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Volume 2 begins with an overview of nonlinear estimation techniques (Chapter 36), which are often required when integrating complementary navigation sensors. This chapter also lays the groundwork for the estimation strategies that are described in subsequent chapters.

The next group of chapters covers a variety of RF‐based complementary navigation techniques. Many of the principles and algorithmic approaches for indoor navigation are summarized in Chapter 37, as well as a survey of different types of indoor navigation sensors and phenomenologies. This is followed by several chapters which describe in detail a variety of RF signals, including cellular (Chapter 38), terrestrial navigation beacons (Chapter 39), digital television (Chapter 40), low‐frequency systems (Chapter 41), radar (Chapter 42), and RF signals from low‐Earth orbiting (LEO) satellites (Chapter 43).

There are two chapters that describe inertial technology: a general introduction to INS (Chapter 44) and MEMS inertial systems (Chapter 45). The introduction chapter provides an overview of inertial systems. It describes the fundamental mechanisms of various accelerometers and gyroscopes that are the building blocks of INS, their error characteristics and performances, and outlook of technology advancement. The focus of MEMS inertial sensors is to reduce the cost, size, weight, and power when compared to existing inertial sensors. Doing so would expand the applications in which it is feasible to leverage inertial technology.

It is important to recognize that inertial systems cannot operate without aiding from additional sensors, other than for short time periods. The primary reason for this is that inertial systems are unstable in the vertical channel, so at a minimum they need some sort of aiding of the vertical channel (such as a barometric altimeter or terrain height aiding). Even if the vertical channel is aided, the horizontal directions will drift in an inertial system, with the rate of drift determined by the quality of the system and the accuracy of the initialization of the attitude and position of the system. (Even if an INS had perfect gyroscopes and accelerometers, there would still be growing error due to imperfections in our knowledge of gravity).

Probably the most common sensor used to aid an inertial is a GNSS receiver. Chapter 46 describes classic approaches for integrating GPS with INS, including loose and tight integration. It also describes a different way of thinking about the GPS/INS integration problem, in which there is more emphasis on using carrier‐phase measurements to provide velocity‐like updates to the INS, with additional correction from the pseudorange measurements.

Clock has been an essential sensor for navigation since ancient times. The accuracy and stability of clocks continue the improve in recent decades. Chapter 47 provides an overview of recent technology development in atomic clocks for GNSS.

An approach for using knowledge of the variation in Earth’s magnetic field for absolute positioning using a magnetometer is described in Chapter 48. This method works indoors, on a ground vehicle, and in an aircraft, and this chapter describes the differences between these different environments and shows examples of working systems in each case.

Next, the use of LiDAR for navigation is described in Chapter 49. Various types of LiDARs are considered, as well as different ways in which LiDAR data can be leveraged for navigation purposes. This chapter also describes features that can be identified using LiDAR data, and how those features can be incorporated into an integrated navigation system. Both dead‐reckoning and absolute positioning/attitude approaches are considered.

Chapter 50 describes the many ways in which cameras can be used for navigation. Initially, a mathematical model of a camera is provided, as well as methods for camera calibration. Image features are described as well as algorithms for using these features to relate camera images to position and rotation of the camera. Several methods for image navigation are described, and as with LiDAR, both dead‐reckoning and absolute positioning/attitude approaches are considered. Another chapter (51) is dedicated to the topic of photogrammetry, which also uses a camera, but lays more emphasis on using the camera in order to develop knowledge about the scene that is viewed by one or more cameras. The vision navigation and photogrammetry chapters can be thought of as opposite sides of the same coin. With vision navigation, the desire is to figure out where the camera is, based on some knowledge of the scene. With photogrammetry, the desire is to figure out information about the scene, based on some knowledge of the camera position (and perhaps orientation).

As mentioned earlier in this introductory chapter, any measurement that changes when the sensor position changes can potentially be used as a navigation source. A good example of this is X‐ray pulsar‐based navigation which is described in Chapter 52, along with other variable celestial sources for navigation. The fundamental premise here is that if we can accurately measure the time of arrive of the periodic signal coming from several X‐ray‐emitting pulsars, we can use this information to determine our location. Additionally, methods for performing X‐ray pulsar‐based attitude determination are given.

In contrast to all of the technology‐based approaches describe thus far, Chapter 53 focuses on brain neural processing in order to perform various navigation tasks. While these neurological approaches are quite difference from the approaches that engineers normally take to develop navigation systems, the way in which navigation is done by the brain suggest possibilities that we can attempt to implement with various forms of computing. Chapter 54 further describes various ways in which animals are able to navigate and orient without the use of the modern sensors described elsewhere in this volume.

Volume 2 then concludes with several chapters that describe specific applications that make heavy use of navigation systems. Many of these applications did not exist prior to the arrival of GNSS, and those that did exist have seen large increases in capabilities by leveraging both GNSS and complementary navigation approaches.

The applications covered include survey and mobile mapping (Chapter 55), precision agriculture (Chapter 56), wearable navigation technology (Chapter 57), driverless vehicles (Chapter 58), train control (Chapter 59), unmanned aerial systems (Chapter 60), aviation (Chapter 61), spacecraft navigation and orbit determination (Chapter 62), spacecraft formation flying and rendezvous (Chapter 63), and finally Arctic navigation (Chapter 64).

Taken together, Volume 2 shows the incredible value of navigation systems and the variety of approaches that are available in cases where GNSS is not sufficient. Whether we realize it or not, our day‐to‐day lives are heavily dependent on the ability of many systems that interact with (or that are behind the scenes) to determine time and position, and there is an increasing number of creative options and opportunities for precise navigation and time that can meet the needs of current and future applications.