (RTK), and Precise Point Positioning (PPP).
1. Global Navigation Satellite System (GNSS)
GNSS refers to all satellite navigation systems, including global, regional, and augmentation systems. These systems provide 3D coordinates, velocity, and time information to users anywhere on the Earth's surface or in near-Earth space, regardless of weather conditions.
USA
GPS
N/A
WAAS (Wide Area Augmentation System)
Russia
GLONASS
N/A
SDCM (Satellite-based Differential Correction Message)
China
COMPASS
N/A
BDSBAS (BeiDou Satellite-based Augmentation System)
Europe
Galileo
N/A
EGNOS (European Geostationary Navigation Overlay Service)
Japan
N/A
QZSS
MSAS (MTSAT Satellite-based Augmentation System)
India
N/A
IRNSS
GAGAN (GPS Aided GEO Augmented Navigation)
GNSS positioning relies on pseudo-range measurements, which are prone to errors such as ephemeris errors, satellite clock errors, and atmospheric disturbances. Since the measurements include four unknowns—three coordinates for the receiver and the satellite clock error—at least four satellites are required to obtain a solution.
GNSS signals are composed of three primary components:
Carrier Wave: GPS operates on two frequency bands, L1 (15410.23 MHz) and L2 (12010.23 MHz).
Pseudo-random Noise Code: This includes the C/A code (1.023 MHz, 1023 bits for civilian use) and P code (10.23 MHz for military use).
Navigation Information: This includes satellite health data, ephemeris data, clock corrections, and ionospheric model parameters.
The theoretical lower limit for GNSS single-point positioning accuracy using the C/A code is typically around 10m CEP due to various error sources, including alignment and atmospheric effects.
2. GNSS Differential Augmentation Systems
To improve positioning accuracy, differential augmentation systems such as Ground-Based Augmentation System (GBAS) and Satellite-Based Augmentation System (SBAS) are used.
GBAS: By installing numerous reference stations with known coordinates, GBAS provides corrections for errors like ephemeris, clock errors, and ionospheric disturbances.
SBAS: Similar to GBAS, SBAS uses a network of reference stations to collect satellite data. This data is processed to correct errors, which are then transmitted to users via a geostationary satellite, improving positioning accuracy.
RTK is a carrier-phase differential positioning technique that uses real-time carrier phase measurements from a reference station with known coordinates. These measurements correct common errors and are broadcast to mobile stations, improving positioning accuracy. RTK can achieve centimeter-level accuracy, typically around 1 cm + 1 ppm.
RTK accuracy is influenced by the time resolution of the signal. The L1 carrier frequency is much higher than that of the C/A code, allowing for a finer range measurement and greater accuracy.
While RTK technology delivers high accuracy, its performance can degrade over long distances between base stations. Precise Point Positioning (PPP) overcomes this limitation by utilizing a single dual-frequency GNSS receiver along with satellite ephemeris and clock products from the International GNSS Service (IGS). This technique allows for accurate single-point positioning, even in the absence of a local reference station.
Principle
Single-point Positioning
Differential positioning C/A code
Differential positioning C/A code
Differential positioning Carrier phase
Single-point Positioning Carrier phase
Accuracy
Meter-level
Decimeter-level
Decimeter-level
Centimeter-level
Decimeter-level
Received Content
GNSS Signals (C/A code)
GNSS Signals (C/A code) The total error correction from base station
GNSS Signals (C/A code) The total error correction from base station
GNSS Signals (Carrier phase) The total error correction from base station
GNSS Signals (Carrier phase) The precise ephemeris and clock error products from IGS
Applicable Scenarios
Global
Within a radius of 40 km from the reference station
Global
Within a radius of 40 km from the reference station
Global
Initialization Time
Second-level (No need to set up base stations)
Second-level (After the base station network is established)
Second-level (After the base station network is established)
Second-level (After the base station network is established)
30 min (No need to set up base stations)
Disadvantages
Low positioning accuracy
Requires the deployment reference stations
Incomplete coverage in high latitude areas
Requires the deployment reference stations
Long initialization time
