Global Navigation Satellite Systems
How It Works
The term GNSS stands for Global Navigation Satellite System(s). A GNSS typically consists of three segments: the satellites orbiting the Earth, stations on the ground to track and monitor the satellites, and users who rely on the satellites to compute their position and motion. There are several independent GNSS in operation today:
- GPS – Operated by the U.S.
- GLONASS – Operated by Russia
- BeiDou – Operated by China
- Galileo – Operated by the European Union
- IRNSS – Operated by India (Regional Navigation Satellite System)
- QZSS – Operated by Japan (Regional Navigation Satellite System)
GPS (Global Positioning System, originally called Navstar GPS) is based on a constellation of 24 satellites operated by the United States Department of Defense (DoD). The satellites circle 22,000 km above Earth twice a day in precise orbits. The satellites constantly transmit right-hand circularly polarized signals back to GPS receivers on the ground.
The signals are transmitted at two frequencies, L1 and L2. The main GPS carrier signal L1, at 1575.42 MHz, is modulated by two codes: the coarse/acquisition (C/A) code also known as civilian code and the precision/secure (P/Y) code, reserved by cryptographic techniques for military and authorized civilian users. The GPS L2 signal, at 1227.6 MHz, only contains the precise code and was established to provide a second frequency for ionospheric group delay correction.
In 2005, the GPS modernization program began with the launch of the first IIR-M satellite, and with it came two new transmitted signals. L2C for civilian users and a new military signal (M cord) in both L1 and L2 to provide better jamming resistance than the previous Y code. Additionally, a new radio frequency link called L5, at 1176.45 MHz, for civilian usage has been activated.
There are two GPS services:
• Precise Positioning Service (PPS): DoD reserves for itself and authorized partners for security reasons
• Standard Positioning Service (SPS): Available free for all worldwide civilian users
Before this update, the DoD would intentionally degrade the accuracies of the SPS or civilian GPS systems to maintain a strategic advantage. However, this practice stopped in the year 2000 to encourage more development of GPS technology which in turn would yield economic and social benefits.
GLONASS (GLObal NAvigation Satellite System) is the satellite navigation system operated by the Russian Aerospace Defense Forces. There are approximately 24 GLONASS satellites in operation orbiting at a similar altitude as GPS satellites. However, GLONASS satellites orbit in a way that provides better coverage at higher latitudes compared to other GNSS.
GLONASS currently broadcasts on two frequencies (G1 and G2) but will expand to three frequencies (adding G3) with future satellite launches. G1 transmits at a frequency of 1602 MHz, G2 at 1246 MHz, and once operational, G3 at 1204 MHz.
There are two GLONASS services:
• Precise Positioning Service (PPS): Used solely by the military and authorized users
• Standard Positioning Service (SPS): Available free for all worldwide civilian users
The BeiDou Navigation Satellite System is operated by the China National Space Administration. Sometimes called COMPASS or BeiDou-2, BeiDou has both regional and global satellites in space. The regional satellites are visible over the eastern hemisphere while the global satellites orbit Earth similar to other GNSS. BeiDou is still being deployed, but provides operational coverage in areas like Asia, Australia, New Zealand, India, Russia, Africa, and Europe. The completed system will have 5 geostationary satellites (GSO) and 30 non-GSO satellites. 27 of the non-GSO satellites will be in the Medium Earth Orbit and 3 in the Inclined Geosynchronous Orbit, and all will help provide worldwide coverage.
The Galileo system is civilian-controlled and operated by the European Space Agency, unlike GPS, GLONASS, BeiDou, IRNSS, and QZSS, which are all military-controlled and operated. The Galileo constellation, when complete, will consist of 30 satellites, transmitting signals on several frequencies that overlap those used in other GNSS.
Galileo will have four frequencies transmitting with three in the lower-band between 1164 MHz and 1300 MHz (E5a, E5b, and E6), and one in the upper-band between 1559 MHz and 1591 MHz (E1).
There are five Galileo services:
• Open Service (OS): Meter-level positioning accuracy, accessible to, and targets mass market and intended for motor vehicle navigation and location-based mobile telephone services
• Contribution to Integrity Monitoring: Integrity monitoring services aimed at users of Safety-of-Life applications
• Commercial Service (CS): Centimeter-level positioning accuracy, for professional or commercial use
• Public Regulated Service (PRS): Restricted to government authorized users
• Search and Rescue Service (SAR): Worldwide search and rescue service to help forward distress signals to rescue coordinates by detecting emergency signals
The Indian Regional Navigational Satellite System (IRNSS) is a Regional Navigation Satellite System owned and operated by the Indian government and developed by the Indian Space Research Organization (ISRO). The system will provide a service area of approximately 1,500 km around India and will have two kinds of services: Special Positioning Service (SPS) and the Precision Service (PS). There are 7 satellites planned to transmit signals on the S-band.
The Quasi-Zenith Satellite System (QZSS) is a Regional Navigation Satellite System owned and operated by the Japanese government. QZSS will consist of 3 satellites in the Highly Elliptical Orbit (HEO) and a fourth in a geo-stationary orbit. There are 6 signals planned for QZSS:
• L1-C/A (1575.42 MHz): Used by combining with GNSS; increase availability of PNT services
• L1C (1575.42 MHz): Used by combining with GNSS; increase availability of PNT services
• L2C (1227.6 MHz): Used by combining with GNSS; increase availability of PNT services
• L5 (1176.45 MHz): Used by combining with GNSS; increase availability of PNT services
• L1-SAIF (1575.42 MHz): Sub-meter class augmentation; interoperable with GPS & SBAS
• LEX (1278.75 MHz): QZSS experimental signal for high-precision (3-cm level) service; compatible with Galileo E6 signal
Always Growing. Always Expanding.
About GNSS Technology
How It Works
A GNSS receiver recognizes where each satellite is in its orbit and compares it with the time required to receive each satellite’s signal. The receiver uses these measurements to calculate its specific position on Earth.
A GNSS receiver can only track satellites orbiting above the horizon. Typically, there are between 6 to 12 satellites visible above the horizon at any one time. The receiver tries to track all visible satellites. If some satellites become blocked or “shaded” by tall buildings or other major obstacles, the receiver automatically attempts to reacquire the blocked signals. Although a GNSS receiver needs at least four satellites to provide a three-dimensional solution (latitude, longitude, and altitude), it can maintain a (latitude-longitude) position using three satellites.
GNSS constellations are designed to provide worldwide positioning services with an accuracy ranging from 5 to 15 meters. More precise accuracies are not possible with standard GNSS, due to minor timing errors and satellite orbit errors, plus atmospheric conditions that affect the signals and their arrival time on Earth. However, there are ways to improve GNSS accuracy using additional services. There are four primary services available, each capable of improving position accuracies to better than one meter:
- Radiobeacon Differential GPS (DGPS) Corrections
- Space-Based Augmentation Systems
- Privately Owned ‘L-Band’ Satellites
- Real-Time Kinematic Positioning
The U.S. Coast Guard and Army Corps of Engineers have established a network of radiobeacons that constantly broadcast differential GPS (DGPS) corrections to receivers. The network covers the U.S. from coast to coast. The Canadian Coast Guard provides similar beacon coverage along its coast lines, the Great Lakes, and the St. Lawrence River. There are similar beacon networks in many other regions worldwide.
One of the main advantages of radiobeacon DGPS is the DGPS corrections are free to anyone with the appropriate equipment, and the equipment is relatively inexpensive. The long-range signals penetrate valleys and urban canyons and travel around obstacles, providing service where other services cannot. The corrections are continuously monitored to ensure their integrity.
Currently most radiobeacon stations transmit only corrections for GPS satellites, although stations in Russia also transmit GLONASS corrections.
Whereas radiobeacon stations broadcast DGPS correction signals from the ground, Space-Based Augmentation Systems (SBAS) broadcast correction signals from geostationary satellites.
SBAS corrections rely on networks of base stations on the ground to monitor GPS satellites. Rather than broadcasting corrections directly to users, the networks send signals up to geostationary satellites, which broadcast the signals back to individual SBAS-capable receivers on Earth.
Like radiobeacon differential signals, SBAS differential signals are free to anyone with appropriate equipment. Instead of calculating corrections at each base station applicable to the area surrounding each site, WAAS, EGNOS, MSAS, and GAGAN determine corrections for very large areas by using all base station data together. This facilitates more uniform corrections, often continent-wide.
For example, the WAAS network of 38 reference stations includes base stations in Canada and Mexico. It can provide relatively uniform accuracy and coverage from the Arctic Ocean to Hawaii to the mid-Caribbean to the mid-Atlantic and the shores of Greenland. By comparison, approximately 100 radiobeacon DGPS stations operated by the U.S. Coast Guard, the Army Corps of Engineers, and the Canadian Coast Guard provide coverage coast to coast in the U.S. and some of Canada’s coastal waters, but no coverage into Mexico.
Currently SBAS corrections only contain full correction information for GPS satellites.
Privately owned satellite systems provide differential correction signals to anyone subscribing to their services. ‘L-band’ refers to the operating frequency range of 1000 – 2000 MHz in the radio spectrum that the private ‘L-band’ satellites use. Their signals are available almost worldwide. As the content of the messages are controlled by the private companies, it is at their discretion if they carry corrections for more than just GPS satellites.
Hemisphere GNSS’ Atlas® GNSS global correction service is an L-band-based correction service that provides scalable corrections for GPS, GLONASS, and BeiDou.
Real-Time Kinematic (RTK) is the highest level of accuracy for GNSS positioning and navigation and comes from a technique relying on a nearby, stationary GNSS reference receiver, called a ‘base’, and a radio link. The base provides more data to the user’s receiver, called the ‘rover’, than other correction methods such as SBAS or beacon corrections. The additional information is called ‘carrier-phase information’ and is the basis for high-accuracy positioning.
RTK positioning requires the GNSS base data to be sent to the rover every second. Typically, a digital radio link is used to transmit from the base to the rover. The rover then solves for a carrier-phase solution. This phase solution is accurate to about one centimeter in most situations. For RTK to work, the base typically must be within 30 to 50 km of the rover. RTK data can contain corrections for any or all GNSS constellations.
Space Weather Status
Conditions, Maps, & Weather Status
Space weather refers to the changing environmental conditions in near-Earth space and the space between the sun and the Earth’s atmosphere. Space weather is influenced by occurrences such as solar flare activity, ionospheric variability, and energetic particle events.
The energy emitted from our sun fluctuates over time. Scientists studying the sun’s activities refer to it as the solar cycle. During times of greater solar activity in the solar cycle, heightened ionospheric activity is observed including periods of scintillation, which can cause GNSS signals to be distorted. This reduces the performance of GNSS receivers. Ionospheric scintillation is most pronounced near the Earth’s magnetic equator, most notably in Brazil.
Below are resources that can be used to track space weather conditions:
Current Scintillation Maps:
Current TEC (Total Electron Content) Maps:
Space Weather Conditions: