Why GNSS and GPS Do Not Function Properly Indoors

GNSS (Global Navigation Satellite Systems) and GPS (Global Positioning System) provide positioning and location information for people, vehicles, and objects in motion all over the world. They have revolutionized the way we navigate our world and made location data a critical piece of many information systems (although they do have vulnerabilities).


In this article, we explore how GNSS and GPS work, their applications and also their limitations, including why they do not function properly indoors.


What is GNSS and how does it work ?


List of GNSS systems


GPS (Global Positioning System) – Developed by the United States Department of Defense, the GPS system was initially dedicated to military applications but has since opened itself to civilian use. GPS is the most widely used GNSS system worldwide. It comprises a constellation of at least 24 satellites.


Galileo – Operated by the European Union, Galileo is a rapidly expanding GNSS system with more than 30 satellites planned for its complete constellation by end of 2024. It offers global coverage and high-precision positioning.


GLONASS (Global Navigation Satellite System) – Developed by Russia, GLONASS is another significant global navigation system. It consists of 24 active satellites and provides coverage primarily over Russian territory.


BeiDou – Developed by China, BeiDou is another expanding GNSS system with global ambitions, featuring a constellation of 35 satellites. It is especially prominent in Asia and neighboring regions.


IRNSS/NavIC (Indian Regional Navigation Satellite System) – Developed by India, NavIC is designed to serve the Indian subcontinent, offering high-accuracy navigation and positioning services via 8 satellites.


QZSS (Quasi-Zenith Satellite System) – A regional satellite system launched by Japan in 2018 with 4 satellites to enhance the GPS in Asia. It is planned to expand to 11 satellites.



How GNSS Works


GPS is the most familiar GNSS system, so let’s dive deeper into its workings:


Satellite Constellation: The GPS constellation consists of at least 24 satellites, evenly distributed across six orbital planes. The constellation’s geometry ensures that a minimum of four satellites is always visible from any point on Earth.


Synchronization: All GPS satellites are precisely synchronized and transmit signals that include information about their location and the time they sent the signal.


Trilateration: GPS relies on a trilateration method to determine a receiver’s position. This process involves measuring the time it takes for signals from multiple satellites to reach the receiver. By measuring the time it took for the signals to travel from each satellite to the receiver, the GPS receiver can compute its distance from each satellite.


Receiver Calculation: The GPS receiver uses the information from the satellites to calculate its position in three dimensions (latitude, longitude, and altitude). It does this by comparing the time at which the signals were sent by the satellites with the time the receiver received them.



GNSS and GPS Interchangeability

The term “GPS” is often used as a generic term to refer to all “GNSS” systems in common conversation. It is important to understand that GNSS is the broader concept, encompassing multiple satellite systems, and GPS is just one of them. GNSS systems all contribute to provide global positioning and navigation services, especially as most receiver chipsets decode signals from 3 to 4 GNSS constellations.



RTK : real time corrections for increased precision

Real Time Kinematic or RTK is a technique that has been developed to increase the precision of GNSS receivers from a few meters down to a few centimeters. It relies on error correction parameters that are broadcast in real time to high end GNSS receivers via networks of terrestrial stations to refine the receiver measurements.



What are the main applications of GNSS and GPS ?


One of the most obvious applications of GNSS and GPS is navigation and wayfinding. Whether you’re using a smartphone to find your way in an unfamiliar city or steering a massive container ship across the open ocean, GNSS technology plays a pivotal role in ensuring precise and accurate navigation.


Aircraft navigation

Commercial and private aircraft rely on GNSS for navigation, approach, and landing procedures. GNSS technology has significantly improved safety in the aviation industry.


Marine navigation

Ship navigation has been revolutionized by GNSS, enabling vessels to navigate with pinpoint accuracy, even in the most challenging conditions. GNSS is crucial for route optimization and collision avoidance and it is used in the Automatic Identification System (AIS) that helps monitor maritime traffic.


Automotive navigation

The integration of GPS in modern vehicles has transformed the driving experience. Navigation apps offer turn-by-turn directions and real-time traffic updates. GNSS and RTK will also play a pivotal role in the rise of autonomous vehicles. Precise and real-time positioning information is mandatory for self-driving cars, trucks, and drones.


Surveying and Mapping

GNSS technology has greatly improved surveying and mapping processes, providing accurate and efficient methods for land surveyors, cartographers, and geologists. GNSS RTK receivers are used to measure precise locations and elevation changes for property boundaries, construction sites, and infrastructure projects.

GNSS data is also a fundamental component of Geographic Information Systems (GIS), facilitating the creation of detailed and accurate maps and spatial analysis.


Environmental Monitoring

Researchers use GNSS technology to track environmental changes, monitor wildlife, and study geological phenomena.


Precision farming

GNSS RTK-equipped tractors and machinery allow farmers to map soil properties and plant, cultivate, and harvest crops with high precision. This results in increased yields and reduced resource consumption.


Timing and Synchronization

The accuracy of GNSS timing signals is crucial for various applications where clock accuracy is essential, such as telecom networks, financial transaction platforms, energy networks and data centers. In scientific fields such as seismology, astronomy and climate science, GNSS data is also crucial.



Do GNSS systems have any Vulnerabilities ?


GNSS systems have become critical but they do have inherent vulnerabilities:


Satellite Constellation Health

The reliability of GNSS systems relies on the health of the satellite constellations, and any failures or disruptions on one or several satellites can have widespread consequences.


Cyber Risks

GNSS systems can be targets of cyber-attacks as they support many mission-critical information systems.


Jamming and Spoofing

GNSS signals are vulnerable to jamming and spoofing, which can disrupt or mislead navigation systems. Jamming is frequently used in modern conflict zones, and cases of spoofing have endangered air traffic on several occasions.



To address these challenges, researchers and industry leaders are continually working on improving GNSS technology, on several fronts.


Multi-Constellation Integration

Many GNSS receivers are now designed to work with multiple constellations, providing more robust and accurate positioning.


Augmentation Systems

Ground-based augmentation systems, such as RTK, are being developed to enhance the accuracy and reliability of GNSS signals.


Continued Expansion of Constellations

GNSS systems like Galileo and BeiDou are actively expanding their constellations, increasing global coverage and redundancy.


Improved Anti-Spoofing and Anti-Jamming Technologies

Efforts are ongoing to develop more resilient GNSS signals that can withstand deliberate interference. Encryption and authentication mechanisms are being developed to protect the integrity of positioning data. Military-grade GPS signals (M-Code) employ encrypted signals to prevent spoofing attempts.



Understanding Indoor Limitations of GNSS and GPS


GNSS signals struggle to penetrate buildings and other structures, limiting their effectiveness indoors. Several factors come into play.


Signal Attenuation

Satellite signals, such as those used in GNSS and GPS, are relatively weak by the time they reach the Earth’s surface. The primary reason for this weakness is the long distance the signals must travel from the satellites, which are orbiting at altitudes of approximately 20,000 kilometers. By the time these signals reach a receiver, they are susceptible to attenuation or weakening when they pass through solid structures like walls, roofs, and even large objects within buildings.


The L-band frequencies used by GPS (1.57542 GHz for the L1 signal) have limited penetration capabilities through dense materials. When these signals encounter barriers like concrete walls or metal roofs, their strength diminishes considerably, making it difficult for indoor receivers to detect and process them accurately.


Multipath Effects

Multipath propagation is a phenomenon where GNSS signals bounce off surfaces such as walls, floors, and ceilings before reaching the receiver. This results in multiple signal paths arriving at slightly different times, causing interference and errors in position calculations.


Multipath effects introduce significant errors in the time-of-arrival calculations that GPS receivers rely on to determine accurate positions. In outdoor environments, the open sky allows for a direct line of sight to multiple satellites, minimizing multipath issues. Indoors, however, the signal reflections create a complex environment that makes positioning erratic.


Line-of-Sight Requirements

GNSS systems require a direct line of sight to at least four satellites to triangulate a precise position. Indoor environments, with their numerous obstacles and lack of open sky view, hinder this requirement.


Signal Interference

Electronic devices commonly found indoors, such as Wi-Fi routers, Bluetooth devices, and other RF-emitting equipment, can interfere with GNSS signals. This interference further complicates the receiver’s ability to lock onto the satellite signals and maintain a reliable connection.


The RF spectrum is crowded with various signals, and the close proximity of electronic devices can introduce noise and signal overlap, disrupting the delicate GPS signal reception. This issue is exacerbated in environments with high concentrations of electronic equipment.



Alternatives to GNSS and GPS for Indoor Positioning


Given these limitations, alternative technologies have been developed to provide indoor positioning solutions. These include:


Wi-Fi Positioning Systems (WPS)

Utilize existing Wi-Fi infrastructure to triangulate the position of a device based on signal strength from multiple access points.


Bluetooth Low Energy (BLE) Beacons

They use low-power beacons to broadcast signals that can be picked up by receivers to determine location with moderate accuracy.


Ultra-Wideband (UWB)

They offer high-precision positioning by using short pulses over a wide frequency spectrum, minimizing interference and multipath effects.


Inertial Measurement Units (IMU)

They combine accelerometers, gyroscopes, and magnetometers to measure their own movement and position themselves, they are often used in conjunction with other technologies.





In the world of navigation and positioning, GNSS and GPS have evolved into indispensable tools, transforming industries such as transportation and becoming critical for many other business segments.


Their vulnerabilities are being addressed, but their intrinsic indoor limitations need to be acknowledged and dealt with. For indoor positioning and navigation, look for positioning technologies that complement GNSS and GPS.


More information on Sysnav’s indoor positioning solutions.