What is GNSS, how does it work and why do we use it?

1. WHAT IS GNSS?

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Global Navigation Satellite System (GNSS) is a general term for any satellite constellation that provides global or regional positioning, navigation and timing services. GNSS devices are used in practically all fields in the modern world. Be it recording the route of sports activities, using navigation in a car, aviation, marine traffic, tracking, tracing, surveying and more.

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2. HOW DOES GNSS WORK?

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The operation of GNSS devices is quite complex. To understand how they work, we need to break them down into at least 3 sets and understand the basic principles of operation of each of them.
 

2.1. Space segment
It refers to satellites in Earth's orbit launched by the countries listed below. A satellite constellation is arranged in evenly spaced orbital planes, with at least four satellites in each plane. This arrangement ensures that at least four satellites are visible at least 15° above the Earth's horizon at almost any time, from any point on the planet, although in reality there are more.

Despite the fact that the satellites differ in age and design, their principle of operation remains the same. Each satellite contains four highly accurate clocks with a base frequency of 10.23 MHz that continuously transmit two L-band carriers that travel back to Earth at the speed of light. These carrier waves are called L1 and L2. Carrier waves are important because they bring information from the satellite back to Earth, and it is this information that allows our GNSS receiver to determine where we are (GPS.GOV, 2022)
 

2.2. Control segment

The control segment refers to the many Earth stations located around the world that are used to track, control and send information to each of the GNSS satellites. This is an important role as it is critical that the clocks in each satellite are synchronized. The entire system depends on time.Timing).

Also, the orbital information transmitted to each satellite is critical, as we need it to determine where the satellite was when the information was sent. All information is transmitted to the satellites and transferred to your GNSS receiver in the L1 carrier navigation message (GPS.GOV, 2022)
 

2.3. User segment

The user segment is the part that interests most people. This segment includes everything related to a GNSS receiver: satellite navigation, mobile phones, drones, law enforcement and the like. So how does it work?

 

As we wrote before, there is a constellation of satellites that orbit above us and send a constant stream of information back to Earth at the speed of light. Understanding how this helps to pinpoint our location is quite complex, but it is based on a process called trilateration(GPS.GOV, 2022).

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We should also change a common misconception. At no time does the GNSS receiver in your satnav or phone send any information to the satellites. The receivers we use today are completely passive - they only receive information and do not transmit it. Only the European Galileo system has slightly different receivers, as there is an emergency function that sends information when activated, but this does not apply to general use.

Simply explained, a GNSS receiver receives signals from satellites and determines where it is located. When it knows its position, it sends this information using some other system, for example a GSM data connection, back to a specific monitoring station (GPS.GOV, 2022).

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2.4. TYPES OF GNSS SYSTEMS

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          _cc781905-5cde-3194 -bb3b-136bad5cf58d_         BeiDou Navigation Sattelite System (BDS)

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BeiDou or BDS is a global GNSS owned and operated by the People's Republic of China. The BDS was officially launched in 2020. The operational system consists of 35 satellites. BDS was previously called Compass.

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          _cc781905-5cde-3194 bb3b-136bad5cf58d

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Galileo is a global GNSS owned and operated by the European Union. The EU announced the launch of Galileo services in 2016 and   established a system of more than 24 satellites by 2021 (GPS.GOV, 2022).

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          _cc781905-5cde-3194 -bb3b-136bad5cf58d_     GLONASS

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GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema or Global Navigation Satellite System) is a global GNSS owned and operated by the Russian Federation. A fully operational system consists of 24+ satellites (GPS.GOV, 2022).

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          _cc781905-5cde-3194 -bb3b-136bad5cf58d_        Indian Regional Navigation Satellite System (IRNSS) / Navigation Indian Constellation          _cc781905-5cde-3194 -bb3b-136bad5cf58d_          ( NavIC)

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IRNSS is a regional GNSS owned and operated by the Government of India. IRNSS is an autonomous system designed to cover the Indian region and a 1500 km belt around the Indian mainland. The system consists of 7 satellites. In 2016, India renamed IRNSS as the Navigational Indian Constellation (NavIC), which means "sailor" or "navigator" (GPS.GOV, 2022).

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          _cc781905-5cde-3194 Quasi-Zenith Satellite System (QZSS)

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QZSS is a regional GNSS owned by the Japanese government and operated by QZS System Service Inc. (QSS). QZSS complements GPS to improve coverage in East Asia and Oceania. Japan announced the official launch of QZSS services in 2018 with 4 operational satellites and plans to expand the constellation to 7 satellites by 2023 for autonomous capability (GPS.GOV, 2022).

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          _cc781905-5cde-3194 GPS

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The Global Positioning System (GPS), originally Navstar GPS, is a satellite radio navigation system owned by the United States government and operated by the United States Space Force. It is one of the Global Navigation Satellite Systems (GNSS) that provides geolocation and timing information to a GNSS receiver anywhere on or near Earth where there is an unobstructed view of four or more GPS satellites. The GPS project was started in 1973 by the US Department of Defense. The system consists of more than 24 satellites (GPS.GOV, 2022).

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3. WHY DO WE USE GNSS?

 

At Precizni vinogradi, we initially use a geodetic GNSS receiver to improve the absolute and relative accuracy of the analyzes and models. The accuracy of the GNSS receiver, which is built into the unmanned aircraft itself, is relatively low. According to the manufacturer's instructions, it is up to +- 1.5 mv horizontal and +- 0.5 mv vertical. Using the geodetic GNSS receiver and the method of Earth control points (eng.Ground Control Points– GCP) the absolute and relative accuracy increases to a few centimetres.

When using a geodetic GNSS receiver, we use the RTK measurement method (Real-Time Kinematic positioning) with the aim of achieving centimeter accuracy. The latter is based on receiving position corrections in real time from 16 permanent stations, which are mostly evenly distributed across the Slovenian territory. In addition, GNSS stations from neighboring countries (Austria, Hungary and Croatia) are also used to improve position accuracy. For the purpose of relatively constant availability of the RTK measurement method, the distances between permanent GNSS stations are lower than 70 km. Permanent GNSS stations operate 24 hours a day and 365 days a year. Their operation is monitored by the monitoring center in Ljubljana (GURS, 2022). 

Receiving position corrections can be done in a variety of ways. A very common method that we also use at Precizni vinogradi is the use of an NTRIP server (eng. Networked Transport of RTCM via Internet Protocol), which enables the exchange of time and consequently location data between our receiver (eng. .Rover) and the nearest permanent GNSS station (eng.Bass). Data exchange takes place via a SIM card (3.5 G network), which is built into our geodetic GNSS receiver. In this way, we achieve measurement accuracy down to one centimeter.

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Figure 1: Locational distribution of permanent GNSS stations Signal networks operated by GURS

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Source: GURS, 2022.

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We also use our equipment with advantage for other services, such as the display of plot boundaries based on the spatial data of the land cadastre and for other geodetic measurements. We determine the course of the boundaries of built-up, forest or agricultural plots with an accuracy of up to one centimeter, and we can also carry out geodetic measurements with the same accuracy. .

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4. RESOURCES

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• GPS.GOV, 2022. US Space Force. URL: https://www.gps.gov/systems/gps/ (cited 05/05/2022).

• Signal Network, 2022. Geodetic Administration of the Republic of Slovenia [GURS]. URL: https://gu-signal.si/ (cited 05.05.2022).