Global Navigation Satellite System (GNSS) refers to a constellation of satellites providing signals from space that transmit Positioning, (Velocity) and Timing (PVT) / Positioning, Navigation and Timing (PNT) data to GNSS receivers. The receivers then use this data to determine location.
- Space Segment: Consist of the satellites in the constellation. The space segment are to generate and transmit code and carrier phase signals, and to store and broadcast the navigation message uploaded by the control segment.
- Control: The control segment is responsible for the proper operation of the satellites within the constellation. Its basic functions are:
- to control and maintain the status and configuration of the satellite constellation
- to predict ephemeris and satellite clock evolution
- to keep the corresponding GNSS time scale (through atomic clocks)
- to update the navigation messages for all the satellites
- User Segment: GNSS antenna and receiver. The receivers calculates the raw GNSS measurements (pseudorange, carrier phase, Doppler, carrier-to-noise-density ratio (C/N0) which is used to calculate PVT based on the PNT signals received from the satellites.
GNSS receivers used a multilateration to calculates the receiver's position and time based on pseudorange measurements and the calculated satellite position using the ephemeris parameters contained in the respective GNSS constellations navigation message.
Positioning, Navigation and Timing (PNT)
The U.S. Department of Transportations defines PNT as
- Positioning: The ability to accurately and precisely determine one's location and orientation two-dimensionally (or three-dimensionally when required) referenced to a standard geodetic system (such as WGS84)
- Navigation: The ability to determine current and desired position (relative or absolute) and apply corrections to course, orientation, and speed to attain a desired position anywhere around the world, from sub-surface to surface and from surface to space; and
- Timing: The ability to acquire and maintain accurate and precise time from a standard (UTC), anywhere in the world and within user-defined timeliness parameters. Timing also includes time transfer.
For more on GNSS see for instance ESA navipedia. ESA also have published a Data Processing Book. See
- GNSS DATA PROCESSING - Volume I: Fundamentals and Algorithms
- GNSS DATA PROCESSING - Volume II: Laboratory Exercises
- ESA Tools
for more and free download.
Constellations and systems
In context of GNSS, each satellite system managed by a different country or coalition of countries is referred to as a satellite constellation. List of GNSS constellations:
|NAVSTAR Global Positioning Systems (GPS). Oldest GNSS. Owned by the United States government and operated by the United States Space Force. GPS's standard positioning service started in 1993. For more see GPS ESA navipedia.
|Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS). Owned and operated by the Russian Federation. GLONASS-K2 (next satellite design) scheduled to enter service in 2022. An announcement predicts the deployment of a group of communications and navigational satellites by 2040. Is FMEA based, but there are plans to switch to CDMA. For more see GLONASS Wikipedia, GLONASS ESA navipedia.
|European Union. For more see Galileo ESA navipedia.
|China. For more see BeiDou ESA navipedia.
Time, Timing and Time Systems
|International Atomic Time (TAI). A high-precision atomic coordinate time standard. It is a continuous scale of time, without leap seconds. TAI is the basis for UTC.
|Coordinated Universal Time (UTC). Used for civil timekeeping all over the Earth's surface. UTC deviates from TAI by a number of whole seconds. UTC is a discontinuous time scale due added leap seconds. As of 1st January 2017 UTC is exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972. UTC = TAI − leap seconds. Real- time estimates of UTC are computed and provided by different centres, such as UTC (USNO), from the United States Naval Observatory (USNO); UTC(NIST), from the National Institute of Standards and Technology (NIST); and UTC(SU) from Russia.
|GPS Time (GPST) is a continuous time scale (no leap seconds) defined by the GPS control segment on the basis of a set of atomic clocks. It starts at 0h UTC (midnight) of 6th (transition from 5th to 6th) January 1980. GPST is synchronized with UTC(USNO) at the 1 ms level (modulo 1 s), but actually kept within 25 ns. As of 2022 the is a 18 second difference between UTC and GPST (Wikipedia leap seconds). GPS time is provided to the user as a combination of time of week (TOW) and the current GPS week.
|Glonass Time (GLNT) is generated by the Glonass Central Synchronizer. The difference between UTC(SU) and GLNT should not exceed 1 ms plus 3h. Unlike GPS, Galileo or Beidou, the Glonass time scale implements leap seconds, like UTC.
|Galileo System Time (GST) is a continuous time scale maintained by the Galileo central segment and synchronized to TAI with a nominal offset below 50 ns. The GST start epoch is 0h UTC on Sunday, 22 August 1999.
|BeiDou Time (BDT) is a continuous time scale starting at 0h UTC on 1 January 2006, and is synchronized to UTC within 100 ns (modulo 1 s).
|Local Zone Time (LZT). LZT provides the time offset value between UTC time and the local time.
Other time related parameters
|A leap second is a one-second adjustment that is occasionally applied to UTC, to accommodate the difference between precise time TAI and the imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation.
|TOW or iTow is the cumulative number of decimal seconds in a week starting on Sunday at 00:00:00. It ranges from 0 to 604799, which corresponds to the following Saturday at 23:59:59. Therefore TOW rollovers to 0 each week on Sunday at 00:00:00. Each GNSS satellite broadcasts the current time using this concept of the TOW expressed in seconds and the week number.
|Number of the current GPS week
|Week number rollover
|The GPS week number rollover is a phenomenon that happens every 1024 weeks, which is about 19.6 years. The Global Positioning System (GPS) broadcasts a date, including a week number counter that is stored in only ten binary digits, whose range is therefore 0–1023. After 1023, an integer overflow causes the internal value to roll over, changing to zero again
|Pulse per second
|Time of Validity (TOV) is the time when a sensor measurement is valid
|Time of Arrival (TOA) is the time when a sensor measurement is fully transferred to the receiver (as in the opposite of transceiver) of the measurement.
|Time of Transport TOT is the time the first byte of the package was transferred to the receiver (opposite of transceiver) of the measurement.
GNSS performances are described in terms of
- accuracy of an estimated or measured PVT at a given time is the degree of conformance of that PVT with the true PVT
- integrity is the measure of the trust that can be placed in the correctness of the information supplied by a navigation system. Integrity includes the ability of the system to provide timely warnings to users when the system should not be used for navigation
- availability of a navigation system is the percentage of time that the services of the system are usable by the navigator
- continuity of a system is the ability of the total system to perform its function without interruption during the intended operation
Satellite-based Augmentation System (SBAS)
A Satellite-based Augmentation System (SBAS) is a civil aviation safety-critical system that supports wide-area or regional augmentation through the use of geostationary satellites broadcasting augmentation information. The augmentation information provided by SBAS covers:
- corrections and integrity for satellite position errors
- satellite clock/time errors and
- errors induced by the estimation of the delay of the signal while propagating through the ionosphere
The SBAS compatible GNSS receiver can also use pseudorange measurements from SBAS satellites.
There are currently three SBAS systems operational + several systems in the planning or development stage:
|The European Geostationary Navigation Overlay Service (EGNOS) is the European SBAS service that augments the PVT services provided by the U.S. GPS. Future evolutions such as EGNOS V3 are expected to augment both GPS and Galileo constellation data. EGNOS consist of three GEO satellites. From 23rd March 2020 onwards, PRN123 and PRN136 are operational while the PRN126 is in test mode. On 1st January 2019 PRN 120 was decommissioned. For more see EGNOS ESA navipedia.
|For more see WAAS ESA navipedia.
|For more see MSAS ESA navipedia.
|Others incl. system are under feasibility studies, in development or implementation phase are GAGAN, SDCM, BDSBAS (formerly SNAS), SACCSA and SouthPAN. StarFire is privately operated wide-area differential GPS developed by John Deere's NavCom and precision farming groups. For more see ESA Other SBAS navipedia.
|Indian Regional Navigational Satellite System (IRNSS) is a regional satellite navigation system owned by the Indian government. Renamed to Navigation Indian Constellation (NAVIC) in 2016. For more see NAVIC ESA navipedia.
|The Quasi-Zenith Satellite System (QZSS), is a regional time transfer system and enhancement for GPS what is receivable within and around Japan. It consist of five satellites, where one will replace the initial and oldest QZSS satellite. The constellation is planned be be increased from four to seven. For more see QZSS ESA navipedia.
Differential GNSS, RTK and PPP
Differential GNSS (DGNSS) is one type of GNSS Augmentation technique that is used to improve the primary (standard positioning service) GNSS PVT solution by the use one base/reference station or a network of ground-based base/reference stations (base station(s)). The base station(s) are broadcasting differential information to the user (also known as rover) to improve the accuracy of the user's/rovers' position (integrity is not necessarily assured). The broadcast of the differential information can be done using the internet wirelessly (4G/5G), using radio communication such as UHF or with wired serial communication for moving baseline applications. More on moving baseline is given below.
|The classical dGNSS technique is an enhancement to a primary GNSS system and consist of a rover and a reference/base station. With known (accurate) position of the reference/base station, the deviation the measured pseudoranges to each of the individual satellites can be calculated based on the base stations estimated and know position. These corrections can thereby be used for the correction of the measured positions of other user receivers (rovers).
|Real-Time Kinematics (RTK) is a differential GNSS technique which provides high positioning performance in the vicinity of a base station (within 10th of kilometers). The technique is based on the use of carrier measurement and the transmission of raw GNSS measurements from the base station (with known location) to the rover. By double differencing the measurements taken by the rover with the measurements received from the base the main errors, common errors affecting the base and the rover (that drive the stand-alone positioning error) cancel out.
|Moving baseline RTK (MB RTK) is technique that is similar to RTK, but where the base station is moving with the rover. This enables the rover to estimate its position relative to the base in Earth (ECEF) or local coordinates (NED/ENU). By knowledge the relative mounting vector of the base's and the rover's respective antenna, one can calculate the heading the the platform the base and the rover is mounted on.
|Precise Point Positioning (PPP) is a global precise positioning service using all the available GNSS constellations. PPP requires the availability of precise reference satellite orbit and clock products using a network of GNSS reference stations distributed worldwide. Combining the precise satellite positions and clocks with a dual-frequency GNSS receiver, PPP is able to provide position solutions at centimeter level. Accuracy performance can improved with to a sub-centimeter level in post-processing or using static mode. For more see PPP ESA.
|Post-Processed Kinematic(PPK). Same as RTK but the solution is obtained post process.
Reference frames and procedures
|The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface.
|An International Terrestrial Reference Frame (ITRF) is a realization of the ITRS. ITRF was developed and is maintained by the International Earth Rotation and Reference Systems Service (IERS) (formerly the International Earth Rotation Service). This is a very accurate reference frame. Used for research.
|The World Geodetic System 1984 (WGS84) is the spatial reference system of GPS satellites with an error of less than 2 centimeter.
|Glonass Reference Frame (PZ-90).
|Galileo Reference Frame (GTRF).
|Beidou Reference Frame (CGCS2000).
WGS-84, PZ-90, GTRF and are Earth-fixed-Earth-centered reference frames. For more see Reference Frames in GNSS.
Other terms and abbreviations
Ephemeris: At set of parameters that can be used to calculate the trajectory of celestial objects or satellites. In context of GNSS these are parameters broadcasted in the navigation message (GPS,GLONASS,Galileo) used by the receiver to calculate a respective satellite's position and velocity. See GPS and Galileo Satellite Coordinates Computation for more.
PRN: Pseudo-Random Noise (PRN). Used in context of PRN sequences or codes.
- DOP: Dilution of precision (DOP) is used in satellite navigation to specify the position or time error as function of the navigation satellite geometry on positional and time precision. For position: User equivalent range error x DOP = position precision. Relevant DOPs are
- HDOP – horizontal dilution of precision
- VDOP – vertical dilution of precision
- PDOP – position (3D) dilution of precision
- TDOP – time dilution of precision
- GDOP – geometric dilution of precision
- LOS: Line of sight. Used in context of the direction (unit vector) to a given satellite seen from the user/rover/receiver
- GEO: Geostationary orbit
- CDMA: Code-division multiple access (CDMA) is a channel access method used by various radio communication. CDMA allows multiple access, where several transmitters can send information simultaneously over a single communication channel where each transmitter uses a unique code. GPS and Galileo used this technique.
- FMEA: Frequency-division multiple access (FDMA) is a channel access method used in some multiple-access protocols. FDMA allows multiple users to send data through a single communication channel by dividing the bandwidth of the channel into separate non-overlapping frequency sub-channels and allocating each sub-channel to a separate user. GLONASS uses this technique.
The Radio Technical Commission for Maritime Services (RTCM) is a non-profit international standards organization. Regarding GNSS, Special Committee (SC) 104 on Differential Global Navigation Satellite Systems (dGNSS) provides the relevant standards that are often used in dGNSS and RTK operations. The RTCM SC-104 standard is a communication protocol for sending differential dGNSS (dGPS initially) to a GNSS receiver from a secondary/base station GNSS. The RTCM protocol specifications ara available at http://www.rtcm.org
As of Q1 2022, RTCM 3.3 is the latest message protocol and supports multi constellation, multi frequency measurement satellite navigation corrections and is advised for use.
Receiver Independent Exchange Format (RINEX) is a data interchange format for raw satellite navigation system data. This allows the user to post-process the received/logged data from independent receiver type. The format hence supports different type of receivers for the base and the rover in RTK and dGNSS operations. Also receivers from different vendors.
Other Coordinate systems
|(Earth centered) Inertial. Inertial, not rotating, with respect to the stars. Useful to describe motion of celestial bodies and spacecraft. There exist several approximated ECI frames. One is the J2000 frame. For the J2000, the z-axis upwards from the Earth's center to the North pole along the Earth's rotation axis. The x-axis is aligned with the mean equinox. The y-axis complete the frame.
|Earth-fixed-Earth-centered (ECEF). Not inertial, accelerated, rotating w.r.t stars. Useful to describe motion of objects relative the Earth. Measurements from GNSS systems are given in this frame. There exist different definitions. GPS and the WGS-84 datum defines: The z-axis upwards from the Earth's center to the North pole along the Earth's rotation axis. The x-axis points towards the zero meridian. The y-axis complete the frame.
|North-East-Down (NED). Moving frame. x-axis points North, y-axis points East, z-axis points Up.
|East-North-Up (ENU). Moving frame. x-axis points East, y-axis points North, z-axis points Up.