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Radio‐based navigation is another popular technique in this category. It was first developed in the early twentieth century and its application was widely developed in the World War II. More recently, it was considered as a reliable backup of the global positioning system (GPS) in the United States, and could reach a navigation accuracy of better than 50 m. One of the most common aiding techniques in this category is Global Navigation Satellite System (GNSS), where a satellite constellation is implemented in the space as “landmarks,” transmitting radio waves for navigation purposes. The navigation accuracy of GNSS for civilian use is currently about 5 m along the horizontal direction, and about 7.5 m along the vertical direction. Long‐term evolution (LTE) signals have also been proposed and demonstrated to be used for navigation purposes. The principle of LTE‐based navigation is similar to GNSS, except that the landmarks are the LTE signal towers instead of satellites. The greatest advantage of LTE over GNSS is its low cost, since no special signal towers has to be established and maintained. Currently, a horizontal navigation accuracy of better than 10 m has been reported.

In the case of short‐range navigation aiding techniques, Ultra‐Wide Band (UWB) radio, WiFi, Bluetooth, and Radio‐Frequency Identification (RFID) have all been explored. They are typically used in indoor navigation due to their short signal propagation range. Unlike radio‐based navigation, in which the radio frequency is fixed, UWB radio occupies a large bandwidth ( MHz), thus increasing capability of data transmission, range estimation accuracy, and material penetration. WiFi and Bluetooth devices are popular in smartphones, and therefore utilizing them as aiding techniques in indoor navigation does not require any additional infrastructures. RFID has also been proposed due to its low cost for implementation. More recently, 5G and millimeter‐wave communication infrastructure have been explored as a potential source of signals for navigation [27]. For all these aiding techniques, there are two kinds of methods to perform localization: Received Signal Strength (RSS) and fingerprinting. RSS‐based localization algorithm takes advantage of the fact that the strength of the received signal drops as the distance between the source and the receiver increases. Therefore, the strength of the received signal can be used as an indicator of ranging information. Fingerprinting localization algorithm is based on comparing the measured RSS values with a reference map of RSS. Table 1.1 summarizes the non‐self‐contained aiding techniques with artificial signals.

Table 1.1 Summary of non‐self‐contained aiding techniques.

Aiding technique	Applicable area	Positioning accuracy (m)	Notes
GPS	Above earth surface	5	Large signal coverage area Unavailable below the Earth's surface and in complex urban areas Susceptible to jamming and spoofing
LTE/5G	Mostly in urban areas	10	No extra infrastructure needed Rely on cellular signal coverage
Radar	In the air	50	Cheap and robust to different weathers Very large effective range Signal can penetrate insulators but will be obstructed by conductive material
UWB	Mostly indoor	0.01	Very accurate distance measurement in a short range Simple hardware with low power consumption Susceptible to interference
Lidar	In the air	0.1	Accurate position and velocity measurement Affected by the weather, such as strong sunlight, cloud, and rain
WiFi	Indoor	1	A priori knowledge of WiFi router is needed Algorithm is needed to compensate for signal strength fluctuations
Bluetooth	Indoor	0.5	Moderate measurement accuracy with very low power hardware Short range of measurement (<10 m)
RFID	Indoor	2	Easy deployment Very short range of measurement