What is GNSS and how does it bring you precise positioning 

Wondering how to improve your mapping project accuracy with GNSS? This powerful technology leverages signals from multiple satellite constellations to provide positioning with centimeter-level precision. So, let’s explore how GNSS can take your projects to the next level.

What does GNSS stand for?

GNSS stands for Global Navigation Satellite Systems, which are used to determine the precise geographic coordinates of objects. These systems consist of three key components:

• The space segment, or a specific GNSS constellation, consists of satellites evenly distributed across orbital planes.
• The control, or ground, segment is a network of reference and monitor stations that manage the navigation satellite constellation and oversee its core functions.
• The user segment is a variety of user devices designed to receive and process GNSS signals. This segment spans a wide range of devices—from typical consumer products like smartphones, to specialized navigation gear and professional-grade equipment such as RTK GNSS receivers, designed to deliver centimeter-level precision for surveying and mapping.

GNSS ensures reliable and precise positioning, even in challenging environments, by using signals from various satellite constellations, such as GPS, GLONASS, Galileo, and BeiDou.

What are the main satellite systems?

Today, different satellite systems within GNSS operate independently from each other. They were initiated by single countries or regions, with the goal of providing global or regional coverage: 

Satellite system	Country	Number of satellites
GPS: Global Positioning System	United States	31 satellites
GLONASS: Global Navigation Satellite System	Russia	24 satellites
Galileo: Europe’s global navigation satellite system	European Union	23 satellites
BeiDou: BeiDou Navigation Satellite System	China	44 satellites
QZSS: Quasi-Zenith Satellite System	Japan	4 satellites
IRNSS: Indian Regional Navigation Satellite System	India	7 satellites

However, that doesn’t mean each system is limited to use in a specific country or region.

GNSS coverage and orbit characteristics

GNSS satellites orbit at a distance of 20,000 km above the Earth’s surface; therefore, a single orbit around the Earth takes about 12 hours. Most modern satellite systems consist of multiple satellites to provide global coverage and allow satellite signals to be received anywhere on Earth.

Combining different satellite constellations enhances positioning precision, provides redundancy in case of signal loss, and ensures better performance in challenging environments.

Using multiple GNSS constellations—like GPS, GLONASS, Galileo, and BeiDou—not only improves positioning accuracy and reliability but also helps ensure better coverage depending on your location. Using more than one global navigation satellite system can increase dynamic positioning accuracy and provide independence and redundancy.

Regional strengths of GNSS systems

Reach receivers support all major GNSS constellations, and it’s generally recommended to keep them all enabled, regardless of where you’re working. While you won’t run into issues by leaving them on, understanding each constellation’s regional strengths can help you optimize performance:

• GPS and GLONASS enhance coverage in northern regions such as Canada, the northern United States, and northern Eurasia. Their satellite orbits are specifically optimized to perform well at higher latitudes, though not at extreme polar regions.
  
• Galileo is particularly effective in Europe, where its satellites are frequently overhead and more readily supported by local ground infrastructure, delivering strong signal availability and high accuracy.
  
• BeiDou, while originally focused on China and its neighbors, performs especially well across Southeast Asia, Oceania, and much of the Southern Hemisphere, including Africa and Latin America.
  
• QZSS is a valuable addition for users in Japan and Australia.

In most cases, using multiple constellations gives your receiver access to 30–40+ satellites, which is beneficial for the resulting receiver’s positioning accuracy.

How does GNSS manage to provide global coverage?

GNSS satellites don’t calculate their position on the fly—they’re placed into precisely planned orbits and continuously tracked by ground control. Earth-based stations monitor satellites’ trajectories, correct for any orbital drift, and upload updated position data, known as ephemeris.

Satellites’ position is defined within a standardized coordinate system called WGS84. This shared reference frame allows satellites, control stations, and receivers to “speak the same language,” ensuring consistent and accurate global positioning.

Each GNSS satellite communicates with GNSS receivers on Earth, continuously broadcasting a radio signal with the data needed for positioning. At the core of this signal is the carrier wave—the transport layer for delivering essential information to the receivers on Earth. The information sent includes two key elements:

• Ranging code. A unique binary sequence that allows the GNSS receiver on Earth to determine how long the signal took to reach it. This time delay is used to estimate the distance to the satellite.
  
• Navigation data contains the satellite’s orbital information, clock correction data, and status. This lets the GNSS receiver know exactly where the satellite was when it transmitted the signal.

Together, they form the foundation of GNSS positioning. Without accurate time and position data from the satellites, the receiver would not be able to compute its location reliably.

How receivers calculate a position from GNSS signals

While this is a simplified overview, the general process is as follows. When the receiver picks up the signal from a GNSS satellite, it compares the transmission time with the time of reception to calculate how long the signal took to arrive. Because GNSS signals travel at the speed of light (~299,792 km/s), the receiver can then compute the distance to the satellite using the following formula:

                                    Distance = Speed of Light × Time Delay

For example, if the signal takes 0.07 seconds to arrive, the satellite is about 20,985 km away. Even a 1-microsecond timing error (one millionth of a second) can introduce a 300-meter positioning error, highlighting why precise timing is critical to GNSS accuracy.

To calculate a 3D position (latitude, longitude, and altitude), any receiver needs signals from at least four satellites. This method is called trilateration. So, each satellite sends a signal with its exact position and time, and your receiver calculates the following:

1. With the distance to one satellite, you know you’re somewhere on a sphere around it.

2. With two satellites, you’re somewhere on the circle where two spheres intersect.

3. Three satellites narrow it down to two possible points.

4. The fourth satellite helps resolve which point is correct and fixes the receiver’s clock error.

In general, any receiver calculates distances from multiple satellites and finds the intersection point, determining its exact position. The more satellites involved, the easier it is for the receiver to spot and correct bad data. However, even using this method, the regular receivers like GPS in our smartphones have the accuracy of only several meters.

What are the factors affecting location accuracy?

When a satellite signal travels through space to the Earth, it can face various factors that affect signal accuracy.

• Atmospheric conditions, including atmospheric pressure in the troposphere, result in a significant positional error.
  
• Signal blockage from obstructions, such as high buildings, prevents a clear line of sight to a satellite, affecting signal accuracy.
  
• Multipath happens when part of the signal reaches the receiver after bouncing off a surface or object.

How to improve location and timing precision?

Various types of GNSS-enabled devices have different capabilities to mitigate factors influencing GNSS accuracy. As mentioned earlier, typical consumer devices, such as smartphones, are limited to meter-level positioning accuracy. In contrast, GNSS receivers like Emlid Reach, equipped with advanced technologies and specialized algorithms, can achieve centimeter-level precision.

For example, the following two methods help improve location and timing precision, resulting in centimeter-accurate measurements.

Real-time kinematic and post-processed kinematic

• Real-Time kinematic (RTK) technique corrects common errors in current satellite navigation systems. The main difference between RTK and standard GNSS positioning is that RTK achieves a higher level of accuracy by using two GNSS receivers.
  
  At the heart of RTK is a two-part system: a moving GNSS rover and a stationary GNSS base. The base station acts as a fixed reference point, as it knows its own position—it can be pre-defined and pre-set. It can be a local GNSS receiver or a remote reference station connected to the rover to transmit the data over the internet or radio in real time. In either case, its role remains the same—to transmit its satellite observations and position to the rover as raw data. This allows the rover to account for satellite signal errors on the go—assuming they’re the same at both base and rover—and compute its position with centimeter-level accuracy.
  
• Post-processed kinematic (PPK) is an alternative technique to RTK. With the PPK workflow, the base and rover receivers don’t have an ongoing connection; therefore, accurate positioning doesn’t happen in real time. All algorithms are applied afterward. Both base and rover record raw GNSS logs, which are then processed in a dedicated PPK software.

To achieve centimeter-level accuracy in RTK or PPK, consider a few critical conditions: baseline length, satellite visibility, and electromagnetic interference.

Baseline

Baseline is distance between the base and the rover. RTK and PPK assume that both the base and rover operate under similar environmental conditions when calculating and correcting satellite signal errors.
The recommended maximum baseline for RTK with Emlid Reach receivers is about 8 km (5 miles), assuming a clear line of sight, while for PPK, the optimal baseline is up to 30 km (18 miles).

However, as the baseline length increases, the mentioned assumption becomes less accurate—atmospheric interference and other errors vay over distance, reducing overall accuracy. That’s why we recommend keeping the distance between the base and rover as short as possible whenever feasible.

Good satellite visibility

Open sky is key for centimeter precision. Trees, buildings, or other obstructions can block or reflect signals, slowing down or breaking the signal reception on the receiver.

Absence of electromagnetic interference

Avoid placing the receiver near sources of electromagnetic interference (EMI), such as high-voltage power lines, radio or cell towers, generators or engines, and nearby electronics. EMI generates unwanted radio frequency noise that can distort or weaken satellite signals, making it harder for the receiver to track satellites reliably.

Emlid GNSS receivers for centimeter-accurate solutions

Global navigation satellite systems paired with GNSS receivers and antennas enable high-accuracy applications across industries.

Emlid’s GNSS receivers track signals from multiple GNSS constellations, including GPS, GLONASS, BeiDou, Galileo, and QZSS and ensure reliable performance even in challenging environments. Designed for RTK, PPK, and PPP surveying, Emlid Reach receivers can be used for precise surveying, drone mapping, and GIS applications.

Tim Durham’s recent drone mapping project demonstrates how Emlid’s GNSS receivers help to achieve accurate drone mapping results. To create a 3D model of an old barn scheduled for demolition, Tim used a Reach RS2+ base to stream RTK corrections to his DJI RTK drone (used as a rover). This allowed Tim to capture precisely located aerial data, which he then processed using photogrammetry software to create an accurate 3D model.

Ready to achieve centimeter precision in your projects? Explore Emlid’s GNSS receivers and elevate your surveying, mapping, and GIS workflows with precision and reliability.

FAQs

What is the difference between GPS and GNSS?

GPS (Global Positioning System) is a specific satellite navigation system operated by the United States. While GNSS (Global Navigation Satellite System) is a broader term that includes multiple satellite systems such as GPS, GLONASS (Russia), Galileo (Europe), and BeiDou (China). GNSS can utilize signals from multiple constellations, offering better accuracy and reliability than single GPS, which relies solely on its own network.

How many GPS satellites are there?

The GPS constellation typically consists of 31 operational satellites. These satellites work together to provide precise positioning, navigation, and timing services worldwide.

How accurate is GNSS positioning?

Standard GNSS positioning can provide accuracy within several meters. Advanced techniques like RTK (real-time kinematic) and PPK (post-processing kinematic), supported by Emlid GNSS receivers, enhance this to centimeter-level precision.

How do Emlid receivers help mitigate GNSS inaccuracies?

Emlid receivers use advanced correction methods, such as RTK and PPK, to counter common issues affecting GNSS accuracy. They also support multi-constellation tracking, which ensures reliable signals even in challenging environments.

Can I use GNSS receivers for drone mapping?

Yes! Emlid GNSS receivers are ideal for drone mapping. When used as a base station for an RTK/PPK drone, the Emlid Reach receiver enables precise image geotagging and supports workflows such as creating orthophotos and 3D models.