Positioning, Navigation, and Timing

PNT is a combination of three distinct, constituent capabilities: positioning, navigation, and timing. Positioning is defined as the ability to accurately and precisely determine one's location and orientation in two or three dimensions and referenced to a standard geodetic system. Navigation is defined as the ability to determine position, either relative or absolute, and apply corrections to course, orientation, and speed to arrive at a desired position, from sub-surface to surface and from surface to space. And timing is defined as the ability to acquire and maintain accurate and precise timing from a standard anywhere in the world and within user-defined parameters.

When PNT is used in combination with different types of data and information (such as map, weather, or traffic data), the result is a recognized service known as Global Positioning System (GPS). PNT often encompasses more than navigational functions of which GPS is a component. This model consists of three segments: the space segment, the control segment, and the user segment.

In this model, the space segment refers to a system of twenty-four satellites circling the earth, also known as a global navigation satellite system (GNSS). The control segment refers to the monitoring stations that monitor and maintain the satellites. And the user segment refers to the military, governmental, commercial, and other entities (such as private citizens) that rely on GPS systems.

PNT is used to understand where users are, determine from that position how to get to another position, and synchronize networks for time stamping. The technology can be found almost everywhere, including in consumer products such as cellphones and smartwatches and in industries such as surveying, mapping, farming, mining, and road construction. And the timing component has been key for telecommunications, financial, and energy sectors, while transport relies on positioning and navigation for improved efficiency. This can offer greater functionality, such as weather forecasting, earthquake monitoring, and sea-level measurements.

Part of space-based PNT systems, global navigation satellite systems (GNSS) are systems comprised of a constellation of satellites that beam down positioning and timing information to GNSS receivers, which analyze data to establish location. This information is used by military, governmental, and commercial entities. GNSS offers two types of services: an open service and an authorized service. Open service is, as the name suggests, available to any user. While authorized service is available only to authorized users and often provides better performance. These authorized services support defense military operations of the United States and Russia. Open services are used in commercial and civil operations, such as the security operations of police and civil protection.

Multiple nations, besides the United States and Russia, are developing and deploying regional or global GNSS systems. These regional systems, known as regional navigation satellite systems (RNSS), provide regional coverage only unlike GNSS. And most countries remain dependent on the GPS or GLONASS GNSS services.

Despite the reliance of multiple industries on GNSS and PNT systems, the space-based signals involved in these systems are often low-power and unencrypted. This makes those signals susceptible to both intentional and unintentional disruption. There are some programs to increase the resiliency of this infrastructure and reduce or remove these vulnerabilities. Two of these programs are the PNT Integrity Library and the Epsilon Algorithm Suite.

The PNT Integrity Library is intended for GNSS receivers and GNSS-based timing servers for use in future development or integration into existing products and platforms. The program provides spoofing detection capabilities for GNSS sources, using available PNT solutions and observables. And when possible, this includes other measurements and data available in the antenna or receiver processing chain. The PNT Integrity Library also works to provide a scalable framework for GNSS manipulation detection and offers varying levels of protection based on the available data.

The Epsilon Algorithm Suite works to detect inconsistencies in position, velocity, and clock observables commonly provided by GPS receivers. And it allows an end-user to have basic spoofing detection capabilities without modifications to the existing GPS receiver.

Image of GPS III satellite.

The first GPS satellites were launched for the U.S. Department of Defense in 1978, but over years many organizations have played a role in their refinement. Those original GPS satellites were built by Lockheed Martin for the Air Force, and eight of the original twenty-one GPS IIR satellites launched were modernized and designated as GPS IIR-M. The fleet of GPS IIR and IIR-M satellites makes up the majority of the GPS constellations.

However, as the demands for GPS and related PNT services have increased, this has led to the development of GPS III, with satellites being built to deliver signals three times more accurate than the current generation for better accuracy for consumers and military users, offering signals up to eight times more powerful to improve jamming resistance and availability for critical missions worldwide. The new satellites are designed to have a lifespan of fifteen years, whereas the original GPS IIR satellites were designed with a seven-and-a-half-year lifespan.

Further, the new GPS III satellites are designed to adapt to new technology developments and changes in mission objectives and to increase the expansion of GPS technology to more people. This expansion is made possible by the L1C civilian signal, which is interoperable with other GNSS satellites. The L1C signal shares the same center frequency as Europe's Galileo networks, Japan's QZSS, and China's BeiDou satellites. The signal teams from Japan and Europe worked with Lockheed Martin to ensure the GNSS satellite systems were compatible. This allows all GNSS systems to increase their accuracy in tracking.

M-code VersaPNT receiver designed by Orolia.

M-code is a new military signal used in the L1 and L2 (1575.42 MHz and 1227.60 MHz) GPS bands, which is designed to improve the security and anti-jamming properties of military navigation using GPS. These signals can be delivered to specific regions using spot beam transmissions, with much greater satellite power in the given region, and are expected to be around 20 dB more powerful than conventional full-Earth coverage beam. The M-code is further developed to be resistant to jamming, while also allowing the military to selectively jam the commercial GPS L1 C/A signal while continuing to receive the signal from friendly military forces. The M-code signals are encrypted to allow receivers to detect and reject false signals.

The M-code is unusual because it is designed to allow a military receiver to determine its position with the M-code alone, while the traditional signal must first acquire the C/A code to do so. Further, it is also spread across 24 MHz of the bandwidth.

Satellite-based augmentation systems (SBAS) operate to improve the accuracy, integrity, continuity, and availability of GNSS by correcting signal measurement inaccuracies. SBAS augment GNSS constellations by providing GEO ranging, integrity, and correct information. The main goal is to provide integrity assurance and increase the accuracy with position errors below 1 meter. The augmentation information provided by SBAS also covers corrections and integrity for satellite position errors, satellite clock and time errors, and errors induced by the estimation of the delay of the signal while crossing the ionosphere. For delays caused by the troposphere, the user can apply a tropospheric delay model.

Several countries have developed and implemented their own satellite-based augmentation systems:

• The European Union's European Geostationary Navigation Overlay Service (EGNOS)
• The United States' Wide Area Augmentation System (WAAS)
• Japan's Michibiki Satellite Augmentation System (MSAS)
• India's GPS-aided GEO-Augmented Navigation (GAGAN)
• China's in-development BeiDou SBAS (BDSBAS)
• South Korea's Korea Augmentation Satellite System (KASS)
• Russia's System for Differential Corrections and Monitoring (SDCM)
• ASECNA's SBAS for Africa and Indian Ocean (A-SBAS)
• Australia's and New Zealand's Southern Positioning Augmentation Network (SPAN)

Despite the popularity and ubiquity of GEO satellites for GNSS systems, the increasing popularity of LEO satellites has driven growth in satellite-based GNSS systems. These systems can be important when used in industries such as air travel where navigation needs to be precise, with a need for 30 cm or smaller level of resolution in order to achieve around one failure in every 1 billion miles. LEO GNSS satellites have been tested and could offer stronger signals more resistant to interference and the rapidly moving geometry could yield tighter resolutions, closer to 10cm 95 percent of the time to reduce the failure rate.

GNSS could be included on LEO satellite constellations for increased PNT coverage.

Especially with the growth of autonomous vehicles, the need for greater precision in PNT systems has increased, as these transit systems will not rely on GNSS alone but will employ sensors for different types of spatial resolution and relative positioning. This could include combinations of cameras, LiDAR, radar, ultrasonic sensors, and wheel-speed sensors, among others. For absolute positioning, GNSS is integrated with these systems, with the expectation for resolution lower than 30 cm to decrease failure rates.

Similar to SBAS, a ground-based augmentation system (GBAS) augments existing GNSS systems by providing corrections. These systems improve the accuracy, integrity, continuity, and availability of GNSS by correcting signal measurement inaccuracies, and through operational data. Unlike SBAS, a GBAS is based terrestrially, with a network of ground stations across regions to maximize coverage. Often, GBAS are used to provide corrections to aircraft in the vicinity of an airport to improve these aircrafts' GPS navigational positions. As well, they are used to provide an alternative to the Instrument Landing System (ILS) supporting the full range of approach and landing operations.

Another augmentation to existing GNSS systems is hybrid and autonomous PNT systems (HAPS). They represent a broader selection of solutions of improving PNT capabilities, offering a re-tasking of cellular and other networks to cover gaps in the "vision" of GPS or for hybrid PTN systems that can use alternate-frequency methods. The ultimate goal is to develop a completely autonomous PNT source within a device or for a vehicle that does not require positioning and timing from external sources but offers PNT at the same or higher quality than existing systems.

Ubiquitous Positioning, Navigation, and Timing systems are being developed to provide redundancy to GNSS systems or else to extend the range of GNSS systems. Any ubiquitous PNT system is required to be able to deliver GNSS-like performance anywhere, anytime, and under any operating conditions and to exceed the performance levels of GNSS for safety and liability for critical applications. One method of developing ubiquitous PNT has included a proposal for the use of Wi-Fi positioning to offer room-level granularity. Most applications of this type of ubiquitous PNT might be required to locate persons in a certain room or section of a building; however, room level granularity, or region-level granularity, might also be sufficient for pedestrian-based location services. Otherwise, ubiquitous PNT services independent of GNSS have been suggested for use in the following:

• Location-based services
• Personal and pedestrian navigation
• Intelligent transportation systems
• Air traffic management
• Asset location and tracking
• First responders and fire-fighters
• Unmanned autonomous vehicles for mapping and surveillance
• Emergency response and rescue operations in large warehouses, multi-story buildings, train or metro stations, or airports
• Dismounted soldier navigation
• Vehicle collision avoidance systems
• Navigation and guidance of teams of robots
• Precision farming

Assured Positioning, Navigation, and Timing (A-PNT) are any systems that offer A-PNT systems that enhance the effectiveness of existing systems and use alternative systems to conventional PNT systems. GPS is a form of A-PNT that offers accurate location and allows users to plan their movements. However, as powerful as GPS is, it has a variety of problems:

• GPS coverage is not always available, and signals can be jammed
• The accuracy of GPS signals can be problematic when dealing with vertical locations and varied elevation
• Leaders in the Department of Defense are concerned with an over-reliance on a single form of technology, which creates a vulnerability

Example of an A-PNT system mounted on a ground vehicle.

This has led to the development of new or alternative A-PNT systems to enhance the effectiveness of GPS or to provide similar PNT services of conventional GPS when those systems are unavailable. One such system is the AC²ES, which is a converged computer for A-PNT services designed to provide PNT services and information at all times. The AC²ES is embedded on a widely used data distribution unit - expandable (DDUx) II. The DDUx II and military variants are fielded on over 150,000 vehicles so the capability can be implemented without additional space or weight requirements. Convergence with DDUx II also allows for ease of use as A-PNT to provide users with PNT information without standard GPS PNT sources. The system works with technologies to provide a reliable, GPS/GNSS-denied navigation solution during real-world jamming or spoofing attacks. These technologies include the following:

• Anti-jam technology
• Anti-spoof technology
• M-Code receivers
• Image-based terminal
• Inertial measurement units

Example of an A-PNT system for soldiers.

As noted above, A-PNT solutions are of special interest to the military, as they could provide a source to support communications, command and control, logistics, targeting, and effects. These systems are not expected to replace GPS receivers but could provide stand-alone capacity if need be and be equipped on soldiers as part of their gear to give them PNT capabilities when GPS may be limited or denied. These systems are designed to offer maintenance for the integrity of positioning and timing in GPS-contested environments and to keep pace with current and future threats and technologies. Further, many of these systems, such as those designed by Collins Aerospace, use two-line replaceable-unit (LRU) systems to replace existing navigation systems and offer easier upgrades and sustainability for future development. These new units are also developed to include military code (M-Code) capability and improved levels of reliability through Modernized Signal Tracking (MST) to enhance GPS integrity.

Example of an inertial navigation system.

One such A-PNT system is an inertial navigation system, which works to calculate the direction moved over time, with a varying degree of drift, and with the use of high-precision oscillators to provide continuity of time. One advantage of this system is that it cannot be spoofed and jammed. An individual platform can include a number of sensors, each of which offers a potential source of complementary information to support navigation. This complementary information can provide greater trust in the system, similar to the level of trust offered by traditional GPS systems. Further, many of these systems, such as those explored above, are developed with open architecture that can integrate data from diverse sensors, both GPS and non-GPS, in a single hub for distribution throughout the platform.

One development in A-PNT is non-GNSS or non-GPS PNT. Many of these technologies have been recommended for technologies such as 3D digital twins, real-time mapping, metaverse, or flying autonomous taxi drones, which all have an underlying reliance on GPS for the critical "where" and "when." GPS/GNSS does not always work where it should and can stop in unpredictable situations; to solve for accurate location or elevation, especially where GNSS is denied or degraded, many solutions have been developed. One solution is the use of terrestrial beacons and meteorological sensors to serve cities and regions with non-GPS PNT. Another driver for reliable A-PNT technology is in situations where GNSS does not work well, such as indoors, in dense urban canyons, or where heavy interference exists.

Example of the NextNav MBS system.

One such system has been developed by NextNav, called the Metropolitan Beacon System (MBS). It consists of networks of towers or connected towers that synchronize signals for end-user devices to determine location. Depending on the density of such a network and local weather conditions, they have been capable of delivering a resolution of 5 meters to 10 nm in urban environments. Further, these systems have been developed with small atomic clocks to keep them updated and synchronized in their timing. And the towers can communicate with GNSS satellites and those atomic clocks to further increase synchronicity. The towers also transmit signals in the 900 MHz range, to strengthen signals against potential jamming and spoofing.

Further, the system includes navigational computations for vertical and horizontal axis, while offering barometers to compute differentials in air pressure and calibrate those barometers to understand how they move between elevations. These systems communicate with an end-user device and the barometers and pressure sensors in those devices to compare with the barometers in the system in order to provide 3D navigation in dense urban environments.

Another aspect of A-PNT solutions is developing resilient PNT solutions, especially, as noted above, any disruption in GNSS availability, reliability, and integrity can weaken the infrastructure that relies on this information. For example, in 2015, a blip of thirteen-millionths of a second resulted in outages of GPS services, which disrupted emergency services in parts of the US and cause a two-day outage of the BBC's digital radio service. The power grid was also affected, but these effects were minimized because of the short-term backup timing systems used.

This has led governments, such as the United States government, to take notice of the potential threat to national security and the world economy. This has seen several agencies report on the increasing reliance on PNT services, which led to the Executive Order 13905 of February 2020, which laid a policy for increased resilience in PNT services, such as ensuring that critical infrastructure can withstand disruption or manipulation of PNT services and encouraging the use of A-PNT layers to supplement GNSS. Related reports noted that the solution to these concerns will likely be a proliferation of redundant PNT services, which can work with and supplement GPS. And given the diversity of operational needs, many reports around the need for increased resilience in PNT have concluded that these should be developed in coordination with the industry and include regulatory and financial incentives to encourage adoption.

As PNT and related technologies, such as GNSS and GPS increase in civilian, rescue, and military use, there has been a push to develop cybersecurity systems to ensure the integrity of the systems and the integrity of the signals. GPS has long been vulnerable to accidental and intentional interference, spoofing, and degradation or denial of signals. Additionally, the satellites can be vulnerable to damage or destruction by space debris or intentional attack. This led to the development of PNT cybersecurity to help organizations identify systems, networks, and assets dependent on PNT services; identify appropriate PNT services; detect the disruption and manipulation of PNT services; and manage associated risks.

As part of the developments in security and cybersecurity of PNT, the National Institute of Standards and Technology (NIST) in the United States released a cybersecurity guidance for PNT services. Formally titled Foundational PNT Profile: Applying the Cybersecurity Framework for the Responsible Use of Positioning, Navigation, and Timing (PNT) Services, the document is a part of the NIST's response to the February 12, 2020 Executive Order 13905, Strengthening National Resilience Through Responsible Use of Positioning, Navigation, and Timing Services. NIST sought public input for the development of the profile and regarding the general use of PNT data, which are developed to help mitigate the cybersecurity risks with PNT services.

Under the NIST framework, any organization using PNT services can leverage the PNT profile to help an organization perform the following:

• Identify systems that use or form PNT data
• Identify PNT data sources
• Detect disruption and manipulation of the systems that form or use PNT services and data
• Manage risk regarding the responsible use of these systems

These functions are then aligned against the NIST CSF, which is composed of five high-level functions: identify, protect, detect, respond, and recover.

Part of the process for protecting PNT requires strengthening GPS, developing alternative sources of PNT data, and developing ways of integrating those alternative sources of data into the systems that currently rely on GPS. Further, this framework has looked for complements and backups for the GPS timing components to ensure the availability of uncorrupted and non-degraded timing signals for military and civilian users. This would include the development of a wireless, terrestrial system that is capable of providing wide-area coverage and synchronized with UTC, resilient, and difficult to disrupt or degrade, capable of penetrating underground and inside buildings.

Example of a potential lunar space GNSS system for space and lunar navigation.

In space, PNT is important for navigation, timing, and accurate positioning, which can be important for satellites maintaining their orbit and for the travel of spacecraft through multiple orbits. However, the mapping of space has not gone beyond specific orbits, and PNT can be used to track but not necessarily to navigate. For example, PNT has been used to track Voyager-1 and Voyager-2 since 1977. This PNT has been done using the basic features of electromagnetic theory, such as the use of ranging and the doppler effect to determine the range, position, direction, and velocity through two-way tracking. Through signals sent between the spacecraft and the ground station, the differences in amplitude, phase, and doppler can be determined, and distance, position, and direction can be derived, and those measurements can be used to send commands to the spacecraft for course correction.

Example of a small size atomic clock developed by NASA for deep space exploration.

To increase the capability of PNT for the exploration of space, there has been a development for one-way ranging, for more autonomous navigation, with JPL/NASA working on a prototype Deep Space Atomic Clock, which is intended to provide a clock with accuracy better than 2 nanoseconds and fifty times more accurate than the atomic clocks on GNSS systems. One-way tracking (from Earth to spacecraft) is a good step toward autonomous PNT in deep space. NASA has been researching a galactic positioning system, based on X-ray and ultra-regular oscillations coming from distant millisecond pulsars from neutron stars to derive timing and location. This technology was demonstrated on the International Space Station in 2018. Another possible area for PNT has been the use of optical navigation, using images from the spacecraft for star-based navigation, planetary limb navigation, and terrain relative navigation.

In low-Earth orbit, or below around 1,800 miles, spacecraft have long relied on GNSS signals for PNT data, which has allowed them to calculate their location using these signals to navigate. This is beneficial to these missions and these satellites and spacecraft because they can react and respond to events in real time, which can save missions money and simplify ground operations.

Beyond 1,800 miles in altitude, navigation becomes more challenging. The expanse called the Space Service Volume, or the geosynchronous orbit, extends from 1,800 to 22,000 miles, where the GNSS constellations themselves exist can be navigated; but past the GNSS constellations themselves, the signals have to be received from the opposite side of the Earth. This makes the PNT more difficult since the signals are weaker. However, through using side lobes for signal reception and piecing together their structure and strength to determine if the satellite could meet its PNT requirements.

Engineers have further used that data to develop detailed models of radiation patterns of GPS satellites in an effort called GPS Antenna Characterization Experiment. While documenting these characteristics, NASA explored the feasibility of using the side lobe signals for navigation outside what had been considered the Space Service Volume in lunar space. This has led to the development as well of the Magnetosphere Multiscale Mission (MMS), which has successfully determined its position using GPS signals at distances nearly halfway to the moon.

Example of a micro-chip for PNT.

In the military context, the Department of Defense (DoD) uses PNT for vehicles and munitions, which creates a critical vulnerability for these systems where GPS signals can be degraded or unavailable. This has seen the development of a Micro-PNT program for devices, which offers lower cost, size, weight, and power solutions for precision navigation in harsh environments relevant to the DoD's needs. To achieve this, the sensors being developed are intended to operate under high dynamics, self-calibrate, and develop fully integrated, miniature timing and inertial measurement units for ubiquitous deployment and miniature atom-based inertial sensors for extended operations.

At the same time, the United States Army is looking to develop GPS-independent PNT systems comparable to GPS systems, in that they would be capable of providing precise, localized position data in near real time, using multiple integrated systems to maximize accuracy and to identify and mitigate any drift over time. This requirement could be accomplished by possibly using multiple technologies integrated into networked systems of devices. The potential technologies include, but are not limited to, fiber-optic gyros, magnetometry, and chip-scale atomic clocks that can work to create the PNT data required. And the maximum navigation of timing error of a stand-alone device required by the projects should not exceed 20 meters and 1 microsecond at 1 hour.

An important part of this technology would be a networking protocol that will transmit the PNT data to the nodes in the local network. This could use existing wireless data services on the device, with the networking protocol able to synchronize all nodes using signal propagation delays and maintain synchronization within the error bounds of the system.

Similar to other PNT solutions, especially assured PNT services, fleet services is one area in the military where there is a need for assured PNT services, especially GPS-independent solutions that are non-proprietary and sensor-agnostic solutions. GPS-independent solutions considered for naval use have included celestial navigation, magnetometry, and other signal opportunities. Some newer systems have coupled GPS navigation with inertial navigation systems, which have a symbiotic relationship in which the inertial system is calibrated by the GPS system, and the inertial system can compute positions during a GPS outage. This requires the navy to use precise clocks to maintain position and time during a GPS outage.

Another development has been a move toward a new version of GPS called M-Code. The M-Code is an effort on the part of the U.S. Department of Defense to modernize GPS, involving additional levels of protections for GPS signals through the combined use of high-gain directional antennas and wide-angle antennas to broadcast M-Code from GPS III satellites. And another system has been designed with a modular mounted assured PNT system, which is designed to be modular, scalable, and flexible in order to integrate internal and external IMUs, internal clocks, and other redundant PNT sources for greater PNT resilience.

To expand these efforts, the Naval Information Warfare Center (NIWC) Atlantic opened the Maritime Positioning, Navigation, and Timing (M-PNT) Laboratory. The lab is the Navy's new home for research, development, test, evaluation, integration, and certification for surface and submarine PNT systems. The M-PNT laboratory is designed to support technology development for a GPS or sensor denied environment, such as enhancements to the inertial navigation systems and alternative positioning system technologies to support integrated warfare systems.