Abstract

This article discusses the fundamentals and importance of positioning, navigation, and timing (PNT). The article describes the critical elements of PNT, including global navigation satellite systems (GNSS) such as GPS, and their role in modern technologies and various applications. It also discusses the challenges and limitations of PNT caused by interference, spoofing, lack of line-of-sight between satellites and receivers, and multipath in urban environments. It discusses A-PNT (Assured Position, Navigation, and Timing), which uses additional sensors to improve data accuracy and ensure robustness to errors.

The sections will cover the introduction to PNT, the structure of PNT, and the future of PNT systems. Or what is A-PNT? What are Inertial Labs’ INS capabilities? The conclusion will summarize the benefits of PNT and A-PNT and how Inertial Labs™ solutions can help navigate without a GNSS signal.

Section 1. Introduction to PNT

PNT is an acronym for “Positioning, Navigation, and Timing”. This concept covers technologies and systems that provide positioning, navigation, and time synchronization. In simple terms, any system, network, or technology provides data to calculate or supplement the calculation of longitude, latitude, altitude, transmit time or frequency data, or any combination of these. [1]. An example of such a system is the Global Navigation Satellite System (GNSS) [2]. These are also technologies based on ground stations, such as Differential GPS (DGPS), Figure 1 [3].

Figure 1. National PNT Architecture.

PNT consists of several key components:

• Positioning is the process of determining the location of an object.
• Navigation involves planning and controlling the movement of an object from one point to another.
• Time synchronization is essential to PNT technologies, especially in networks requiring precise timing.

Today, PNT technologies are used in various fields:

• Transport: navigation systems for cars, aircraft, and ships.
• Telecommunications: synchronization of networks and base stations.
• Surveying and cartography: precise measurements to create maps.
• Agriculture: precision farming technologies.
• Smart Cities: Infrastructure and Resource Management.
• Financial sector: management of transactions and operations in the market.
• National security: support of military operations and communications.

Next, let’s look at the structure of PNT and its components.

Section 2. PNT Structure

In the previous section, we looked at the critical components of PNT (positioning, navigation, and time synchronization). Let’s look at each of them in more detail:

• Positioning determines and locates objects in space relative to other objects or coordinate systems. The most popular method of global positioning is GNSS. Major constellations such as GPS (USA), GLONASS (Russia), Galileo (Europe), and BeiDou (China) provide data on the user’s location around the Earth. To improve accuracy and reliability, a network of ground stations is also used, as well as a Satellite-Based Augmentation System (SBAS) or Ground Based Augmentation System (GBAS) [4]. SBAS and GBAS can be used with GNSS for maximum accuracy and reliability. This is especially important in critical applications such as aviation, marine navigation, and surveying. Examples of SBAS systems include WAAS (Wide Area Augmentation System) in the USA and EGNOS (European Geostationary Navigation Overlay Service) in Europe. In addition to this, DGPS methods, real-time Kinematic (RTK), and Precise Point Positioning (PPP) should be applied [5, 6]. These additions to GNSS can significantly reduce positioning errors, making them valuable components in PNT systems.

• Navigation and orientation systems are an integral part of the PNT system. Since GNSS signals can be subject to external influences, such as jamming, spoofing, and space weather [7]. Also, GNSS signals are not available indoors or underground. Therefore, an inertial navigation system (INS) is an autonomous source of information about an object’s speed, coordinates, and orientation. Modern INS are based on MEMS accelerometers, gyroscopes, and magnetometers [8]. These sensors use navigation algorithms to determine an object’s speed, coordinates, and orientation angles when the GNSS signal becomes unavailable (the so-called autonomous mode). However, INS has a significant limitation – they require periodic correction from satellites since gyroscopes and accelerometers have drifted, increasing errors in determining orientation and navigation. In simple terms, the longer there is no GNSS signal, the greater the error. In this regard, additional sensors are used; such integration is called Sensor Fusion, which will be discussed in more detail in the next section.
• Time synchronization. An atomic clock synchronizes time and provides high accuracy and stability of time [9]. GNSS uses nuclear clocks to generate and transmit time signals; time information is transmitted along with location data. This allows receivers to correctly determine their location using time stamps and synchronize their clock with GNSS systems. There are also additional synchronization systems:
• NTP (Network Time Protocol) is a widely used method for allowing devices on a network to synchronize their clock. It relies on the Internet for time reference and provides sufficient accuracy for most industrial applications [10].
• PTP (Precision Time Protocol), on the other hand, provides highly accurate synchronization using hardware timestamps and precise clock calculations. This makes it suitable for time-critical applications requiring millisecond-level accuracy [11].

Synchronization time in PNT systems ensures accuracy and reliability in positioning and navigation. Use high-precision atomic clock, modern protocols synchronization, and integration with other systems to achieve the necessary level of accuracy.

As can be seen, PNTs are complex and voluminous systems that consist of subsystems. The latter, in turn, can also consist of subsystems. Although the classic PNT system is GNSS, it cannot always provide stable signal transmission due to external factors, so additional components, methods, and services are used. Such integration is necessary to ensure maximum accuracy, reliability, and safety.

Section 3. Future of PNT systems. Or what is it, A-PNT?

               As stated earlier, PNT is not necessarily a GNSS-based system. Many modern solutions are based on rejecting GNSS as a vulnerable component. In this regard, an alternative direction for developing PNT has emerged – A-PNT.

The U.S. military has long relied on GPS for navigation, targeting, and communications, but its vulnerabilities are becoming increasingly apparent. Threats such as signal jamming, spoofing, and degradation can significantly impact military operations. To ensure mission success and the safety of service members, the Department of Defense is actively exploring alternative solutions through Assured Positioning, Navigation, and Timing (A-PNT) systems. These systems include a variety of technologies and strategies aimed at providing reliable and accurate positioning and navigation information even when GPS is unavailable or unreliable.

               The critical feature of A-PNT is sensor fusion [12]. The principle of sensor fusion is based on combining data from several sensors to obtain more accurate and reliable information. When there is no GNSS signal, data from all sensors used in the system is collected. These can be different types of sensors: LiDARs (laser range finders), cameras, accelerometers, gyroscopes, ultrasonic sensors, etc. Each sensor measurement can be subjected to the fusion process. Each measurement may be filtered or normalized to remove noise and compensate for errors. For example, this may include camera distortion correction or accelerometer noise filtering. The data is combined using various techniques, such as the Kalman filter. It helps predict the next state of the system based on previous data and updates this information with new measurements. Once the data is merged, a final estimate of the system state (estimation of position, velocity, and object orientation) is created.

A great example of A-PNT systems is autonomous vehicles. In unmanned vehicles, sensor fusion combines data from cameras, lidars, radars, and GNSS or alternative positioning systems. It includes technologies such as LORAN, Pseudolite, and other terrestrial systems that can be used to provide positioning and navigation data [13, 14]. It allows the vehicle to see and understand the environment with high accuracy and safety. Lidars are good at determining distances but poor at distinguishing the color of objects. Cameras can capture visual data, but their accuracy depends on lighting. Combining this data helps to get a more complete picture of the road in the face of GNSS signal loss.

A-PNT is used in military operations and civilian sectors where reliable navigation and time synchronization are critical, such as aviation, logistics, and telecommunications.

Advantages of A-PNT systems:

• Increased security. Inertial navigation systems are not subject to spoofing or jamming. Using gyroscopes, accelerometers, magnetometers, barometers, odometers, and other external data allows for determining the position and orientation of an object in the complete absence or incorrectness of the GNSS signal.
• Improved reliability: Even if the GNSS signal is not suppressed or distorted, in practical situations, the lack of a direct line of sight to GPS satellites and reflections from buildings can cause errors in positioning. Sensor-based systems used with GPS help overcome these limitations by providing highly accurate positioning data in real-time.

An example of such a system is INS from Inertial Labs.

Section 4. Inertial Labs’ INS capabilities

Inertial Labs is using a modular systems-of-systems approach by generating an eco-system of aiding data sources to utilize the technical advantages of its proprietary Kalman filter, which serves as a solid foundation to perform advanced sensor fusion in times of GNSS signal outages, jamming, or spoofing.

Figure 2. The Inertial Labs Sensor Fusion Platform.

Inertial Labs has identified a significant problem associated with traditional PNT solutions that mainly concerns over-reliance on multi-constellation Global Navigation Satellite System (m-GNSS) aided navigation systems that are not able to satisfactorily complement other alternative PNT (Position, Navigation, Timing) sources such as inertial, magnetic, barometric, machine vision and multi-source alternative RF (Radio Frequency) signals. In an era of electronic warfare, GNSS jamming and spoofing are more apparent than ever, which prompts the need for an infrastructure-less, resilient, alternative Position, Navigation, Timing (alt-PNT) solution that can continuously deliver accurate navigational information and time synchronization outputs within GPS/GNSS outages for extended periods. Complementing and reducing reliance on traditional GNSS-based PNT fusion systems is a critical problem for various defense applications.

The Inertial Labs’ INS has integrated capabilities to accept RF (Radio Frequency) ranging aiding data inputs from advanced, infrastructure-less, MESH network-based tactical Software Defined Radios (SDR) that are developed by DTC (Domo Tactical Communications), Figure 3.

Figure 3. RF-Geolocation.

From an application standpoint, the DTC’s BluSDR-30-B radios serve as base (“beacon”) points within a defined range of up to 30 kilometers from the vehicle’s BluSDR-30 radio, which is mounted on aerial or land systems with the INS. Once powered, the radios automatically establish a secure MESH network configured to send Time of Flight (ToF) packets throughout the MESH (between each radio), supporting up to 144 nodes within the network. Hence, the SDR on board the vehicle shall obtain real-time ToF data from the distant base radios deployed on the ground along with their known, instantaneous 3D position coordinates.

Inertial Labs offers several products that exemplify advancements in sensor fusion technology. Key products include:

• Inertial Measurement Units (IMUs): These devices combine accelerometers, gyroscopes, and magnetometers to measure orientation, velocity, and gravitational forces precisely. Notable models include the IMU-P and IMU-F series.
• Attitude and Heading Reference Systems (AHRS): These systems, such as the AHRS-10, integrate data from multiple sensors to provide accurate orientation and heading information, which is crucial for aviation, marine, and autonomous vehicle applications.
• Inertial Navigation Systems (INS): Products like the INS-D and INS-P series combine IMUs with GPS/GNSS receivers, offering highly accurate positioning and navigation solutions for drones, robotics, and land vehicles.
• Marine and Land Solutions: Inertial Labs’ offerings for marine and land applications, such as the MRU-B and the GPS-Aided INS, enhance navigation and control for ships, underwater vehicles, and land-based systems.

These products demonstrate Inertial Labs’ commitment to leveraging sensor fusion technology to deliver precise, reliable, and versatile solutions across various industries.

Conclusion

Positioning, navigation, and timing (PNT) systems are critical technologies supporting various applications across industries. However, as the core component of PNT, GNSS suffers from several limitations, including vulnerability to interference and signal unavailability in non-line-of-sight or multipath conditions. To improve the reliability and accuracy of PNT, especially in challenging environments, it is essential to use additional sensors and other assistive technologies, which is the next step in PNT development – A-PNT (Assured Position, Navigation, and Timing). This ensures more stable and accurate positioning, critical for future high-tech solutions and infrastructures.

Like Inertial Labs, we can expect even greater precision, efficiency, and versatility in various technological applications. This technology will ultimately transform industries and improve the accuracy and functionality of various systems.

References

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[2] Wikipedia. “Satellite Navigation.” Wikipedia, 19 Mar. 2020, en.wikipedia.org/wiki/Satellite_navigation.

[3] Wikipedia Contributors. “Differential GPS.” Wikipedia, Wikimedia Foundation, 16 Apr. 2019, en.wikipedia.org/wiki/Differential_GPS.

[4] “GNSS Augmentation.” Wikipedia, 17 July 2022, en.wikipedia.org/wiki/GNSS_augmentation.

[5] “Real-Time Kinematic Positioning.” Wikipedia, 5 June 2022, en.wikipedia.org/wiki/Real-time_kinematic_positioning.

[6] Wikipedia Contributors. “Precise Point Positioning.” Wikipedia, Wikimedia Foundation, 1 Apr. 2023, en.wikipedia.org/wiki/Precise_Point_Positioning. Accessed 3 Oct. 2024.

[7] Mendez, Maria. “What Are the Limitations of GNSS?” Inertial Labs, 2 Oct. 2024, inertiallabs.com/what-are-the-limitations-of-gnss/. Accessed 3 Oct. 2024.

[8] Mendez, Maria. “How does an INS work?” Inertial Labs, 10 Oct. 24, https://inertiallabs.com/how-does-an-ins-works/

[9] “Atomic Clock.” Wikipedia, 18 Sept. 2020, en.wikipedia.org/wiki/Atomic_clock. Accessed 17 Oct. 2024

[10] Wikipedia Contributors. “Network Time Protocol.” Wikipedia, Wikimedia Foundation, 7 Sept. 2019, en.wikipedia.org/wiki/Network_Time_Protocol.

[11] “Precision Time Protocol.” Wikipedia, 26 Mar. 2023, en.wikipedia.org/wiki/Precision_Time_Protocol.

[12] Mendez, Maria. “Advancements in Sensor Fusion Technology.” Inertial Labs, 3 June 2024, inertiallabs.com/advancements-in-sensor-fusion-technology/.

[13] “LORAN.” Wikipedia, 28 Oct. 2022, en.wikipedia.org/wiki/LORAN.

[14] Wikipedia Contributors. “Pseudolite.” Wikipedia, Wikimedia Foundation, 18 Feb. 2023, en.wikipedia.org/wiki/Pseudolite. Accessed 3 Oct. 2024.