PNT systems have cemented themselves in the modern world, finding application across almost every domain, from civilian applications such as transportation, banking, energy grids, and air travel to military employment for aircraft, drone, and ship navigation, as well as missile guidance, and precision-guided munitions.

Positioning and timing techniques form the foundation of PNT systems. They can be broadly classified as given in Figure 1.

Figure 1: Position and Timing Techniques Employed in PNT Systems

Source: Author’s own, based on quantum vs classical complementarity PNT.^[1]

Timing Techniques

Timing techniques can be divided into dead reckoning and time synchronisation. In dead reckoning, a timing sensor is employed to estimate time relative to a previously estimated time, such as logs from previous marine navigations.^[2] Time synchronisation employs the more modern approach of estimating time relative to an externally defined time, such as Coordinated Universal Time (UTC).^[3] While the latter is the more accurate and reliable approach, dead reckoning is used in the absence of traditional timing systems like the Global Positioning System (GPS). Accuracy^[a] in dead reckoning tends to reduce over time due to error accumulation (or ‘drift’) by clocks.

Timing systems rely on clocks, which typically consist of an oscillator^[b] and a counter. Earlier oscillators were only made of quartz crystal, but now they are gradually being replaced by Micro-Electromechanical Systems (MEMS) that are more accurate.^[4]

A counter consists of a circuit that counts or accumulates the periodic signals from an oscillator.^[5] Examples include binary ripple counters and decade counters.

Positioning Techniques

Similar to timing techniques, positioning techniques can be divided into dead reckoning and position-fixing. Dead reckoning employs inertial navigation to estimate position, based on a previously known or estimated position.^[6] Inertial navigation relies on inertial measurement units (IMUs), such as accelerometers and gyroscopes, to track acceleration and rotation respectively. This is done by monitoring a moving mass, such as a ball suspended from a spring, and integrating the measured accelerations and rotations at each measurement point to estimate position.^[7] Each subsequent estimation tends to accumulate measurement errors over time. Here, too, dead reckoning is reasonably accurate over short distances, but tends to accumulate errors over longer ranges due to sensor drift. Examples of mature IMUs include ring laser and fibre-optic gyroscopes.

Position-fixing estimates position relative to one or multiple landmarks sharing a common reference frame (like GNSS satellites), terrestrial features, or features from a map. It can be broadly divided into radionavigation and map/feature-matching.^[8]

In radionavigation, radio signals from one or a network of transmitters are used to estimate the receiver’s position. Depending on the transmitter, radionavigation can take on several forms:

1. Satellite Navigation or GNSS: GNSS such as GPS and the Russian Global Navigation Satellite System (GLONASS) consist of a constellation of Medium-Earth Orbit (MEO) satellites that transmit radio signals in the L-band frequency range (1–2 gigahertz) to the receiver, which uses the position data of the satellites contained therein to estimate its position (see figure in the box).^[9]
2. Terrestrial Radionavigation Systems: These consist of ground-based transmitters such as Loran (Long-Range Navigation) and pseudolites,^[c] which provide regional coverage as opposed to global coverage provided by GNSS.^[10]
3. Signals of Opportunity: These signals, such as LTE signals and broadband communication signals from Low-Earth Orbit (LEO) satellites like Starlink, are not typically meant for navigation.^[11] But receivers can estimate their position using the Doppler Effect, or the change in the frequency of the signal caused by relative motion between the receiver and source.

How Does GPS Work? GNSS like GPS require a global constellation of satellites, which transmit radio frequency (RF) signals from MEOs. Some of the GNSS systems are GPS (US), GLONASS (Russia), Beidou (China), Galileo (Europe), QZSS (Japan), and NavIC (India). GPS, in particular, currently consists of a constellation of 31 operational satellites, though it requires a minimum of 24 to function effectively, and signals from four satellites to triangulate the receiver’s position. GPS satellites carry extremely accurate atomic clocks that are used to broadcast a Pseudorandom Noise (PRN) code containing precise timing to the receiver. The time difference between the signal reception and broadcast can give the accurate receiver position after accounting for propagation delays caused by the earth’s ionosphere.

Source: US Federal Aviation Administration.^[12]

In map- or feature-matching, the user’s environmental observations are matched with an existing map or database containing known man-made (such as streets and buildings) or geophysical (such as terrain, gravitational anomalies, or magnetic fields) features. A summary of map-/feature-matching techniques is given in Table 1.

Table 1: Map-Matching Techniques

Source: Author’s own, based on Classical vs Quantum Complementarity PNT.^[13]

In general, estimating position or time for most applications involves using dead reckoning in conjunction with position-fixing and time-synching techniques to rectify the error caused by drift. Some techniques such as Simultaneous Localisation and Mapping (SLAM) inherently involve using multiple methods like dead reckoning and map-/feature-mapping.^[14] The respective accuracies for different techniques are given in Table 2.

Table 2: Accuracies and Limitations of Positioning Techniques 

Source: Author’s own, based on How Quantum Sensing Will Help Solve GPS Denial in Warfare^[15]

Limitations of Classical PNT Systems

The efficacy of classical PNT systems can be curtailed by a variety of factors:

Warfare

Given their pivotal importance for military operations, attacking PNT systems like GPS has become a focal point in modern warfare. These attacks are conducted using counterspace weapons which can be broadly classified into four categories:^[16]

1. Kinetic Weapons: These include direct ascent as well as orbital anti-satellite (ASAT) weapons. Additionally, traditional kinetic weapons like bombs and missiles can also be used to damage terrestrial PNT infrastructure.
2. Non-Kinetic Weapons: These employ radiated energy to destroy or damage space-based systems like GNSS. They include nuclear detonation and directed energy weapons like lasers.
3. Electronic Warfare: In general, electronic warfare (EW) refers to the use of the electromagnetic spectrum to conduct an attack. When it comes to GNSS, EW is confined to jamming and spoofing, as the low-power nature of GNSS signals makes them susceptible to interference and prime candidates for navigation warfare.^[17] Jamming uses an RF transmitter/jammer to overpower GNSS signals on a particular frequency band, preventing nearby receivers from distinguishing true signals from the noise. In spoofing, the adversary creates fake signals bearing the appearance of GNSS signals but containing misleading data.
4. Cyberattacks: The reliance of GNSS systems on software and the internet also renders them susceptible to cyberattacks. For instance, in 2016, an unintentional software-based disruption created a data upload problem, leading to a timing error across the GPS system.^[18]

Adversarial attacks on PNT systems can have severe ramifications for a nation’s military. For instance, in the Russia–Ukraine conflict, Russian EW systems designed to jam GPS signals have had a marked impact on the effectiveness of typically reliable US-made satellite-guided weapons used by Ukraine, such as Excalibur shells, JDAM-ER missiles, and HIMARS launchers, reducing their hit rate from over 50 percent to less than 10 percent.^[19],[20] EW has also been employed by Israel in its conflict with Hamas and Hezbollah, and led to at least one civilian aircraft nearly entering enemy airspace as it was following a spoofed signal.^[21] GPS jamming has been employed during the Iran War, too, increasing the risk of collision in vessels travelling along the Strait of Hormuz due to widespread Automatic Identification System (AIS) malfunction.^[22]

Furthermore, due to increasing commercial availability of jamming and spoofing technologies, EW is also being employed by non-state groups and terrorists in countries such as Ukraine, Syria, Afghanistan, South Korea, and India.^[23]

GPS-Denied Environments

Location is a critical factor in determining GPS signal reception. GPS signals become significantly weaker and inaccurate in remote areas or areas under dense foliage.^[24] They cannot penetrate underground or underwater. Hence, alternative techniques are required for navigation in these areas.

Natural Events

Natural events such as solar flares and Coronal Mass Ejections (CMEs)^[d] can significantly degrade GNSS performance.^[25] Terrestrial events like hurricanes can also damage or nullify alternative PNT systems like the Loran.

Other Factors

Other inadvertent factors like negligence, space debris, and GNSS technical failure can also severely handicap PNT capabilities.^[26]

In the absence of PNT systems, precision-guided munitions are essentially reduced to regular artillery shells. Tasks requiring precision timing and navigation, such as encrypted communication and preventing friendly fire, would be affected. Therefore, every nation’s military relies on a combination of PNT techniques, depending on the situation, with each technique involving a trade-off between accuracy and availability, leading to the concept of PNT resilience. However, no classical PNT system is completely free of vulnerabilities and any of them can, in principle, be disrupted. On the other hand, quantum PNT systems are not susceptible to these disruptions, since they are passive and do not rely on external signals.^[27]